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PRESENTED    1!Y 


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LECTURES 


ON 

NATURAL  AMD  EXPERIMENTAL 

CONSIDERED  IN  1Tb  PRESENT  STATE  OF  IMPROVEMENT. 

DESCRIBING,   IN    A    FAMILIAR    AND    EASY   MANNER, 

THE  PRINCIPAL  PHENOMENA  OF  NATURE  j 

AND    SHOWING 

THAT  THEY  ALL  CO-OPERATE  IN  DISPLAYING  THE  GOODNESS, 
WISDOM,  AND  POWER  OF  GOD. 

BY    THE    LATE 

GEORGE   ADAMS, 

MATHEMATICAL   INSTRUMENT    MAKER    TO    HIS    MAJESTY,   &C. 

IN  FOUR  VOLUMES, 

Illustrated  with  forty-three  co/ifier/ilates,  elegantly  engraved, 

THIS  AMERICAN  EDITION, 

PRINTED    FROM    THE    LAST    LONDON    EDITION,    EDITED    BY 

WILLIAM  JONES,  MATHEMATICAL  INSTRUMENT  MAKER, 

IS  CAREFULLY  REVISED  AND  CORRECTED  BY 

ROBERT  PATTER  SO  JV, 

Professor  of  Mathematics,  and  Teacher  of  Natural  Philosophy,  in  the 
University  of  Pennsylvania. 

TO  THIS  VOLUME  IS  SUBJOINED 

AN  APPENDIX  j 

Containing- 

"A  BRIEF  OUTLINE  OF  PHYSICS,    OR  NATURAL  PHILOSOPHY,    IN    THE 
FORM    OF    A    COLLEGIATE   EXAMINATION  :" 

BY  THE  AMERICAN  EDITOR. 
VOL.  IV. 

PRINTED  FOR  WILLIAM  W.  WOODWARD, 

N°.  52,  CORNER    OF   CHESNUT    AND    SECOND    STREETS, 

PHIL  AD  EL  PHIA. 
1807. 


TABLE  OF  CONTENTS. 


VOL.  IV. 


LECTURE  XXXVII. 

Page 

OF  the  Copernican  system 1 

Summary  view  of  the  solar  system    .....       4 
Tables  of  the  diameters,  distances,  &c.  of  the  planets  31 

LECTURE  XXXVIII. 

Explanation  of  the  seasons,  &c.  on  the  Copernican 

system 39 

Of  the  shape  or  figure  of  the  earth 40 

Of  the  diurnal  motion  of  the  earth 43 

Of  the  phenomena  occasioned  by  the  diurnal  motion  45 
Of  the  annnal  motion  of  the  earth    .     .     .     .     .     49 

Of  the  apparent  motion  of  the  sun 50 

Of  the  seasons  of  the  year 55 

LECTURE  XXXIX. 
An  explanation  of  the  phenomena  of  the  planets  ac- 
cording to  the  Copernican  system       ....  64 

Of  the  inferior  planets 67 

Of  the  superior  planets 76 

Of  the  secondary  planets  or  satellites     .     .     .     .  79 

Of  the  moon's  motion 84 

LECTURE  XL. 
Of  eclipses *.....     91 

Of  the  eclipses  of  the  moon    .......     92 

Of  the  eclipses  of  the  sun    . 98 

Of  the  limits  of  the  solar  and  lunar  eclipses    .     .104 

(  f  the  period  of  eclipses 106 

The  darkness  at  our  Saviour's  crucifixion  superna- 
tural  .107 


IV  CONTENTS. 

LECTURE  XLI. 

Page. 

Of  parallax  and  refraction,  &c Ill 

Of  refraction 113 

Of  parallax Ill 

Of  the  aberration  of  light 119 

Of  the  precession  of  the  equinoxes      .     .     .     .  121 

LECTURE  XLII. 

Of  solar  and  sidereal  time,  &c.     ;     .     .     .     .     .  1 23 

Of  mean  and  apparent  time        128 

Of  the  equation  of  time 130 

LECTURE  XLIII. 

On  the  planetarium,  &c 134 

Of  the  planetarium 136 

Of  the  tellurian .147 

Of  the  lunarium 155 

Of  the  new  terrestrial  globe        .     .     .     .     .     .  161 

Of  a  celestial  globe 165 

APPENDIX  TO  LECTURE  XLIII. 

BY  THE  E,  EDITOR. 

A  further  description  of  Martin's  orrery        .     .  171 

Comparative  observations  on  globes     .     .     .     .  176 

Advantages  peculiar  to  the  new  mounted  globes  178 

Advantages  peculiar  to  the  common  mounted  globes  1 79 

Description  of  the  equatorial  or  universal  sun-dial  180 

LECTURE  XLIV. 

Of  the  fixed  stars     .     .      .     .      .      .     .      .     .     .  187 

Herschei  on  the  construction  of  the  universe,  &c.  192 

Of  the  telescopic  appearance  of  the  planets  .     .  199 

Of  comets     .     . "...  207 

Of  a  plurality  of  worlds 210 

LECTURE  XLV. 

Of  physical  astronomy    .........214 

Of  the  motion  of  the  elements 215 

Of  the  mathematical  principles  of  astronomy     .  225 

Of  deflecting  forces 226 


CONTENTS.  Y 

Page. 

Of  the  gravitation  of  the  moon     -  230 

Of  the  gravitation  of  the  primary  planets       -  238 

Of  the  centre  of  the  solar  system            -         -  241 

Of  the  approach  and  recess  of  the  planets      -  245 

Of  the  moon's  irregularities           ...  248 

LECTURE  XLVI. 

Of  electricity        -         -         -         -         .         -  257 

Electrical  appearances           -  261 

Of  electrical  attraction  and  repulsion     -         -  263 

Of  the  electrical  machine     -  267 

Experiments  on  electrical  attraction,  &c.          -  273 

Of  imitating  the  planetary  motions         -         -  278 

Electrical  fluid  universally  disseminated            -  280 

Of  the  Franklinian  theory             -         -         -  282 
Of  the  electric  spark,  and  of  the  influence  of 

points     -         -          -                   -          -         -  285 

Of  the  diffusion  and  subdivision  of  the  electric  fluid  292 

LECTURE  XL VII. 

Of  the  Leyden  phial 293 

Of  the  theory  of  the  Leyden  phial         -         -  296 

Experiments  on  the  theory  of  ditto         -         -  299 
Experiments  on  the  the  contrary  electricities  of  the 

Leyden  jar 304 

Of  the  electrical  battery        -         -         -         -  313 

LECTURE  XLVIII. 

Of  lightning  and  conductors,  &c.     -         -         -  317 

Of  conducting  rods 326 

LECTURE  XLIX. 

Of  the  nature  of  electricity      -         -         -         -  331 

Of  animal  electricity             ....  354 

Of  the  later  experiments  on  animal  electricity  362 

LECTURE  L. 

Of  magnetism 374 

To  ascertain  whether  a  body  has  any  iron       -  -       377 

Of  the  poles  of  a  magnet     -  '      -         -         -  379 

Of  the  action  of  the  magnetic  poles       -         -  380 


VI  CONTENTS. 

Page. 

Of  magnetic  centres 383 

To  render  iron  magnetic       -  385 

To  touch  a  horse- shoe  magnet       -  387 

To  make  a  magnet  with  several  poles    -         -  388 

Of  armed  magnets       -         -          -         -         -  391 

Of  the  magnetism  of  the  earth      -  392 

Of  the  directive  power  of  a  magnet       -         -  394 

Of  the  variation  of  the  needle       -  397 

Of  the  dip  of  the  needle      -  400 

Of  the  influence,  of  the  aurora  borealis            -  401 

Similarity  between  electricity  and  magnetism  402 

Of  the  theory  of  magnetism         -         -          -  403 

LECTURE  LI. 

Of  meteorology      -.----  405 

Of  the  barometer 411 

Of  some  of  principal  the  requisites  of  a  good  ba- 
rometer           ------  413 

To  boil  the  quicksilver  in  a  barometer  tube    -  414 

Of  the  nonius     -         - 417 

Of  the  portable  barometer   -         -         -         -  418 

Of  the  thermometer    -----  422 

Of  the  rain-gage »  430 

Of  the  hygrometer      -         -         -         -         -  431 

LECTURE  LII. 

Of  rain -         -  4S9 

Of  the  nature  of  clouds         -  446 

Of  the  duration  of  clouds     -  448 

Of  hail 452 

Of  thunder 452 

Of  winds 455 

Of  trade-winds  and  monsoons       -         -         -  458 

Of  the  aurora  borealis          -         -         -         -  465 

Of  the  sources  of  heat          -  466 

Of  the  sources  of  cold         -  468 

Of  evaporation 470 

Of  annual  temperature         -         -         -         -  471 

Of  atmospherical  electricity           -  474 

Of  prognostic  signs  of  the  weather        -         -  477 

Of  prognostics  by  the  barometer                      -  479 


CONTENTS.  .       Vll 

Page. 

Further  indications  of  the  weather  by  the  barometer  481 
From  the  thermometer,  &c.  ...         482 

From  clouds       -         -         -         --         -         483 

Jones  on  the  superiority  of  the  northern  hemisphere  484 
Conclusion -         487 


APPENDIX  TO  LECTURE  LIE 

BY  THE  E.  EDITOR. 

Of  a  barometer  to  measure  the  heights  of  moun- 
tains, &c 489 

Of  de  Luc's  hygrometer .     .     .  491 

Of  Six's  improved  thermometer 495 

Of  the  rain-gage 495 

Of  the  wind-gage   .     .  , 496 

CONTENTS  OF  APPENDIX, 

BY  THE  A.  EDITOR. 

Physics,  or  natural  philosophy  defined      .     .     .  491 

Rules  of  philosophizing 491 

General  properties  of  matter 492 

Laws  of  motion      .........  •»  .  463 

Impact  of  bodies 493 

Central  forces -     .     .     .     .  495 

Attraction  of  cohesion 497 

Attraction  of  gravitation 498 

Projectiles 499 

Centre  of  gravity 501 

Mechanic  powers    . 502 

The  lever 503 

The  axis  and  wheel 504 

The  pulley  and  tackle 504 

The  inclined  plane     .     .' 505 

The  wedge •     .  505 

The  screw 505 

Friction    . :     .     .  506 

Motion  of  bodies  on  inclined  planes     ....  506 


Vlll  CONTENTS. 

Page. 

Pendulums 507 

Hydrostatics       .     .     • 508 

Specific  gravities 509 

Pneumatics 510 

Sound 512 

Hydraulics 514 

Hydraulic  machines 515 

Optics 516 

Catoptrics    .     . 517 

Dioptrics 520 

The  eye  and  vision 522 

The  rainbow 523 

Microscopes      .     - 524 

Telescopes 426 

Magnetism 529 

Electricity      .     .    , 532 

Astronomy 537 

Eclipses        ...........  546 

Table  of  the  planets'  motions,  distances,  &c.  548 

Tides  .     .     . 550 

Winds      .    ' 552 

Chronology 555 


A  TABLE 

OF 

JREFERENCES  TO  THE  PLATES 

IN  VOLUME  IV. 


Fig.  1,  p.  5. 


ASTRONOMY — Plate  L 


ASTRONOMY— Plate  II. 


Fig.  1,  p.  42.  Fig.  3,  p.  47.  Fig.  4,  p.  54 

Fig.  2,  p.  42. 

ASTRONOMY.— Plate  III. 
Fig.  3,  p.  8,  Fig.  4,  p.  9. 


ASTRONOMY.— Plate  IV. 

Fig.  1,  p.  51.  Fig.  3,  p.  126.  Fig.  5,  p.  131. 

Fig.  2,  p.  125.  Fig.  4,  p.  130. 


ASTRONOMY.— Plate  V. 
Fig.  1,  p.  56,  57,  59.         Fig.  2,  p.  66. 


ASTRONOMY.— Plate  VI, 

Fig.  1,  p.  61. 

Fig.  2,  p.  67,  68,  69. 
ASTRONOMY— Plate  VII. 

Fig.  3,  p.  76. 

Fig.  1,  p.    77. 
Fig.  2,  p.  116. 

Fig.  3,  p.  112. 
ASTRONOMY — Plate  VIII. 

Fig.  4,  p.  114. 

Fig.  1,  p.  75. 
Fig.  2,  p.  75. 
Fig.  3,  p.  87. 

Fig.  4,  p.  88. 
Fig.  5,  p.  93. 

ASTRONOMY Plate  IX. 

Fig.  6,  p.  93. 
Fig.  7,  p.  93. 

Fig.  1,  p.  89. 

Fig.  2,  p.  89. 
ASTRONOMY— Plate  X. 

Fig.  3,  p.  84o 

Fig.  1,  p.  94. 
VOL.  IV: 

Fig.  2,  p.  95. 

A 

Fig.  3,  p.  100, 

ASTRONOMY.— Plat_e  XI. 

Fig.  1,  p.  135, 172.  Fig.  2.  p.  142,  173.  Fig.  3,  p.  142,  IT'S. 

ASTRONOMY.— Plate  XII. 

Fig.  1,  p.  135, 147, 172,    Fig.  2,  p.  135,  155,  174. 
173. 

ASTRONOMY.— Plate  XIII. 

Fig.  2,  p.  161, 163, 176,    Fig.  3.  p.  165, 176,  178.    Fig.  4,  p.  164. 
178. 

ASTRONOMY— Plate  XIV. 

Fig,  2,  p.  171, 181. 

ASTRONOMY — Plate  XV. 

Fig.  1,  p.  119.'  Fig.  8,  p.  249.  Fig.  14,'  p.  252. 

Fig.  2,  p.  120.  Fig.  9,  p.  250.  Fig.  15,  p.  252. 

Fig.  3,  p,  227.  Fig.  10,  p.  250.  Fig.  16,  p.  252. 

Fig.  4,  p.  228.  Fig.  11,  p.  250.  Fig.  17,  p.  253. 

Fig.  5,  p.  233,  235.  Fig.  12,  p.  251.  Fig.  18,  p.  253. 

Fig.  6,  p.  245.  Fig.  13,  p.  251.  Fig.  19,  p.  253. 

Fig,  7,  p.  248. 

ELECTRICITY— Plate  I. 

Fig.  1,  p.  265,  279.  Fig.  8,  p.  294.  Fig.  14,  p.  309. 

Fig.  2,  p.  265.  Fig.  9,  p.  294.  Fig.  15,  p.  311,  316. 

Fig.  3,  p.  266.  Fig.  10,  p.  302.  Fig.  16,  p.  313. 

Fig.  4,  p.  266.  Fig.  11,  p.  303.  Fig.  17,  p.  296. 

Fig.  5,  p.  267,  269.  Fig.  12,  p.  294.  Fig,  18,  p,  312. 

Fig.  6,  p.  277.  Fig.  13,  p.  308. 
Fig.  7,  p.  275. 

ELECTRICITY  and  MAGNETISM Plate  II. 

Fig.  1,  p.  280. 

Fig.  2,  p.  288. 

Fig.  3,  p.  329. 

Fig.  4,  p.  289. 

Fig.  5,  p.  291, 

Fig.  6,  p.  289. 


Fig.    7,  p.  380. 

Fig.  13,  p.  386. 

Fig.    8,  p.  381. 

Fig.  14,  p.  391. 

Fig.    9,  p.  382. 

Fig.  15,  p.  386. 

Fig.  10,  p.  382. 

Fig.  16,  p.  378. 

Fig.  11,  p.  383. 

Fig.  17,  p.  401. 

Fig.  12,  p.  383,  398. 

LECTURES 


ON 


NATURAL  PHILOSOPHY. 


ON 


ASTRONOMY. 


LECTURE  XXXVII. 


OF    THE    COPERNICAN    SYSTEM. 


HAVING  shown  you  the  appearance  of  the  hea- 
venly bodies  as  seen  from  the  earth,  it  will  be  now  pro- 
per to  show  you  why  the  motions  of  the  planets  appear 
to  us  so  different  from  what  they  really  are.  One  of  the 
ends  for  which  man  was  formed,  is  to  correct  appear- 
ances and  errors  by  the  investigation  of  truth  ;  whoever 
considers  him  attentively  from  infancy  to  manhood,  and 
from  manhood  to  old  age,  will  find  him  ever  busy  in 
endeavouring  to  find  some  reality  to  supply  the  place  of 
those  false  appearances,  by  which  he  has  hitherto  been 
deceived.  Thus,  it  is  the  business  of  the  present  lecture 
to  correct  those  errors  that  arise  from  appearances  in 

VOL.  IV.  13 


2  THE    COPERNICAN    SYSTEM. 

the  heavens,  and  to  prove  the  truth  of  the  Copernican 
System,  which  is  generally  received,  because  it  rationally 
accounts  for,  and  accords  with,  the  phenomena  of  the 
heavens.  In  this  system,  the  sun  is  placed  in  the  centre, 
and  the  earth  and  other  planets  revolve  round  him  as 
their  centre. 

There  are,  however  strong  reasons  for  believing,  that 
some  of  the  sages  of  antiquity  were  acquainted  with  the 
true  solar  system  as  revived  by  Copernicus.  It  was  the 
universal  doctrine  of  the  Pythagorean  school,  and  is 
clearly  marked  out  as  such  by  Aristotle  :  for  these,  says 
he,  assert,  that  fire  is  in  the  midst  of  the  world,  and 
that  the  earth  is  one  of  the  heavenly  bodies.  He  after- 
wards speaks  of  a  set  of  men,  who  held  a  system  essen- 
tially similar  to  that  of  the  modern  Semitychonic.  Eu- 
demus,  in  his  history  of  astronomy,  as  cited  by  Anatolius, 
says,  that  Anaximander  was  the  first  who  discovered  the 
earth  to  be  one  of  the  heavenly  bodies,  and  to  move 
round  the  centre  of  the  world.  Aristar chits  held,  that 
the  earth  is  carried  round  the  sun,  in  the  circumference 
of  a  circle,  of  which  the  sun  itself  is  the  centre ;  and 
that  the  sphere  of  the  fixed  stars  is  so  immense,  that  the 
circle  of  the  earth's  annual  orbit  bears  no  greater  pro- 
portion to  it,  than  the  centre  of  any  sphere  bears  to  its 
whole  surface.  Pbilolaus,  and  others,  declared  the  mo- 
tion of  the  sun  round  the  earth  to  be  only  apparent. 
They  saw  and  felt  the  importance  of  this  globe  over  ours, 
and,  supposing  its  influence  to  extend  to  much  larger 
bounds  than  that  of  the  earth,  they  placed  it  in  the  centre 
of  the  universe.  Among  the  Romans  we  find,  that  Numa 
built  a  temple  to  represent,  as  Plutarch  interprets  it,* 
the  system  of  the  heavens,  with  a  sacred  fire  in  the 
centre  of  it. 

Thus  also  in  the  Jewish  tabernacle,  the  seven  lights 
had  a  reference  to  the  seven  chief  lights  of  the  hea- 


*  Those  who  \v:mt  forth-  r  ii  if  .nation  on  this  head,  muy  consult  the 
n^tes  to  Sydejiham's  Trans)  itym  of  the  Rivals  of  Phito,  IJu/en\s  Inquiries 
fnt&the  Origin  of  the  D&crtvertes  attributed  to  the  Mbgerng;  Jones  Es- 

H  iv  <  w  mo  First  Principles  oj  N .»;ur«l  Philos  ,.;>hy  ;  Baih'ir  lii<io\vc  de  i"As- 
troiiomie  Aucienne. 


THE    CORPERNICAN    SYSTEM.  3 

vens.  Hence  also  the  heavens  are  called,  in  sacred  writ, 
the  tabernacle  of  the  sun  ;  the  whole  of  our  system 
dwelling  within  his  influence.  The  foregoing  citations* 
are,  we  presume,  sufficient  to  show,  that  the  ancients 
were  not  ignorant  of  the  true  solar  system. 

But  still  it  was  no  general  persuasion,  nor  does  it 
seem  ever  to  have  been  mentioned  after  the  time  of 
Ptolemy i  who  adopted  that  system  which  now  goes  un- 
der his  name ;  his  system,  though  erroneous,  was  in- 
genious ;  and  with  it  the  world  was  content  for  many 
ages.  It  was  then  considered  as  founded  upon  invinci- 
ble demonstration ;  as  a  sacred  truth  that  could  not  be 
weakened  by  the  powers  of  controversy,  or  shaken  by 
the  fluctuations  of  opinion. 

"  But  at  the  time  appointed,  when  it  pleased  the 
Supreme  Dispenser  of  good  gifts  to  restore  light  to  a 
bewildered  world,  and  more  particularly  to  manifest 
his  wisdom  in  the  simplicity  as  well  as  the  grandeur  of 
his  works,  he  opened  the  scene  with  a  revival  of  sound 
astronomy."* 

This  observation  of  the  President  of  the  Royal  Socie* 
ty,  is  well  worthy  your  attention  ;  it  will  enlarge  your 
views  of  Divine  Providence — a  topic  that  ought  to  be 
set  in  every  possible  light  that  can  make  it  either  more 
clearly,  or  more  generally  understood.  If  you  look 
through  the  history  of  past  ages  from  the  early  periods 
of  the  pastoral  and  patriarchal  life,  you  will  see  arts  and 
sciences  progressively  advancing  ;  sometimes  indeed  bu- 
ried for  a  long  interval,  but  again  reviving  with  new 
splendour.  You  see  philosophy  and  religion  advancing, 
and  though  sometimes  deformed  by  superstition,  and 
unnatural  systems  of  atheism,  yet  successively  recover- 
ed from  the  dreams  of  enthusiasts,  and  the  subtilty  of 
the  atheist.  If  you  see  scepticism  and  infidelity  mak- 
ing frequent  attacks  upon  sacred  truth,  you  may  rest 
satisfied,  that  these  attempts  will,  in  due  time,  magni- 
fy its  power,  increase  its  honours,  and  advance  its  tri- 
umphs. 


*  hJr  John  Pnngic*  Six  Di;  courses,  p.  97. 


4  SUMMARY    VIEW    OF 

There  is  no  man  who,  with  respect  to  the  arts  and 
improvements  of  life,  does  not  look  back  with  pity  on 
past  times,  compared  with  his  own  ;  and  philosophy  ne- 
ver extended  the  province  of  human  knowledge  so  far 
and  wide  as  within  the  last  century.  It  is  thus  also  with 
divine  knowledge  ;  this  had  the  same  gradation  and 
order  of  progression,  nature  and  law ;  the  type  and  the 
archetype ;  the  shadow  and  the  substance :  the  king- 
dom  of  God  is  still  advancing,  and  the  evidences  of  his 
administration  and  attributes  still  opening  ;  every  thing 
evinces  that  a  grand  design  has  been  carrying  on  from 
the  earliest  account  of  history  by  a  remarkable  course 
of  Providence,  for  the  benefit  of  the  human  race. 


SUMMARY    VIEW    OF    THE    SOLAR    SYSTEM. 

I  shall  now  proceed  to  give  you  a  summary  view  of 
the  solar  system,  or  that  which  was  revived  and  drawn 
from  oblivion  by  Nicholas  Copernicus,  about  the  year 
1500.  It  is  called  the  solar  system,  because  here  the  sun 
is  supposed  to  be  fixed  in  the  centre,  with  our  earth,  and 
several  bodies  similar  thereto,  revolving  round  him  at 
different  distances. 

The  bodies  within  our  system  that  revolve  round  the 
sun,  appear  bright  by  reflecting  the  light  they  receive 
from  him,  and  are  divided  by  astronomers  into  three 
kinds,  primary  planets,  secondary  planets,  and  comets. 

The  primary  planets  are  those  bodies  which,  in  revolv- 
ing round  the  sun,  respect  him  only  as  the  centre  of 
their  courses ;  the  motions  of  these  are  regularly  per- 
formed in  tracks  or  paths  that  are  found  to  be  nearly 
circular  and  concentric  to  each  other. 

A  secondary  planet,  commonly  called  a  satellite  or  moon% 
is  a  body,  which,  while  it  is  carried  round  the  sun,  does 
also  revolve  round  a  primary  planet,  which  it  respects 
as  a  centre. 

Comets  are  bodies,  which  are  also  supposed  to  revolve 
round  the  sun  ;  the  planets  appear  permanent  in  the 
system,  but  comets  only  appear  accidentally :  they  are 
named  comets  from  their  being  usually  attended  with 


THE    SOLAR    SYSTEM.  5 

long  tails,  fancied  by  some  to  resemble  hair.  The  theo- 
ry of  their  motions  amounts  at  present  to  little  better 
than  rude  conjecture. 

The  path  described  by  a  planet  in  its  motion  round 
the  sun,  is  called  its  orbit. 

In  speaking  of  orbits,  nothing  more  is  meant  than  an 
imaginary  circle  defining  the  path  they  describe,  and  m 
which  they  are  retained  by  a  celestial  but  continuous 
mechanism. 

There  are  seven  primary  planets  usually  reckoned  in 
order  from  the  sun  ;  their  names  and  marks  are, 

Mercury.  Venus.  The  Earth.   Mars.   Jupiter.  Saturn.  Georgium  Sidus. 
$  9  ©  t  %  h  ¥ 

Mar,  Jupiter,  Saturn,  and  the  Georgium  Sidus,  are 
called  superior  planets,  because  their  orbits  include  that 
of  the  Earth. 

Venus  and  Mercury  are  called  inferior  planets,  be- 
cause their  orbits  are  contained  with  the  Earth's. 

By  the  assistance  of  telescopes,  secondary  planets  have 
been  discovered  ;  the  Earth  is  attended  by  one,  Jupiter 
by  four,  Saturn  by  seven,  and  the  Georgium  Sidus  by 
six. 

This  diagram,  plate  1 ,  jig,  1 ,  Astronomy,  represents 
the  solar  system  ;  O,  in  the  centre,  represents  the  sun, 
A  B  the  circle  described  by  Mercury  in  moving  round 
the  sun,  C  D  that  in  which  Venus  moves,  F  G  the  or- 
bit of  the  Earth,  HK  that  of  Mars,  IN  that  of  Jupiter, 
O  P  the  path  of  Saturn,  O  R  the  orbit  of  the  Georgium 
Sidus. 

Every  primary  planet  is  supposed  to  have  two  mo- 
tions,  1 .   The  annual  ;   2.  The  diurnal. 

The  annual  motion  of  a  planet  is  that  whereby  it  is 
carried  in  its  orbit  round  the  sun,  which  in  every  one  is 
found  to  be  in  the  same  direction,  namely,  from  west 
to  east. 

This  motion,  as  you  have  seen,  is  discovered  by  the 
planets  changing  their  places  in  the  celestial  sphere, 
where  they  appear  to  move  among  the  fixed  stars  ;  and 
in  certain  times  to  return  to  the  same  stars  from  which 
they  were  seen  to  depart,  and  so  on  continually. 


6  SUMMARY    VIEW    OF 

The  diurnal  motion  of  a  planet  is  that  by  which  it  turns 
or  revolves  about  its  axis  ;  this,  like  their  annual  motion, 
is  also  from  west  to  east. 

This  motion  is  discovered  by  the  spots  that  are  seen  by 
telescopes  on  the  surface  of  the  planets ;  before  the  dis- 
covery of  telescopes,  it  was  not  suspected  that  the  planets 
had  a  rotatory  motion. 

By  continued  observation,  the  spectator  finds,  that 
these  spots  change  their  places,  and  move  from  one  side 
of  the  planet  to  the  other  ;  then  disappear  for  a  certain 
space  of  time ;  after  which,  they  again,  for  a  while,  be- 
come visible  on  the  side  where  they  were  first  seen,  always 
continuing  the  same  motion  nearly  in  a  uniform  manner. 
The  distance  between  the  spots  increases  as  they  advance 
from  the  edge  towards  the  middle  of  the  planet,  and  then 
diminishes  again  as  they  pass  from  the  middle  to  the  other 
edge.  The  time  they  are  seen  on  the  planet's  disk,  is 
somewhat  less  than  the  time  of  their  disappearance  ;  they 
are  first  seen  on  the  eastern  margin  of  the  planet,  and 
disappear  on  the  western. 

From  these  circumstances  it  is  concluded,  first,  that 
these  spots  adhere  to  the  body  of  the  planet ;  and,  se- 
condly, that  each  planet  is  a  globe  turning  on  its  axis. 

It  may  not  be  improper  to  observe  to  you,  that  the 
axis  of  a  planet  is  only  an  imaginary  line  conceived  to 
be  drawn  through  its  centre,  and  about  which  it  is  con- 
ceived to  turn  in  the  course  of  its  revolution  round  the 
sun.  A  ball,  whirled  from  the  hand  in  the  open  air,  turns 
round  upon  a  line  within  itself,  while  it  is  moving  for- 
ward ;  such  a  line  as  this  is  meant  when  we  speak  of  the 
axis  of  a  planet. 

The  sun  and  moon,  the  stars  and  planets,  appear  to  be 
all  at  an  equal  distance  from  us  ;  though  it  is  highly  pro- 
bable, that  some  of  the  stars  are  many  millions  of  times 
nearer  to  us  than  others.  The  sun  is  demonstrated  to  be 
nearer  than  any  of  the  stars.  The  moon  and  some  of  the 
planets  are  known,  by  ocular  proof,  to  be  nearer  to  us 
than  the  sun,  because  they  sometimes  come  between  it 
and  our  eye,  and  hide  the  whole,  or  a  great  part  of  his 
disk  from  our  view.  They  all,  however,  appear  equally 
distant,  and  as  if  placed  in  the  surface  of  a  sphere,  where- 


THE    SOLAR    SYSTEM.  \ 

of  our  eye  is  the  centre.  In  whatever  place,  therefore,  a 
spectator  resides,  whether  it  be  on  this  earth,  in  the  sun, 
or  in  the  regions  of  Saturn,  he  will  consider  that  place  as 
the  centre  of  the  world  ;  since  it  will  be  to  him  the  cen- 
tre of  a  spherical  surface,  in  which  all  distant  bodies  ap- 
pear to  be  placed  ;  while  he  remains  in  the  same  place, 
he  cannot  judge  properly  of  the  distance  of  surrounding 
objects,  at  least  of  those  which  are  placed  beyond  the  or- 
dinary reach  of  his  view ;  for,  beyond  that  distance,  all 
the  principles  by  which  we  form  our  general  judgment 
fail  us,  and  we  can  only  tell  which  is  nearest,  or  which  is 
farthest,  by  our  own  motion,  or  that  of  the  objects. 

To  illustrate  this,  let  us  suppose  a  number  of  lamps 
to  be  placed  irregularly  at  different  distances  from  the  eye 
in  a  dark  night.  Now,  if  in  this  case  we  suppose  the 
darkness  to  be  so  complete  that  no  intermediate  objects 
can  be  seen,  no  difference  in  colour  observed,  nor  any 
perception  of  a  convergence  towards  the  point  of  sight, 
our  judgment  could  not  assist  us  in  distinguishing  the 
distance  of  one  from  that  of  another  ;  they  would  there- 
fore all  seem  to  be  at  an  equal  distance  from  the  specta- 
tor. 

Each  planet  is  observed  to  pass  through  the  constella- 
tions, Aries,  Taurus,  Gemini,  Cancer,  Leo,  Virgo,  Li- 
bra, Scorpio,  Sagittarius,  Capricornus,  Aquarius,  and 
Pisces ;  and  it  also  appears,  that  every  one  has  a  track 
peculiar  to  itself,  and  that  they  never  move  out  of  a  cer- 
tain space  or  zone  of  the  heavens,  which  is  called  the 
zodiac. 

By  observing  the  planets  in  their  periodic  revolutions 
among  the  fixed  stars,  it  is  found,  that  the  paths  of  the 
planets  are  not  all  in  the  same  plane,  but  that  they  cross 
each  other  in  different  parts  of  the  heavens.  As  they  thus 
move  in  planes  that  are  differently  inclined  to  each  other, 
it  became  necessary  to  refer  them  all  to  one  plane,  in  or- 
der both  to  judge  more  accurately  of  their  inclination,  and 
to  avoid  the  intricacies  of  calculation  ;  this  plane  thus  be- 
came a  standard,  was  considered  as  having  no  obliquity, 
and  all  the  rest  were  referred  to  it.  For  this  purpose  as- 
tronomers have  fixed  upon  the  ecliptic,  or  orbit  of  the 
earth. 


8  SUMMARY    VIEW    OF 

The  plane  of  the  ecliptic  is  supposed  to  divide  the  ce- 
lestial sphere  into  two  equal  parts,  called  the  northern  and 
southern  celestial  hemispheres ;  and  any  body  in  either  of 
these  hemispheres,  is  said  to  have  north  or  south  latitude, 
according  to  the  hemisphere  it  is  in.  The  latitude  of  a 
celestial  object  is  its  nearest  distance  from  the  ecliptic  ta- 
ken on  the  sphere. 

The  planets  are  observed  to  be  sometimes  on  the  north- 
ern and  sometimes  on  the  southern  side  of  the  ecliptic,  so 
that  their  respective  planes  cut  the  ecliptic  in  two  oppo- 
site points,  called  nodes ;  or,  in  other  words,  the  nodes  of 
a  planet's  orbit  are  the  two  points  where  it  intersects  the 
ecliptic.  Thus,  let  A  B  C  D,  plate  3,  Jig.  3,  represent 
the  ecliptic,  B  E  D  F  the  orbit  of  a  planet,  the  points,  B 
and  D,  are  the  two  nodes. 

One  is  called  the  ascending  node,  and  is  usually  marked 
thus  Si  ;  it  is  that  through  which  the  planet  passes  when 
it  moves  out  of  the  southern  into  the  northern  hemisphere. 
The  other  node  through  which  the  planet  passes  in  going 
out  of  the  northern  into  the  southern  hemisphere,  is  call- 
ed the  descending  ?iode9  and  marked  thus  r  . 

During,  therefore,  every  revolution,  each  planet  must 
describe  half  its  orbit  above  the  plane  of  the  ecliptic,  the 
other  half  below.  They  have  therefore  a  north  latitude, 
while  they  describe  one  half  of  their  orbit ;  and  a  south 
latitude,  while  they  describe  the  other  half. 

The  several  orbits  do  not  cross  the  ecliptic  at  the  same 
point,  or  in  the  same  angles ;  their  nodes  are  at  differ- 
ent parts  of  the  ecliptic. 

A  right  line,  joining  the  two  nodes  of  any  planet,  is 
called  the  line  of  the  nodes. 

The  line  of  the  nodes  passes  through  the  sun,  for  as  the 
motion  of  every  planet  is  in  a  plane  passing  through  the 
sun,  consequently,  the  intersection  of  these  planes,  that 
is,  the  line  of  the  nodes,  must  also  pass  through  the  sun. 

You  may  render  the  inclination  of  the  planet's  orbits 
to  each  other  familiar  to  your  mind,  by  taking  as  many 
hoops  as  there  are  planets,  with  a  wire  thrust  through 
each,  and  thereby  joined  to  that  hoop  which  represents 
the  ecliptic,  and  you  may  then  set  the  other  hoops  more 
or  less  obliquely  to  the  representative  of  the  ecliptic. 


THE    SOLAR    SYSTEM.  9 

I  before  mentioned  to  you,  that  the  planets  revolve 
round  the  sun  in  orbits  nearly  circular  and  concentric, 
for  their  several  phenomena  show,  that  they  are  not 
strictly  so.  And  astronomers  have  found,  that  the  only 
curve  they  can  move  in,  to  reconcile  all  the  various  ap- 
pearances, is  an  ellipsis ;  so  that  the  orbits  of  the  pri- 
mary planets  are  ellipses  of  different  curvatures,  having 
one  common  focus  in  which  the  sun  is  fixed ;  but  every 
secondary  planet  respects  its  primary,  round  which  it 
revolves  as  the  focus  of  its  elliptic  motion. 

To  describe  an  ellipsis,  let  a  thread,  tied  together  at 
both  ends,  be  put  over  two  pins  fixed  upright  upon  a 
plane,  at  any  distance  from  each  other  less  than  the  string 
thus  tied  will  reach,  a  pen  carried  round  within  the  string, 
so  as  to  keep  it  always  stretched  out  with  the  same  ten- 
sion, will  describe  upon  the  plane  a  curve,  which  is  the 
periphery  or  circumference  of  an  ellipsis.  Either  of  the 
points,  S,  N,  plate  3,  fig.  4,  where  the  pins  are  fixed  in 
the  plane,  is  called  the  focus  of  the  ellipsis.  The  farther 
the  foci  are  from  one  another,  the  more  oblong  will  the 
ellipsis  described  with  the  thread  be  ;  and  the  nearer  the 
foci  are  to  each  other,  the  nearer  will  the  ellipsis  be  to  a 
circle. 

A  line,  P  A,  plate  3,  fig.  4,  drawn  through  the  foci 
both  ways,  till  it  reach  the  circumference,  is  called  the 
greater  axis,  or  longest  diameter.  A  point,  G,  taken  in 
this  line  equally  distant  from  the  foci,  is  called  the  centre 
of  the  ellipsis.  A  line,  T  V,  drawn  through  the  centre 
perpendicular  to  the  longest  diameter,  till  it  reach  the 
circumference  both  ways,  is  called  the  lesser  axis,  or 
shortest  diameter.  The  distance  between  the  centre  and 
either  of  the  foci,  C  N  or  C  S,  is  the  excentricity  of  the 
ellipsis,  which  is  greater  or  less,  as  the  ellipsis  is  more  or 
less  oblong. 

The  orbit  of  every  planet  is  an  ellipsis,  having  the  sun 
in  one  of  its  foci.  The  axis,  P  A,  of  any  planet's  ellipsis, 
is  called  the  line  of  the  apsides  ;  the  point  A.  where  the 
planet  is  at  its  greatest  distance  from  the  sun,  is  its  aphe- 
lion,  or  higher  apsis ;  the  point  P,  where  it  is  at  its  least 
distance  from  the  sun,  its  perihelion,  or  lower  apsis  ;  the 
extreme  points  of  the  shortest  diameter,  T  V,  are  the 
VOL.  IV.  C 


10  SUMMARY    VIEW,    &C. 

places  of  its  middle  or  mean  distance  from  the  sun.  A 
line,  ST  or  S  V,  drawn  from  either  of  those  points  to  the 
sun,  is  the  line  of  its  mean  distance.  To  estimate  the  ex- 
centricity  of  any  planet,  we  suppose  the  line  of  its  mean 
distance,  ST,  to  be  divided  into  1000  equal  parts,  and 
say  the  excentricity  is  such  a  number  of  those  parts. 

The  motion  of  the  planets  in  their  orbits  is  not  equa- 
ble ;  but  every  planet  observes  this  rule,  that  a  line  drawn 
from  the  sun  to  the  planet  sweeps  over  equal  areas  upon 
the  plane  of  its  ellipsis  in  equal  times ;  therefore  every 
planet  moves  swiftest  in  its  perihelion,  slowest  in  its  aphe- 
lion, and  with  a  middle  or  mean  motion  at  its  mean  dis- 
tance. 

Thus,  in  the  figure,  A  is  the  place  of  the  aphelion,  P 
the  place  of  the  perihelion,  P  A  the  line  of  apsides,  P  A 
is  the  transverse  diameter  of  the  ellipsis,  T  V  the  conju- 
gate diameter. 

The  mean  distance  of  a  planet  from  the  sun,  is  its  dis- 
tance from  him  when  the  planet  is  at  either  extremity  of 
the  conjugate  diameter,  and  is  equal  to  half  the  transverse 
diameter. 

When  two  planets  are  seen  together  in  the  same  sign 
equally  advanced,  they  are  said  to  be  in  conjunction  ;  but 
when  they  are  in  direct  opposite  parts  of  the  zodiac,  they 
are  said  to  be  in  opposition. 

The  place  that  any  planet  appears  to  occupy  in  the  ce- 
lestial hemisphere,  when  seen  by  an  observer  supposed  to 
be  placed  in  the  sun,  is  called  its  heliocentric  place. 

The  place  it  occupies  when  seen  from  the  earth,  is 
called  its  geocentric  place. 

A  motion  in  the  heavens  in  the  order  of  the  signs,  as 
from  Aries  to  Taurus,  Sec.  is  said  to  be  in  consequential 
and  such  are  the  real  motions  of  all  the  planets,  though 
their  apparent  motions  are  sometimes  contrary,  and  then 
they  are  said  to  move  in  antecedentia. 

The  points  where  the  celestial  equator  cuts  the  ecliptic 
are  found  to  have  a  motion  in  antecedentia  of  about  fifty 
seconds  every  year.  This  change  of  place  of  the  first  point 
of  the  ecliptic,  from  whence  the  signs  are  counted,  occa- 
sions a  like  change  in  the  signs  themselves;  which,  though 
scarce  sensible  for  a  few  years,  has  now  become  very  con- 


FIGURE    AND    LIGHT    OF    THE    PLANETS.  11 

siderable.  Thus,  since  the  time  that  astronomy  was  cul- 
tivated by  the  Greeks,  that  is,  about  2000  years  ago,  the 
first  point  of  the  ecliptic  has  removed  backward  about  a 
whole  sign  ;  and  though  it  was  then  about  the  middle  of 
the  constellation  Aries,  it  is  now  about  the  middle  of  Pis- 
ces. Notwithstanding  this  alteration,  the  signs  still  retain 
their  ancient  names  and  marks. 

The  longitude  of  a  phenomenon  in  the  heavens  is  the 
number  of  degrees  counted  from  the  first  point  of  Aries 
on  the  ecliptic,  to  the  place  where  a  circle  of  latitude  drawn 
through  the  phenomenon  would  cut  the  ecliptic  at  right 
angles. 

Thus,  every  phenomenon  in  the  heavens  is  referred  to 
the  ecliptic  by  the  circles  of  latitude,  as  the  longitude  of 
terrestrial  places  are  referred  to  the  equator  by  the  meri- 
dians ;  and  whatever  sign  the  circle  of  latitude  passes 
through,  the  phenomenon  is  said  to  have  its  place  in  that 
sign. 

OF  THE  FIGURE  AND  LIGHT  OF  THE  PLANETS. 

That  the  sun  and  planets  are  spherical  bodies,  is  evi- 
dent from  all  the  observations  that  have  been  made  on 
them  ;  and  that  the  earth  is  of  the  same  figure,  is  not  only 
deducible  from  analogy,  but  it  is  also  proved  by  obser- 
vation, as  I  shall  show  in  the  process  of  these  lectures. 
Astronomers,  when  they  say  that  the  planets  are  spheri- 
cal bodies,  do  not  mean  a  geometrical  sphere,  but  a  figure 
called  an  oblate  spheroid,  which  is  something  like  the  figure 
that  a  flexible  sphere  would  be  formed  into  by  gently  press- 
ing it  at  its  poles.  Observations  have  determined  this  in 
Jupiter ;  and  it  is  known  that  the  earth  is  of  this  figure, 
both  from  observation  and  actual  mensuration. 

That  the  planets  are  all  opake,  or  dark  bodies,  and  conr 
sequently  shine  only  by  the  light  they  receive  from  the  sun, 
is  plain,  because  they  are  not  visible  when  they  are  in  such 
parts  of  their  orbits  as  lie  between  the  sun  and  the  earth  -> 
that  is,  when  their  illuminated  side  is  turned  from  us. 

The  sun  enlightens  only  half  a  planet  at  once  ;  the 
illuminated  hemisphere  is  always  that  which  is  turned 
towards  the  sun,  the  other  hemisphere  of  the  planet  is 


12  OF   THE   SUN. 

dark.  To  speak  with  accuracy,  the  sun  being  larger 
than  any  of  the  planets,  will  illuminate  rather  more 
than  half ;  but  this  difference,  on  account  of  the  great 
distance  of  the  sun  from  any  of  the  planets,  is  so  small, 
that  its  light  may  be  considered  as  coming  to  them  in 
lines  physically  parallel. 

Like  other  opake  bodies,  they  cast  a  shadow  behind 
them,  which  is  always  opposite  to  the  sun.  The  line 
in  the  planet's  body  which  distinguishes  the  lucid  from 
the  obscure  part,  appears  sometimes  straight,  sometimes 
Crooked.  The  convex  part  of  the  curve  is  sometimes 
towards  the  splendid,  and  the  concave  towards  that 
which  is  obscure  ;  and  vice  versa,  according  to  the  situ- 
ation of  the  eye  with  respect  to  the  planet,  and  to  the 
sun  which  enlightens  the  planet. 


OF    THE    SUN. 

The  sun  is  the  centre  of  the  system,  round  which  the 
rest  of  the  planets  revolve.  It  is  the  first  and  greatest 
obj  ^ct  of  astronomical  knowledge,  and  is  alone  enough 
to  stamp  a  value  on  the  science  to  which  the  study  of  it 
belongs.  The  sun  is  the  parent  of  the  seasons ;  day 
and  night,  summer  and  winter  are  among  its  surprising 
effects.  All  the  vegetable  creation  is  the  offspring  of  its 
beams  ;  our  own  life  is  supported  by  its  influence.  Na- 
ture revives,  and  puts  on  a  new  face,  when  it  approaches 
nearer  to  us  in  spring ;  and  sinks  into  a  temporary 
death  at  its  departure  from  us  in  the  winter. 

Hence  it  was  with  propriety  called  by  the  ancients  cor 
cali,  the  heart  of  heaven  ;  for,  as  the  heart  is  the  cen- 
tre of  the  animal  system,  so  is  the  sun  the  centre  of  our 
planetary  system.  As,  the  heart  is  the  fountain  of  the 
blood,  and  the  centre  of  heat  and  motion  ;  so  is  the  sun 
the  life  and  heat  of  the  world,  and  first  mover  of  the 
mundane  system.  When  the  heart  ceases  to  beat,  the 
circuit  of  life  is  at  an  end  ;  and  if  the  sun  should  cease 
to  act,  a  total  stagnation  would  take  place  throughout 
the  whole  frame  of  nature. 


OF    THE    SUN.  13 

*  Bv  his  magnetic  beam  he  gently  warms 
The  universe,  and  to  each  inward  part 
With  gentle  penetration,  though  unseen, 
Shoots  invisible  virtue." 

The  sun  is  placed  near  the  centre  of  the  orbits  of  all 
the  planets,  and  turns  round  his  axis  in  25\  days.  It  is 
inclined  to  the  ecliptic  in  an  angle  of  eight  degrees.  His 
apparent  diameter,  at  a  mean  distance  from  the  earth,  is 
about  thirty-two  minutes  twelve  seconds. 

Those  who  are  not  accustomed  to  astronomical  calcu- 
lation, will  be  surprized  at  the  real  magnitude  of  this 
luminary ;  which,  on  account  of  its  distance  from  us, 
appears  to  the  eye  not  much  larger  than  the  moon, 
which  is  only  an  attendant  on  our  earth.  When  look- 
ing at  the  sun,  you  are  viewing  a  globe,  whose  diameter 
is  about  890,000  English  miles  ;  whereas  the  earth  is 
not  more  in  diameter  than  7970  miles  :  so  that  the  sun 
is  ajDout  1,392,500  times  bigger  than  the  earth.  As  it 
is  the  fountain  of  light  and  heat  to  all  the  planets,  so  it 
also  far  surpasses  them  in  its  bulk.  In  proportion  as 
science  has  advanced,  and  more  accurate  instruments 
have  been  made,  the  magnitude  of  this  luminary  has 
been  found  to  exceed  considerably  the  limits  of  former 
calculations. 

If  the  sun  were  every  where  equally  bright,  his  rota- 
tion on  his  axis  would  not  be  perceptible  ;  but,  by  means 
of  the  spots  which  are  visible  on  his  pure  and  lucid  sur- 
face, we  are  enabled  to  discover  this  motion. 

When  a  spherical  body  is  near  enough  to  appear  of 
its  true  figure,  this  appearance  is  owing  to  the  shading 
upon  the  different  parts  of  its  surface  :  for,  as  a  flat  cir- 
cular piece  of  board,  when  it  is  properly  shaded  by  paint- 
ing, will  look  like  a  spherical  body,  so  a  spherical  body 
appears  of  its  true  shape  for  the  same  reason  that  the 
plane  board,  in  the  present  instance,  appears  spherical. 
But,  if  the  sphere  be  at  a  great  distance,  this  difference  of 
shading  cannot  be  discerned  by  the  eye,  and  consequent- 
ly the  sphere  will*  no  longer  appear  of  its  true  shape ; 
the  shading  is  then  lost,  and  it  seems  like  a  flat  circle. 


14  OF    THE    SUtt. 

It  is  thus  with  the  sun  ;  it  appears  to  us  like  a  bright 
flat  circle,  which  flat  circle  is  termed  the  sun's  disk.  By 
the  assistance  of  telescopes,  dark  spots  have  been  ob- 
served on  this  disk,  and  found  to  have  a  motion  from 
east  to  west  :  their  velocity  is  greater  when  they  are  at 
the  centre,  than  when  they  are  near  the  limb.  They 
are  seen  first  on  the  eastern  extremity,  by  degrees  they 
come  forwards  towards  the  middle,  and  so  pass  on  till 
they  reach  the  western  edge,  they  then  disappear;  and, 
after  they  have  lain  hid  about  the  same  time  that  they 
continued  visible,  they  will  appear  again,  as  at  first.  By 
this  motion  we  discover  not  only  the  time  the  sun  em- 
ploys in  turning  round  its  axis,  but  also  the  inclination 
of  its  axis  to  the  plane  of  the  ecliptic* 

The  page  of  history  informs  us,  that  there  have  been 
periods  when  the  sun  has  wanted  of  its  accustomed 
brightness,  and  shone  with  a  dim  and  obscure  light  for 
the  space  of  a  whole  year.  This  obscurity  has  been 
supposed  to  arise  from  his  surface  being  at  those  times 
covered  with  spots.  Spots  have  been  seen  that  were 
much  larger  than  the  earth. 

The  sun  is  supposed  to  have  an  atmosphere  round  it, 
which  occasions  that  appearance  which  is  termed  the 
zodiacal  light.  This  light  is  seen  at  some  seasons  of  the 
year,  either  a  little  after  sun-set,  or  a  little  before  sun- 
rise. It  is  faintly  bright,  and  of  a  whitish  colour,  re- 
sembling the  milky  way.  In  the  morning  it  becomes 
brighter  and  larger,  as  it  rises  above  the  horizon,  till 
the  approach  of  day,  wh:ch  diminishes  its  splendour, 
and  renders  it  at  last  invisible.  Its  figure  is  that  of  a 
flat  or  lenticular  spheroid  seen  in  profile.  The  direc- 
tion of  its  longer  axis  coincides  with  the  plane  of  the 
sun's  equator.  But  its  length  is  subject  to  great  varia- 
tion, so  that  the  distance  of  its  summit  from  the  sun 
varies  from  45  to  1 20  degrees.     It  is  seen  to  the  best 


*  The  observer  may  view  the  spots  of  the  sun  with  a  refracting  tt lescope 
of  two  or  three  feet,  or  a  reflecting  <ne  of  twelve  inches,  eightten  inches, 
or  two  feet,  taking  care  to  guard  the  eye  with  a  dark  glass,  to  take  off  the 
glaring  light ;  or  the  image  or  picture  of  the  sun,  with  his  -p  ts,  ma)  :je 
thrown  into  a  dark  ronrn  through  a  telescope,  and  received  upon  a  piece 
of  paper  placed  nearer  or  farther  from  the  glass  at  pleasure. 


OF    MERCURY.  15 

advantage  about  the  solstices.  It  was  first  described 
and  named  by  Cassini^  in  1683  ;  it  was  noticed  by  Mr. 
Childrey,  about  the  year  1650.* 


OF    THE    INFERIOR    PLANETS,    MERCURY    AND 
VENUS. 


OF    MERCURY.     $ 

Of  all  the  planets,  Mercury  is  the  least ;  at  the  same 
time  it  is  that  which  is  nearest  the  sun.  It  is  from  his 
proximity  to  this  globe  of  light,  that  he  is  so  seldom 
within  the  sphere  of  our  observation,  being  lost  in  the 
splendour  of  the  solar  brightness,  yet  it  emits  a  very- 
bright  white  light.  It  is  oftener  seen  in  those  parts  of 
the  world,  which  are  more  southward  than  that  which 
we  inhabit ;  and  oftener  by  us,  than  by  those-  who  live 
nearer  the  north-pole ;  for,  the  more  oblique  the 
sphere  is,  the  less  is  the  planet's  elevation  above  the 
horizon. 

Mercury  never  appears  but  a  few  degrees  from  the 
sun.  The  measure  of  a  planet's  separation  or  distance 
from  the  sun,  is  called  its  elongation.  His  greatest  elon- 
gation is  little  more  than  twenty-eight  degrees,  or  about 
as  far  as  the  moon  appears  to  be  from  the  sun,  the  se- 
cond day  after  new  moon.  In  some  of  its  revolutions, 
the  elongation  is  not  more  than  eighteen  degrees. 


*  In  the  philosophical  Ti  ansactions  for  1795,  is  given  a  paper  by  Dr. 
Herschel  on  the  physical  construction  of  the  sun  ;  which  he  supposes  to 
have  an  atmosphere  somewhat  like  that  of  the  earth :  the  black  spots  to 
be  the  opake  ground  or  body  of  the  sun,  and  the  luminous  part  the  atmos- 
phere, which,  when  interrupted,  gives  a  transient  glimpse  of  the  sun:  to 
be  most  probably  inhabited  like  the  rest  of  the  planets;  and, in  an  answer 
to  the  objection,  that  the  heat  of  the  sun  renders  it  unfit  tor  habitation,  "  that 
heat  is  produced  by  the  sun's  rays,  only  when  they  act  on  a  calorific  medi- 
um," and  "  that  they  are  the  cause  of  the  production  of  heat  by  uniting 
with  the  matter  of  fire  contained  in  the  substances  heated."  For  further 
particulars  of  this  curious  hypothesis,  I  refer  the  reader  to  the  Trans- 
actions.— E.  Edit. 


16  OF    MERCURY. 

Mercury  is  computed  to  be  at  about  37  millions  of 
miles  from  the  sun,  and  to  revolve  round  him  in  87 
days,  23  hours,  and  nearly  16  minutes,  which  is  the 
measure  of  its  year, — about  one  fourth  of  ours.  As, 
from  the  nearness  of  this  planet  to  the  sun,  we  neither 
know  the  time  it  revolves  round  its  axis,  nor  the  incli- 
nation of  that  axis  to  the  plane  of  its  orbit,  we  are  ne- 
cessarily ignorant  of  the  length  of  its  day  and  night,  or 
the  variety  of  seasons  it  may  be  liable  to.  Mercury  is 
3000  miles  in  diameter,  and  therefore  contains  in  sur- 
face 28,274,400  square  miles.  Its  apparent  diameter, 
at  a  mean  distance  from  the  earth,  is  20  seconds. 

Mercury  is  supposed  to  move  at  the  rate  of  1 10,680 
miles  per  hour.  The  sun  is  about  26,000,000  times  as 
big  as  Mercury  ;  so  that  it  would  appear  to  the  inhabi- 
tants of  Mercury  nearly  three  times  larger  than  it  does 
to  us  ;  and  its  disk,  or  face,  about  seven  times  the  size 
we  see  it.  As  the  other  five  planets  are  above  Mercu- 
ry, their  phenomena  will  be  nearly  the  same  to  it  as  to 
us.  Venus  and  the  Earth,  when  in  opposition  to  the 
sun,  will  shine  in  full  orbs,  and  afford  a  brilliant  ap- 
pearance to  the  Mercurian  spectator. 

Mercury,  like  the  moon,  changes  its  phases,  accord- 
ing to  its  several  positions,  with  respect  to  the  sun  and 
earth.  He  never  appears  quite  round  or  full  to  us,  be- 
cause his  enlightened  side  is  never  turned  directly  to- 
wards us,  except  when  he  is  so  near  the  sun,  as  to  be- 
come invisible.  The  times  for  making  the  most  favour- 
able observations  on  this  planet,  are,  when  it  passes 
before  the  sun,  and  is  seen  traversing  his  disk,  in  the 
form  of  a  black  spot.  This  passage  of  a  planet  over 
the  face  of  the  sun,  is  called  a  transit.  It  happens  in 
its  lower  conjunction,  at  a  particular  situation  of  the 
nodes. 

If  Mercury,  at  his  inferior  conjunction,  come  to 
either  of  his  nodes  about  these  times,  he  will  appear 
to  transit  over  the  disk  of  the  sun.  But  in  all  other 
parts  of  his  orbit  his  conjunctions  are  invisible,  be- 
cause he  either  goes  above  or  below  the  sun. 


[     17     ] 


OF    VENUS.        9 

Venus  is  the  brightest  and  largest,  to  appearance,  of 
all  the  planets,  distinguished  from  them  all  by  a  supe- 
riority of  lustre  :  her  light  is  of  a  white  colour,  and  so 
considerable,  that  in  a  dusky  place  she  projects  a  sensi- 
ble shade. 

The  diameter  of  Venus  is  7,699  English  miles  ;  her 
distance  from  the  sun  is  about  69,500,000  miles,  she 
goes  round  the  sun  in  224  days,  6  hours,  49  minutes, 
moving  at  the  rate  of  90,955  miles  per  hour.  Her  mo- 
tion round  her  axis  has  been  fixed  by  some  at  23  hours, 
22  minutes ;  by  others  at  about  24  days.  She,  like 
Mercury,  constantly  attends  the  sun,  never  departing 
from  him  above  47  or  48  degrees.  Like  Mercury,  she 
is  never  seen  at  midnight,  or  in  opposition  to  the  sun, 
being  visible  only  for  three  or  four  hours  in  the  morn- 
ing or  evening,  according  as  she  is  before  or  after  the 
sun. 

One  would  not  imagine  that  this  planet,  which  ap- 
pears so  much  superior  to  Saturn  in  the  heavens,  is  so 
inconsiderable  when  compared  to  it ;  for  the  diameter 
of  Saturn  is  79,979  miles ;  while,  on  the  other  hand, 
one  would  scarce  imagine,  that  Venus,  which  appears 
but  as  a  lucid  spangle  in  the  heavens,  was  so  large  a 
globe  as  she  truly  is,  her  diameter  being  7,699  miles. 
It  is  the  distance  which  produces  these  effects.  Her 
apparent  size  varies  with  her  distance  ;  at  some  seasons 
she  appears  nearly  32  times  larger  than  at  others. 

When  this  planet  is  in  that  part  of  her  orbit  which  is 
west  of  the  sun,  that  is,  from  her  inferior  to  her  supe- 
rior conjunction,  she  rises  before  him  in  the  morning, 
and  is  called  Phosphorus,  or  Lucifer,  or  the  Morning 
Star,  When  she  appears  east  of  the  sun,  that  is,  from 
her  superior  to  her  inferior  conjunction,  she  sets  in  the 
evening  after  him ;  or,  in  other  words,  shines  in  the 
evening  after  he  sets,  and  is  called  Hesperus,  or  Vesper, 
or  the  Evening  Star. 

VOL.  IV.  D 


18  pF    VENUS. 

The  inhabitants  of  Venus  will  see  the  planet  Mercury 
always  accompanying  the  sun  ;  and  he  will  be  to  them, 
by  turns,  an  evening  and  a  morning  star,  as  Venus  is  to 
us.  To  the  same  inhabitants,  the  sun  will  appear  al- 
most twice  as  large  as  he  docs  to  us. 

Venus,  when  viewed  through  a  telescope,  is  seldom 
seen  to  shine  with  a  full  face  ;  but  has  phases,  just  like 
the  moon,  from  the  fine  thin  crescent  to  the  enlighten- 
ed hemisphere.  Its  illuminated  part  is  constantly  turn- 
ed towards  the  sun  ;  hence  its  horns  are  turned  towards 
the  east  when  it  is  morning  star,  and  towards  the  west 
when  it  is  evening  star.  Some  astronomers  have  thought 
they  perceived  a  satellite  moving  round  Venus ;  but  as 
succeeding  observers  have  not  been  able  to  verify  their 
observations,  they  are  supposed  to  have  originated  in 
error.  In  observing  the  transit  of  Venus,  Mr.  Dunn, 
and  other  gentlemen,  saw  a  penumbra  which  took  place 
about  five  seconds  before  ihe  contact,  preceding  the 
egress  of  the  planet ;  and  from  thence  they  concluded, 
that  it  had  an  atmosphere  of  about  50  geographical 
miles  in  height.* 

We  are  told,  that  when  Copernicus  first  published  his 
account  of  the  solar  system,  it  was  objected  to  him  that 
it  could  not  be  trfce,  because  if  it  were,  the  inferior  pla- 
nets must  have  different  phases,  according  to  their  dif- 
ferent situation  with  respect  to  the  sun  and  earth : 
whereas  they  always  appear  round  to  us.  The  answer, 
said  to  be  made  by  him,  is,  that  they  appear  round  to 
the  eye  by  reason  of  their  distance  ;  but  if  we  could 
have  a  nearer,  or  more  distinct  view  of  them,  we  should 
see  in  them  the  same  phases  we  do  in  the  moon.  The 
invention  of  telescopes  has  verified  this  prediction  of 
Copernicus*  But  it  is  neither  probable,  that  a  defender 
of  the  Ptolemaic  system  should  make  such  an  objection, 
or  Copernicus  such  an  answer  ;  since  in  the  Ptolemaic, 
as  well  as  in  the  Copernican  system,  the  shape  of  these 


*  From  some  late  observations  of  Dr.  Hersc/u I  and  Mr.  Schroeter,  it 
appears  that  Venus  has  an  atmosphere  of  considerable  height  and  density, 
and  that  it  has  also  inequalities  on  its  surface  like  those  ol  other  planets. 

E.  Edit. 


OF    THE    EARTH.  19 

planets  ought  to  change  just  as  the  moon  does;  conse- 
quently, the  mere  change  of  shape  in  the  inferior  pla- 
nets is  an  argument  which,  in  the  common  way  of  urg- 
ing it,  proves  nothing  at  all  as  to  the  truth  or  falsehood 
of  the  Copernican  system.  If,  besides  the  changes  of 
shape  made  in  the  inferior  planets,  we  consider  the  si- 
tuation of  the  planets  with  respect  to  the  sun,  when 
these  changes  happen  ;  this,  indeed,  will  show  us,  that 
the  Ptolemaic  system  is  false,  as  will  be  seen  in  a  subse- 
quent part  of  these  lectures. 

Taking  the  times  in  which  the  planets  move  round 
the  sun,  for  the  length  of  their  years  ;  and  the  times  of 
their  turning  round  their  axes,  for  the  length  of  their 
days  and  nights  together ;  and  assuming,  as  true,  the 
observations  of  Bianchini,  relative  to  the  rotation  of 
Venus  round  her  axis  ;  we  may  say,  that  a  day  and  a 
night  in  Venus  is  as  long  as  23-j  days  and  nights  with  us  ; 
her  axis  inclines  7-5  degrees  from  the  axis  of  her  orbit, 
on  which  account  the  length  of  her  days  and  nights  dif- 
fers much  more  in  proportion,  and  the  variations  of  her 
seasons  are  greater  than  those  of  our  earth.  She  very 
seldom  has  the  forenoon  and  afternoon  of  the  same  day 
an  equal  length.  At  her  equator,  she  has  the  four  sea- 
sons twice  every  year,  with  other  peculiarities,  which 
are  enumerated  in  larger  treatises  on  this  subject. 

Venus  is  sometimes  seen  passing  over  the  disk  of  the 
sun,  as  a  round  dark  spot.  These  appearances,  which 
are  called  transits,  happen  very  seldom  ;  though  there 
have  been  two  within  these  few  years  ;  one  in  June, 
1761,  the  other  in  June,  1769  ;  the  next  will  be  in  the 
year  1874. 


OF    THE    EARTH.     © 

The  next  planet  that  comes  before  us  is  the  Earth 
that  we  inhabit ;  small  as  it  really  is  compared  to  some 
of  the  other  planets,  it  is  to  us  of  the  highest  import- 
ance :  we  wish  only  to  attain  knowledge  of  others, 
that  we  may  find  out  their  relation  to  this,  and  from 
thence  learn  our  connexion  with  the  universe  at  large. 


20  OF    THE    MOON. 

But  when  viewed  with  an  eye  to  eternity,  its  value  to 
us  is  heightened  in  a  manner  that  exceeds  expression, 
and  surpasses  all  the  powers  of  the  human  mind.  He 
alone  can  form  some  idea  of  it,  who,  in  the  regions  of 
celestial  bliss,  is  become  a  partaker  of  the  length,  and 
breadth,  the  depth  and  height,  of  divine  love. 

The  orbit  of  the  Earth  is  placed  between  those  of 
Venus  and  Mars.  The  diameter  of  the  Earth  is  7920 
miles  ;  its  distance  from  the  sun  is  nearly  96  millions  of 
miles,  and  it  goes  round  him  in  a  year,  moving  at  the 
rate  of  68,856  miles  per  hour.  Its  apparent  diameter, 
as  seen  from  the  sun,  is  about  twenty-one  seconds. 

It  turns  round  its  axis,  from  west  to  east,  in  twenty- 
four  hours,  which  occasions  the  apparent  diurnal  mo- 
tion of  the  sun,  and  all  the  heavenly  bodies  round  it, 
from  east  to  west,  in  the  same  time ;  it  is,  of  course, 
the  cause  of  their  rising  and  setting,  of  day  and  night. 

The  axis  of  the  Earth  is  inclined  2Sj  degrees  to  the 
aris  of  its  orbit,  and  keeps  in  a  direction  parallel  to  it- 
self, throughout  its  annual  course,  which  causes  the  re- 
turn of  spring  and  summer,  autumn  and  winter.  Thus 
its  diurnal  motion  gives  us  the  grateful  vicissitude  of 
night  and  day,  and  its  annual  motion,  the  regular  suc- 
cession of  seasons. 


OF    THE    MOON.      3 

Next  to  the  sun,  the  Moon  is  the  most  splendid  and 
shining  globe  in  the  heavens,  the  satellite,  or  insepa- 
rable companion  of  the  earth.  By  dissipating,  in  some 
measure,  the  darkness  and  horrors  of  the  night ;  sub- 
dividing the  year  into  months  ;  and  regulating  the  flux 
and  reflux  of  the  sea  ;  she  not  only  becomes  a  pleasing, 
but  a  welcome  object;  an  object  affording  much  for 
speculation  to  the  contemplative  mind,  of  real  use  to 
the  navigator,  the  traveller,  and  the  husbandman. 
The  Hebrews,  the  Greeks,  the  Romans,  and,  in  ge- 
neral, all  the  ancients  used  to  assemble  at  the  time  of 
New  Moon,  to  discharge  the  duties  of  piety  and  grati- 
tude for  its  manifold  uses. 


OF    THE    MOON.  21 

That  the  Moon  appears  so  much  larger  than  the 
other  planets,  is  owing  to  her  vicinity  to  us  ;  for,  to  a 
spectator  in  the  sun,  she  would  be  scarcely  visible  with- 
out the  assistance  of  a  telescope.  Her  distance  from 
us,  is  but  small  when  compared  with  that  of  the  other 
heavenly  bodies  ;  for  among  these,  the  least  absolute 
distance,  when  put  down  in  numbers,  will  appear  great, 
and  the  smallest  magnitude  immense. 

The  Moon  is  2161  miles  in  diameter ;  her  bulk  is 
about  three-elevenths  of  that  of  the  earth  ;  her  distance 
from  the  centre  of  the  earth  240,000  miles ;  she 
goes  round  her  orbit  in  27  days,  7  hours,  43  minutes, 
moving  at  the  rate  of  2299  miles  per  hour.  The  time 
in  going  round  the  earth,  reckoning  from  change  to 
change,  is  29  days,  12  hours,  44  minutes.  Her  ap- 
parent diameter,  at  a  mean  distance  from  the  earth,  is 
31  minutes  16  seconds  ;  but  as  viewed  from  the  sun, 
at  a  mean  distance,  about  6  seconds. 

Her  orbit,  is  inclined  to  the  ecliptic,  in  an  angle  of  5 
degrees,  1 8  minutes,  cutting  it  in  two  points,  diame- 
trically opposite  to  each  other, — called  her  nodes.  The 
nodes  have  a  motion  westward,  or  contrary  to  the  or- 
der of  the  signs,  making  a  complete  revolution  in  about 
19  years  ;  in  which  time,  each  node  returns  to  that 
point  of  the  ecliptic  whence  it  before  receded. 

If  the  Moon  were  a  body  possessing  native  light, 
we  should  not  perceive  any  diversity  of  appearance ; 
but  as  she  shines  entirely  by  light  received  from  the 
sun,  and  reflected  from  her  surface,  it  follows,  that  ac- 
cording to  the  situation  of  the  beholder  with  respect  to 
the  illuminated  part,  he  will  see  more  or  less  of  her  re- 
flected beams  ;  for  only  one  half  of  a  globe  can  be  en- 
lightened at  once. 

Hence,  while  she  is  making  her  revolution  round 
the  heavens,  she  undergoes  great  changes  in  her  ap- 
pearance. She  is  sometimes  on  our  meridian  at  mid- 
night, and  therefore  in  that  part  of  the  heavens  which 
is  opposite  to  the  sun  ;  in  this  situation  she  appears  as 
a  complete  circle,  and  it  is  said  to  be  Full  Moon.  As 
she  moves  eastward,  she  becomes  deficient  on  the  west 
side,  and  in  about  7^  days  comes  to  the  meridian,  at 
about  six  in  the  morning,  having  the  appearance  of  a 


22  OF    THE    MOON. 

semicircle,  with  'the  convex  side  turned  towards  the  sun  ; 
in  this  state,  her  appearance  is  called  Half  Moon.  Mov- 
ing still  eastward,  she  become  more  deficient  on  the 
west,  and  has  the  form  of  a  crescent,  with  the  convex 
side  turned  towards  the  sun  ;  this  crescent  becomes 
continually  more  slender,  till  about  fourteen  days  after 
the  Full  Moon  she  is  so  near  the  sun,  that  they  cannot 
be  seen,  on  account  of  his  great  splendour.  About  four 
days  after  this  disappearance,  she  is  seen  in  the  evening, 
a  little  eastward  of  the  sun,  in  the  form  of  a  fine  cres- 
cent, with  the  convex  side  still  towards  the  sun  ;  mov- 
ing still  to  the  eastward,  the  crescent  becomes  more  full ; 
and  when  the  moon  comes  to  the  meridian,  about  six  in 
the  evening,  she  has  again  the  appearance  of  a  bright 
semicircle  ;  advancing  still  to  the  eastward,  she  becomes 
fuller  on  the  east  side  ;  at  last,  in  about  29^-  days,  she 
is  again  opposite  to  the  sun,  and  again  full. 

It  frequently  happens,  that  the  Moon  is  eclipsed  when 
at  the  full ;  and  that  the  sun  is  eclipsed  sometime  be- 
tween the  disappearance  of  the  Moon  in  the  morning  on 
the  west  side  of  the  sun,  and  her  appearance  in  the 
evening  on  the  east  side.  The  nature  of  these  pheno- 
mena will  be  more  considered,  when  we  come  to  treat 
particularly  of  eclipses. 

In  every  revolution  of  the  Moon  about  the  earth,  she 
turns  once  round  upon  her  axis,  and  therefore  always 
presents  the  same  face  to  our  view  ;  and  as,  during  her 
course  round  the  earth,  the  sun  enlightens  successively 
every  part  of  her  globe  only  once,  consequently,  she  has 
but  one  day  in  all  that  time,  and  her  day  and  night  to- 
gether are  as  long  as  our  lunar  month.  As  we  see  only 
one  side  of  the  moon,  we  are  therefore  invisible  to  the 
inhabitants  on  the  opposite  side,  unless  they  take  a 
journey  to  that  side  which  is  next  to  us,  for  which  pur- 
pose some  of  them  must  travel  more  than  1500  miles. 

As  the  moon  illuminates  the  earth  by  a  reflected 
light  received  from  the  sun,  she  is  reciprocally  en- 
lightened, but  in  a  much  greater  degree,  by  the  earth ; 
for  its  surface  is  above  thirteen  times  greater  than  that 
of  the  moon  ;  and  therefore,  supposing  their  power  of 


OF    MARS.  25 

reflecting  light  to  be  equal,  the  earth  will  reflect  thirteen 
times  more  light  on  the  moon  than  she  receives  from  it. 

When  it  is  what  we  call  New  Moon,  we  will  appear 
as  a  Full  Moon  to  the  Lunarians  ;  as  it  increases  in 
light  to  us,  ours  will  decrease  to  them  :  in  a  word,  our 
earth  will  exhibit  the  same  phases  to  them  that  she  does 
to  us. 

We  have  already  observed,  that  from  one  half  of  the 
Moon  the  earth  is  never  seen  ;  from  the  middle  of  the 
other  half,  it  is  always  seen  over  head,  turning  round 
almost  thirty  times  as  quick  as  the  Moon  does.  To  her 
inhabitants,  the  earth  seems  to  be  the  largest  body  in  the 
universe,  about  thirteen  times  as  large  to  them  as  she 
does  to  us.  As  the  earth  turns  round  its  axis,  the  se* 
veral  continents  and  islands  appear  to  the  Lunarians  as 
so  many  spots,  of  different  forms  ;  by  these  spots,  they 
may  determine  the  time  of  the  earth's  diurnal  motion ; 
by  these  spots,  they  may,  perhaps,  measure  their  time ; 
they  cannot  have  a  better  dial. 


OF    THE    SUPERIOR    PLANETS. 

Mars,  Jupiter,  Saturn,  and  the  Georgium  Sidus,  are 
called  superior  planets,  because  they  are  higher  in  the 
system,  or  farther  from  the  centre  of  it,  than  the  earth  is. 

They  exhibit  several  phenomena,  which  are  very  dif- 
ferent from  those  of  Mercury  and  Venus  ;  among  other 
things,  they  come  to  our  meridian  both  at  noon  and 
midnight,  and  are  never  seen  crossing  the  sun's  disk. 


of  mars.    % 

Mars  is  the  least  bright  and  elegant  of  all  the  pla- 
nets ;  his  orb  lies  between  that  of  the  Earth  and  Jupi- 
ter, but  very  distant  from  both.  He  appears  of  a 
dusky  reddish  hue  ;  from  the  dullness  of  his  appearance, 
many  have  conjectured,  that  he  is  encompassed  with  a 
thick  cloudy  atmosphere ;  his  light  is  not  near  so  bright 
as  that  of  Venus,  though  be  is  sometimes  nearly  equal 
to  her  in  size. 


24  OF    MARS. 

Mars,  which  appears  so  inconsiderable  in  the  hea- 
vens, is  5,309  miles  in  diameter.  Its  distance  from 
the  sun  is  about  146,000,000  miles.  It  goes  round  the 
sun  in  1  year,  321  days,  23  hours,  moving  at  the  rate 
of  55,287  miles  per  hour.  It  revolves  round  its  axis 
in  24  hours,  39  minutes.  To  an  inhabitant  in  Mars, 
the  sun  would  appear  one  third  less  in  diameter  that  it 
does  to  us.  Its  apparent  diameter,  as  viewed  at  a  mean 
distance  from  the  earth,  is  30  seconds. 

Mars,  when  in  opposition  to  the  sun,  is  five  times 
nearer  to  us  than  when  in  conjunction.  This  has  a  ve- 
ry visible  effect  on  the  appearance  of  the  planet,  caus- 
ing him  to  appear  much  larger  at  some  periods  than  at 
others. 

The  analogy  between  Mars  and  the  Earth  is  by  far 
the  greatest  in  the  whole  solar  system  ;  their  diurnal 
motion  is  nearly  the  same  ;  the  obliquities  of  their  re- 
spective ecliptics  not  very  different.  Nor  will  the  Mar- 
tial year  appear  so  dissimilar  to  ours,  when  we  compare 
it  with  the  long  duration  of  the  years  of  Jupiter,  Saturn, 
and  the  Georgium  Sidus.  It  probably  has  a  consider- 
able atmosphere  ;  for,  besides  the  permanent  spots  on  its 
surface,  Dr.  Herschel  has  often  perceived  occasional 
changes  of  partial  bright  belts,  and  also  once  a  darkish 
one  in  a  pretty  high  latitude  ; — alterations  which  we  can 
attribute  to  no  other  cause  than  the  variable  disposition 
of  clouds  and  vapours  floating  in  the  atmosphere  of  the 
planet. 

A  spectator  in  Mars  will  rarely,  if  ever,  see  Mercury, 
except  when  he  sees  it  passing  over  the  sun's  disk.  Ve- 
nus will  appear  to  him  about  the  same  distance  from 
the  sun,  as  Mercury  appears  to  us.  The  Earth  will  ap- 
pear about  the  size  of  Venus,  and  never  above  48  de- 
grees from  the  sun ;  and  will  be,  by  turns,  a  morning 
and  evening  star  to  the  inhabitants  of  Mars.  It  ap- 
pears, from  the  most  accurate  observations,  that  Mars 
is  a  spheroid,  or  flatted  sphere,  the  equatoreal  diame- 
ter to  the  polar  being  in  the  proportion  of  about  131,  to 
127  ;  and  there  is  reason  to  suppose,  that  all  the  pla- 
nets are,  more  or  less,  of  this  figure. 


C     25     ] 


OF    JUPITER.    % 

Jupiter  is  situate  still  higher  in  the  system,  revolving 
round  the  sun,  between  Mars  and  Saturn.  It  is  the 
largest  of  all  the  planets,  and  easily  distingushed  from 
them  by  its  peculiar  magnitude  and  light.  To  the  naked 
eye  it  appears  almost  as  large  as  Venus,  but  not  alto- 
gether so  bright. 

Jupiter  revolves  round  its  axis  in  9  hours,  56  minutes  $ 
its  revolution  in  its  orbit  to  the  same  point  of  the  eclip- 
tic is  11  years,  314  days,  10  hours.  The  disproportion 
of  Jupiter  to  the  Earth,  in  size,  is  very  great ;  viewing 
him  in  the  heavens,  we  consider  him  as  small  in  magni- 
tude ;  whereas  he  is  in  reality  90,228  miles  in  diameter. 
His  distance  from  the  sun  is  499,750,000  miles  ;  he 
moves  at  the  rate  of  rather  more  than  29,083  miles 
per  hour.  His  apparent  diameter,  as  seen  at  a  mean 
distance  from  the  earth,  is  39  seconds. 

To  an  eye  placed  in  Jupiter,  the  sun  would  not  be  a 
fifth  part  of  the  size  he  appears  to  us,  and  his  disk  25 
times  less.  Though  Jupiter  is  the  largest  of  all  the  pla- 
nets, yet  his  revolution  round  his  axis  is  the  swiftest. 
The  polar  axis  is  shorter  than  the  equatorial  one,  and 
his  axis  perpendicular  to  the  plane  of  his  orbit. 

Jupiter,  when  in  opposition  to  the  sun,  is  much  near- 
er the  earth,  than  when  he  is  in  conjunction  with  him  ; 
at  those  times  he  appears  also  larger,  and  more  luminous 
than  at  other  times. 

In  Jupiter,  the  days  and  nights  are  always  of  an  equal 
length,  each  being  about  five  hours  long.  We  have  al- 
ready observed,  that  the  axis  of  his  diurnal  rotation  is 
nearly  at  right  angles  to  the  plane  of  his  annual  orbit, 
and  consequently  there  can  be  scarce  any  difference  in 
seasons  ;  and  here,  as  far  as  we  may  reason  from  analo- 
gy, we  may  discover  the  footsteps  of  wisdom ;  for,  if 
the  axis  of  this  planet  were  inclined  in  any  considera- 
ble number  of  degrees,  just  so  many  degrees  round  each 
pole  would,  in  their  turn,  be  almost  six  years  in  dark- 
ness ;  and,  as  Jupiter  is  of  such  an  amazing  size,  in  this 
case  immense  regions  of  land  would  be  uninhabitable. 

VOL.  IV.  E 


26  OF    SATURN. 

Jupiter  is  attended  by  four  satellites,  or  moons  ;  these 
are  invisible  to  the  naked  eye,  but  through  a  telescope 
they  make  a  beautiful  appearance.  As  our  moon  turns 
round  the  earth,  enlightening  the  nights  by  reflecting 
the  light  she  receives  from  the  sun,  so  these  also  en- 
lighten the  nights  of  Jupiter,  and  move  round  him  in 
different  periods  of  time,  proportioned  to  their  several 
distances  :  and  as  the  moon  keeps  company  with  the 
earth  in  its  annual  revolution  round  the  sun  ;  so  these 
accompany  Jupiter  in  his  course  round  that  luminary. 

In  speaking  of  the  satellites,  we  distinguish  them  ac- 
cording to  their  places,  into  the  first,  the  second,  and 
so  on  ;  by  the  first,  we  mean  that  which  is  nearest  to 
the  planet. 

The  outermost  of  Jupiter's  satellites  will  appear  al- 
most as  big  as  the  moon  does  to  us  ;  five  times  the  di- 
ameter, and  twenty-five  times  the  disk  of  the  sun.  The 
four  satellites  must  afford  a  pleasing  spectacle  to  the  in- 
habitants of  Jupiter  ;  for  sometimes  they  will  rise  all 
together,  sometimes  be  altogether  on  the  meridian, 
ranged  one  under  another,  and  besides  will  be  frequent- 
ly eclipsed.  Notwithstanding  the  distance  of  Jupiter 
and  his  satellites  from  us,  the  eclipses  thereof  are  of 
considerable  use  for  ascertaining  with  accuracy  the  lon- 
gitude of  places.  From  the  four  satellites  the  inhabi- 
tants of  Jupiter  will  have  four  different  kinds  of  months, 
and  the  number  of  them  in  their  year  no  less  than 
4,500. 

An  astronomer  in  Jupiter  will  never  see  Mercury, 
Venus,  the  Earth,  or  Mars  -y  because,  from  the  immense 
distance  at  which  he  is  placed,  they  must  appear  to  ac- 
company the  sun,  and  rise  and  set  with  him  ;  but  then 
he  will  have  for  the  objects  of  his  observation,  his  own 
four  moons,  Saturn,  his  ring  and  satellites,  and  the 
Georgium  Sidus  with  his  satellites. 

OF    SATURN,    h 

Before  the  discovery  of  the  Georgium  Sidus,  Saturn 
was  reckoned  the  most  remote  planet  in  our  system  ;  he 
shines  but  with  a  pale  feeble  light,  less  bright  than  Jupi- 


OF    SATURN.  27 

ter,  though  less  ruddy  than  Mars.  The  uninformed  eye 
imagines  not,  when  it  is  directed  to  this  little  speck  of 
light,  that  it  is  viewing  a  large  and  glorious  globe,  one 
of  i  he  most  stupendous  of  the  planets,  whose  diameter  is 
79,979  miles*.  We  need  not,  however,  be  surprised  it 
the  vast  bulk  of  Saturn,  and  its  disproportion  to  its  ap- 
]  nance  in  the  heavens  ;  for  we  are  to  consider  that  all 
.  .jeers  decrease  in  their  apparent  magnitude,  in  propor- 
tion to  their  distance :  but  the  distance  of  Saturn  is  im- 
mense ;  that  of  the  earth  from  the  sun  is  96,000,000 
miles,  of  Saturn  916,500,000  miles  ! 

The  length  of  a  planet's  year,  or  the  time  of  its  revo- 
lution round  its  orbit,  is  proportioned  to  its  distance  from 
the  sun.  Saturn  goes  round  the  sun  in  29  years,  167 
days,  6  hours,  moving  at  the  rate  of  rather  more  than 
22,298  miles  per  hour.  His  apparent  diameter  at  a  mean 
distance  from  the  earth  is  16  seconds. 

It  has  not  yet  been  ascertained  by  astronomical  obser- 
vation, whether  Saturn  revolves  or  not  upon  his  axis  : 
we  are  therefore  ignorant  of  the  length  of  his  day  and 
night.  The  sun's  disk  will  appear  ninety  times  less  to 
an  inhabitant  of  Saturn  than  it  does  to'us  ;  but,  notwith- 
standing that  the  sun  appears  so  small  to  the  inhabitants 
of  the  regions  of  Jupiter  and  Saturn,  the  light  that  he 
will  afford  them  is  much  more  than  would  be  at  first 
supposed  ;  for  calculations  have  been  made  from  which 
it  is  inferred,  that  the  sun  will  afford  500  times  as  much 
light  to  Saturn  as  the  full  moon  to  us  ;  and  1600  times 
as  much  to  Jupiter.  To  eyes  like  ours,  unassisted  by 
instruments,  Jupiter  and  the  Georgium  Sidus  would  be 
the  only  planets  seen  fron  Saturn,  to  whom  Jupiter 
would  sometimes  be  a  morning,  sometimes  an  evening 
star. 

One  of  the  first  discoveries  of  the  telescope,  when 
brought  to  a  tolerable  degree  of  perfection,  was,  that 
Saturn  did  not  appear  like  other  planets.  Galileo,  in 
1610,  supposed  it  composed  of  three  stars  or  globes,  a 
larger  in  the  middle,  and  a  smaller  on  each  side ;  and 
he  continued  his  observations  till  the  two  lesser  stars 
disappeared,  and  this  planet  looked  like  the  others. 
Further  observations  showed,  that  what  Galileo  took  for 


'28  OF    THE    GEORGIUM    SIDUS. 

two  stars,  were  parts  of  a  ring.  This  singular  and 
curious  appendage  to  the  planet  Saturn,  is  a  thin, 
broad,  opake  ring,  encompassing  the  body  of  the  pla- 
net, without  touching  it,  like  the  horizon  of  an  artifi- 
cial globe,  appearing  double  when  viewed  through  a 
good  telescope.  The  space  between  the  ring  and  the 
globe  of  Saturn  is  supposed  to  be  rather  more  than  the 
breadth  of  the  ring  ;  the  plane  of  the  ring  is  inclined 
to  the  plane  of  the  ecliptic,  in  an  angle  of  30  degrees, 
and  is  about  21,000  miles  in  breadth.  It  puts  on  dif- 
ferent appearances  to  us,  sometimes  being  seen  quite 
open,  at  others,  only  as  a  line  upon  the  equator.  It  is 
probable,  that  it  at  times  casts  a  shadow  over  vast  re- 
gions of  Saturn's  body.  This  ring  suspended  round 
the  body  of  the  planet,  and  keeping  its  place  without 
any  connexion  with  the  body,  is  quite  different  from  all 
other  planetary  phenomena  with  which  we  are  acquaint- 
ed. But  this  is  rendered  > till  more  surprising  by  the 
discoveries  of  Dr.  Herschel,  who  finds  that  the  planet 
Saturn  has  two  concentric  rings,  of  unequal  dimensions 
and  breadth  situate  in  one  plane,  which  is  probably  not 
much  inclined  to  the  equator  of  the  planet.  These 
rings  are  at  a  considerable  distance  from  each  other, 
the  smaller  being  much  less  in  diameter  at  the  outside, 
than  the  larger  is  at  the  inside ;  the  two  rings  are  en- 
tirely detached  from  each  other,  so  as  plainly  to  permit 
the  open  heavens  to  be  seen  through  the  vacancy  be- 
tween them.  Of  the  nature  of  this  ring,  various  and 
uncertain  were  the  conjectures  of  the  first  observers ; 
though  not  more  perplexed  than  those  of  the  latest.  Of 
its  use  to  the  inhabitants  of  Saturn,  we  are  as  ignorant 
as  of  its  nature. 

Saturn  is  not  only  furnished  with  this  beautiful  ring, 
but  it  has  also  seven  attendant  moons,  the  two  first  next 
his  body  were  lately  discovered  by  Dr.  Herschel. 


OF    THE    GEORGIUM    SIDUS.  9 

From  the  time  of  Huygens  and  Cassini,  to  the  disco- 
very of  the  Georgium  Sidus  by  Dr.  Herschel,  though 


OF    THE    GEORGIUM    SIDUS.  29 

the  intervening  space  was  long,  though  the  number  of 
astronomers  was  increased,  though  assiduity  in  observ- 
ing was  assisted  by  accurrcy  and  perfection  in  the  instru- 
ments of  observation,  yet  no  new  discovery  was  made 
in  the  heavens, — the  boundaries  of  our  system  were  not 
enlarged.  The  inquisitive  mind  naturally  inquires, 
why,  when  the  number  of  those  that  cultivated  the 
science  was  increased,  when  the  science  itself  was  so 
much  improved,  in  practical  discoveries  was  it  so  defi- 
cient ?  A  small  knowledge  of  the  human  mind  will 
answer  the  question,  and  obviate  the  difficulty.  The 
mind  of  man  has  a  natural  propensity  to  indolence  ;  the 
the  ardour  of  its  pursuits,  when  unconnected  with  self- 
ish views,  is  soon  abated,  small  difficulties  discourage, 
little  inconveniencies  fatigue  it,  and  reason  soon  finds 
excuses  to  justify,  and  even  applaud  this  weakness.  In 
the  present  instance,  the  unmanageable  length  of  the 
telescopes  that  were  in  use,  and  the  continual  exposure 
to  the  cold  air  of  the  night,  were  the  difficulties  that 
astronomer  had  to  encounter ;  and  he  soon  persuaded 
himself,  that  the  same  effects  would  be  produced  by 
shorter  telescopes,  with  equal  magnifying  power  ;  here- 
in was  his  mistake,  and  hence  the  reason  why  so  few 
discoveries  have  been  made  since  the  time  of  Cassinu 
A  similar  instance  of  the  retrogradation  of  science  oc- 
curs in  the  history  of  the  microscope,  as  I  have  shown 
in  my  Essays  on  that  instrument. 

The  Georgium  Sidus  was  discovered  by  Dr.  Hers- 
cbel,  in  the  year  1781  ;  for  this  discovery,  he  obtained 
from  the  Royal  Society  the  honary  recompence  of  Sir 
Godfrey  Copley* s  medal.  He  named  the  planet  in  honour 
of  his  Majesty  King  George  III.  the  patron  of  science, 
who  has  taken  Dr.  Herschel  under  his  patronage,  and 
granted  him  an  annual  salary.  By  this  munificence  he 
has  given  scope  to  a  very  uncommon  genius,  and  ena- 
bled him  to  prosecute  his  favourite  studies  with  unre- 
mitted ardour. 

In  so  recent  a  discovery  of  a  planet  so  distant,  many 
particulars  cannot  be. expected.  Its  year  is  supposed 
to  be  more  than  80  of  ours  ;  its  diameter  34,299  miles ; 
distance  from  the  sun  about  1,832  millions  of  miles; 


30  OF    THE    GEORGIUM    SIDUS. 

the  inclination  of  its  orbit  43  minutes  35  seconds  ;  its 
diameter,  compared  to  that  of  the  earth,  as  431,769  to 
1  ;  in  bulk  it  is  8  049,256  times  as  large  as  the  earth. 
Its  light  is  of  a  bluish  white  colour,  and  its  brilliancy 
between  that  of  the  Moon  and  Venus. 

Though  the  Georgium  Sidus  was  not  known  as  a 
planet  till  the  time  of  Dr.  Herschel,  yet  there  are  many 
reasons  to  suppose  it  had  been  seen  before,  but  had 
then  been  considered  as  a  fixed  star.  Dr.  Herschefs 
attention  was  first  engaged  by  the  steadiness  of  its  light; 
this  induced  him  to  apply  higher  magnifying  powers  to 
his  telescope,  which  increased  the  diameter  of  it :  in 
two  days  he  observed  its  place  was  changed  ;  he  then 
concluded  it  was  a  comet ;  but  in  a  little  time  he  with 
others  determined  that  it  was  a  planet,  from  its  vicinity 
to  the  ecliptic,  the  direction  of  its  motion,  being  station- 
ary in  the  time,  and  in  such  circumstances  as  correspond 
with  similar  appearances  in  other  planets. 

With  a  telescope  which  magnifies  about  300  times,  it 
appears  to  have  a  very  well-defined  visible  disk ;  but, 
with  instruments  of  a  smaller  power  it  can  hardly  be 
distinguished  from  a  fixed  star  between  the  sixth  and 
seventh  magnitude.  When  the  moon  is  absent,  it  may 
also  be  seen  by  the  naked  eye. 

Dr.  Herschel  has  since  discovered,  that  it  is  attended 
by  two  satellites  :  a  discovery  which  gave  him  conside- 
rable pleasure,  as  the  little  secondary  planets  seemed  to 
give  a  dignity  to  the  primary  one,  and  raise  it  into  a 
more  conspicuous  situation  among  the  great  bodies  of 
•our  solar  system.* 

As  the  distances  of  the  planets,  when  marked  in 
miles,  are  a  burden  to  the  memory,  astronomers  often 
express  their  mean  distances  in  a  shorter  way,  by  sup- 
posing the  distance  of  the  earth  from  the  sun  to  be  di- 
vided into  ten  parts.     Mercury  may  then  be  estimated 


*  Four  additional  satellites  to  this  planet  have  been  discovered  by  Dr. 
Hascinl,  so  that  it  now  appears  to  have  six.  The  planes  of  their  orbits 
form  such  large  angles  with  that  oi  the  planet,  and  conseque  nth  the  ecliptic, 
as  to  be  almost  perpendicular  to  it ;  and  another  more  singular  dissimilari- 
ty to  that  of  the  old  planets  is,  that  they  move  in  a  retrograde  direction. 
For  further  particulars,  see  Philos.  Trans.  1798,  p.  47. — E.  Edit. 


C   ^    1 

at  four  of  such  parts  from  the  sun,  Venus  at  seven,  the 
Earth  at  ten,  Mars  at  fifteen,  Jupiter  at  fifty-two,  Saturn 
at  ninety-five,  and  the  Georgium  Sidus  at  one  hundred 
and  ninety. 


TABLES    OF    THE    DIAMETERS,    DISTANCES,  &C. 
OF    THE    PLANETS: 

Accompanied  with  various  comparisons  in  order  to  render 
the  ideas  of  these  distances,  &c.  clearer  to  the  mind. 

When  you  endeavour  to  form  any  idea  of  distance, 
magnitude,  or  duration,  by  numbers  only,  you  soon  ex- 
ceed the  limits  of  conception,  and  find  your  faculties 
of  reasoning  as  finite  as  your  senses.  Hence  astrono- 
mers are  frequently  obliged  to  have  recourse  to  mixed 
ideas,  and  make  things  of  different  natures  and  proper- 
ti  >s  assist  each  other,  to  excite  more  adequate  ideas  of 
what  they  would  express.  Some  of  these  methods  I 
shall  now  lay  before  you,  to  assist  your  immagination  in 
forming  ideas  of  the  vast  distance  and  size  of  the  planets. 


Diameters  In  diam. 

Proportion 

Proportion  of 

in  English 

of  the 

>f  surface 

bulk  with  re- 

miles. 

earth. 

with  re- 
spect to 
.he  earth. 

spect  to    the 
earth. 

Sun 

893,522 

113 

12,719 

1,434,400 

Mercury  .  . 

3,261 

2 

3 

j 

6 

l 

Venus  .... 

7,699 

32 

T3 

near  I 

9 
10 

Earth  .... 

7,920 

1 

1 

1 

Moon  .... 

2,161 

3 
TT 

above  T^ 

1 

Mars    .... 

5,312 

1 

4 
9 

T3T 

Jupiter    ... 

90,255 

■4 

129  3 

1,479 

Saturn   •  .  . 

80,012 

10 

102 

1,030 

Georgium  > 
Sidus      ? 

34,217 

a 

m 

.81-1 

C     32     ] 


MEAN    DISTANCES    IN    MILLIONS    OF    MILES. 

The  distances  being  very  great,  the  nearest  million 
only  is  inserted,  that  it  may  be  the  easier  remembered. 


Distances  from  the  sun. 

Millions   of 

miles. 

Sun 

Mercury  . . 

37} 

Venus  .  .  . 

69| 

Earth  .  .  . 

.96 

Moon    .  .  . 

96 

Mars  .... 

1464 

Jupiter  .  . . 

4991 

Saturn  .  .  . 

9161 

Georgium  ? 

1,832 

Sidus    \ 

Difference  between  the  greatest 
and  least  distance  from  the  earth, 
in  millions  of  miles. 


139 


33,570  miles. 
Millions. 
192 
192 
192 

192 


PERIODS    ROUND    THE    SUN    ACCORDING    TO    OUR 
YEARS   AND    MONTHS. 


Progressive  mo- 

tion    in    their 

orbits,     miles 

years. 

days,  hours 

3     | 

min.    sec. 

per  hour. 

Mercury  . . 

— 

87    23 

15      37 

110,680 

Venus  .  .  . 

— 

224    16 

49       12 

80,955 

Earth    .  .  . 

— 

365|      5 

48       45 

68,856 

Mars  .... 

— 

6861  23 

30      63 

55,783 

Jupiter.  .  . 

11 

3141    12 

—      — 

30,193 

Saturn  .  .  . 

29 

167 

5 

—      — 

22,298 

Georgium  > 
Sidus      5 

80 

— 

— 

—      — 

16,411 

days,  hours,  min. 

Moon *s  periodical  revo->             _,     45 
lution  round  the  earth  J 

2,299 

Synodicalrevolv.or  from  }  ori      10      AA 

change  to 

chan 

Se 

V" 

I  A          *•* 

[     33     ] 


Tlie  following  Table  from  Mr.  Vince's  Plan  of  a 
Course  of  Lectures,  may  be  considered  as  more  accu- 
rate ;  it  is  deduced  from  M.  de  la  Lande's  work. 


meandis. 

Sid 

Rev. 

Nod.inl75C 

Incl.  1,786 

Aphelia  1750. 

d.     h.    '     " 

s     °    '      " 

oft' 

s.     °     '     " 

Mercury.         38710 

87  23  15  14 

1  15  20  43 

TOO 

8  13  33  58 

Venus.  .  . 

72333 

224  16  49  11 

2  14  26  lb 

3  23  35 

10     7  46  42 

Earth  . .  . 

100000 

365     6     9  12 

3     8  39  34 

Mars   .  .  . 

152369 

686  23  30  36 

1  17  38  38 

1  51     0 

5     1  28  14 

J    liter.. 

520279 

4332  14  27  11 

3     7  55  32 

I  18  56 

6  10  21     4 

Saturn  ..       954072  10759     1  51  11 

3  21  32  22 

2  29  50 

8  28    9    7 

Ge»  Sidus  1, 9Q81 80  83yr.  I57d.  18h. 

3  12  33  31 

0  46  20 

U  17    6  44 

Revolution  en  its  own  axis  according 

Motion  on  its  axis, 

to  our  da)  s. 

miles  per  hour. 

days.jhouis 

naio. 

s^c. 

1 

Sun 

25   J      6 

3,957 

Mercury  .  . 

unknown. 

unknown. 

Venus    .  .  . 

23 

22 

1,065 

Earth  .... 

^3 

56 

4 

-1,042 

Mars    .... 

24 

39 

556 

Jupiter  .  .  . 

9 

56 

25,920 

Saturn   .  .  . 

unknown. 

unknown. 

Georgiuni  > 

unknown. 

unknown. 

Sidus      ) 

Mo 

on  ...  . 

27 

7 

|4f 

I 

near 

LOJ 

The  rotation  of  Saturn,  agreeable  to  Dr.  Usher's 
computations,  is  10  hours,  12£-  minutes.  A  different 
result  was  however  obtained,  by  taking  the  density  of 
Saturn,  as  stated  by  M.  de  la  Lande.  Dr.  Herschel  has 
settled  the  rotation  of  Saturn's  ring  at  10  hours,  32 
minutes,  16  seconds. 


VOL.  IV. 


[     34     ] 


Light  and  heat    in  proportion  to 
what  the  earth  receives. 


Mercury  ....  7  times  more 

Venus double 

Earth 1 

Mars half 

Jupiter one  27th. 

Saturn one  91 

Georg.  Sidus  .  one  364th 
Moon 1 


Appearances  of  the  sun 
in  proportion  to  what 
it  appears  on  the  earth. 

7  times  greater 

twice  as  great 

1 

half  as  great 

one  27th 

one  91 

one  364th 

1 


DISTANDES    AND    APPARENT    DIAMETERS    OF    THE 
SUN    AND    PLANETS. 


Distances  from  the  earth. 


Sun  .  .  . 
Mercury 
Venus  . 
Earth  .  . 
Mars  •  • 
Jupiter  . 
Saturn  . 
Geor.  Sid. 


Moon 


Greatest, 
mill,  of  miles 

Least.      | 
mill,  of  miles 

m 

0 

94 

58j 

261 

0 

59  5> 

4034 

1.0m 

82  q| 

1.928 

1736 

miles. 

miles. 

256.785 

223.211 

Mean. 


96 

96 

96 

0 

146-1 

4991 

9161 

1132 


miles. 

240.000 


I  Apparent  diameter  viewed 
from  the  earth. 

Greatest     Least.       Mean. 


52    38 

0  11 

1  0 
0  0 
0  22 
0  46 
0  18 
0.3.9 


31  3« 

0  t 

0  H. 

0  f 

0  4 

0  3 

0  H 

0  3.9 


33.36  28.55.30 


52 

5 

0 

7 

0 

17 

0 

0 

0 

7 

0 

37 

0 

27 

0 

16 

3.9 

31 

15 

It  has  been  found  that  a  cannon-ball  moves  about  8 
miles  in  one  minute,  or  704  feet  in  a  second ;  and  that 
sound  moves  about  13  miles  in  one  minute,  or  1144 
feet  in  a  second. 

A  very  high  wind  may  make  sound  move  one  mile  in 
4,4  seconds,  that  is,  in  about  one-twentieth  less  time  than 
in  calm  weather. 

The  most  violent  storm  does  not  move  above  one 
mile  in  a  minute,  or  88  feet  in  a  second. 


DISTANCE    OF    THE    PLANETS. 


35 


From  hence  it  has  been  computed,  that  a  body  issu- 
ing from  the  sun  with  the  swiftness  of  a  cannon-ball, 
that  is,  eight  miles  in  a  minute,  would  employ  the  fol- 
lowing times  in  reaching 


Mercury    

Venus 

Earth 

Mars    

Jupiter 

Saturn 

Gtrorgium  Sidus   .  .  . 

Any  fixed  star  that') 
has  been  accurate-  I 
ly  observed  J 


years. 

8 

«5 
JB 

«-> 
c 
o 

£ 
10 

35 

6 

CO 

S- 
3 
O 

\3 

16 

6 

8 

20 

22 

10 

4 

21 

34 

9 

19 

16 

118 

9 

8 

16 

217 

10 

2 

19 

435 

4 

24 

0 

7,600,000 

c 

'i 

18 
58 
20 
35 
40 
16 
40 


A  ray  of  light  comes  from  the  sun  to  the  earth  in  8 
min.  13  sec.  moves  therefore  11,693,462  miles  in  one 
minute,  or  194,891  miles  in  one  second. 

A  ray  of  light  comes  from  the  moon  in  1,23  seconds. 

From  the  very  accurate  observations  of  Dr.  Bradley, 
it  is  inferred,  that  no  fixed  star,  of  the  great  numbers  ob- 
served by  him,  can  be  at  a  less  distance  from  the  earth 
than  about  400,000  distances  of  the  sun  from  the  earth  ; 
so  that  a  ray  of  light,  which  comes  from  the  earth  in  8 
minutes,  13  seconds,  issuing  from  such  a  star,  must  re- 
quire 6  years  and  3  months  to  reach  the  earth. 

The  following  may  therefore  be  considered  as  propor- 
tional distances  of  the  celestial  bodies  from  the  sun. 


Mercury 28  yards. 

Venus 52  ditto. 

llarth 79  ditto. 

Mars 109  ditto. 

Jupiter 273  ditto. 

Saturn •  684  ditto. 

(ieorgium  Sidus  ....  1357  ditto. 

Moon  6*  inches  from  the  earth,  Syrius  8410  miles, 


[36     ] 


PROPORTIONAL  MAGNITUDE. 

Sun 2  feet  in  diameter. 

Mercury yj  of  art  inch. 

Vemn y  of  an  inch. 

Mara iV  ol  iin  well- 

Jupiter 25  inches. 

Saturn 2Jg  inches. 

Georgium  Sidus  about  1  inch. 

The  distance  of  Syrius  has  been  computed  at  not  less 
than  18,717,442,690,526  miles.  A  cannon  ball,  going 
at  the  rate  of  19.05  miles  per  minute,  would  therefore 
only  reach  it  in  about  1 ,868,307  years.  The  circumfe- 
rence of  its  orbit  would  be  1 17,605,162,638,454  miles; 
if  the  star  moved  through  this  space  in  24  hours,  it  must 
go  at  the  rate  of  361,170,863  miles  per  second. 

Such  is  the  immense  distance  even  of  a  star  of  the  first 
magnitude,  that,  supposing  the  world  to  have  existed 
6000  years,  and  the  distance  to  be  reduced  to  31 1^-  inch- 
es, or  25  feet  1 1  inches,  then  a  cannon  ball,  going  at  the 
rate  of  1143  miles  per  hour,  and  set  in  motion  at  the 
creation  of  the  world,  would  now  have  passed  only  one 
inch  of  that  reduced  space,  because  one  inch  bears  the 
same  proportion  to  31 1.3  as  the  distance  which  the  ball 
would  go  in  6000  years  does  to  the  whole  distance  of  the 
star.     For, 

If  a  cannon  ball  go  1  inch  in  6000  years,  how  far  will 
it  go  in  1,868,307? — Answer ,  311.3o3,  or  26  feet,  11 
inches.     Or, 

If  a  cannon-ball  go  18,717,442,690,526  miles  in 
1,868,307  years,  how  far  will  it  go  in  6000? — Ans. 
60,110,386,645,  which  number  is  to  18717,  &c.  as  1 
inch  is  to  311.333. 

By  the  same  rule,  if  we  suppose  a  star  of  the  second, 
third,  tenth,  one-hundredth  magnitude,  its  distance  will 
be  proportionably  great,  and  the  space  gone  through  pro- 
portionably  small;  that  is,  in  the  same  time  the  ball  would 
have. gone  only  one-halt,  one- third,  one-tenth,  one-hun- 
dredth, &'_•  of  an  inch.  With  respect  to  a  star  of  the 
third  magnitude,  it  would  have  passed  through  a  space 


PROPORTIONAL    DISTANCES,    &C.  37 

equal  to  one  barley-corn ;  with  respect  to  one  of  the 
four-hundredth,  not  the  space  corresponding  to  one 
hair's  breadth,  reckoning  400  hairs  equal  to  one  inch. 

This  supposes  that  a  star  of  the  third,  tenth,  four- 
hundredth,  &c.  magnitude,  is  3,  10,  400  times  as  dis- 
tant as  one  of  the  first ;  and  we  may  also  suppose  that  a 
star,  which  cannot  be  seen  but  with  a  power  of  100, 
1000,  &c.  is  100,  1000,  &c.  times  more  distant  than  one 
which  the  naked  eye  can  just  discover. 

Upontheforegoingsupposition,thedistanceofastarofthe 
second  magnitude  would  be  37,434,885,38 1, 053 r  miles; 
the  diameter  of  its  orbit,  equal  to  74,869,770,762, 1 07, and 
the  circumference  of  its  orbit,  equal  to  235,210,325,276,- 
908.7;  a  degree  of  this  is  653,362,014,658;  a  minute, 
10,889,366,910.75;  and  a  second,  181,489,448^.  A 
cannon  ball  would  therefore  require  65216  years,  86  days, 
6  hours,  17  minutes,  13  seconds,  to  go  through  one  de- 
gree of  this  orbit;  1086  years,  342  days,  2  hours,  30  mi- 
nutes, 17  seconds,  to  pass  through  one  minute;  and  18 
years,  42  days,  4  hours,  50  minures,  30  seconds,  to  go 
over  one  second  of  it. 

The  distance  between  the  stars  marked  &  and  «  in  Orion's 
belt,  is  1  degree,  23  minutes,  1 2  seconds. 


Between  «  and  <?..'..      1     21        9 
Between  3  and  £     ....     2     44     21 

Now  taking  the  whole  years,  and  neglecting  the  frac- 
tions of  the  above  numbers,  a  cannon  ball  would  be  90410 
years  in  passing  from  d  to  £,  88184  in  going  from  *  to  f, 
and  178594  from  *  to  I 

So  likewise,  the  distance  between  <*  and  £,  Ursa  Major% 
or  the  two  hind  wheels  of  the  Placestrum^  is  5°  23'  20*, 
which  a  cannon  ball  would  require  351418  years  to  pass 
over. 

This  evidently  supposes,  that  all  stars  of  equal  magni- 
tude are  equally  distant  (not  that  this  is  certain,  or  even 
probable ;  but  some  data  must  be  assumed)  ;  and  though 
not  accurate,  equally  show  the  wonders  ol  creation. 

The  diameter  of  the  earth  being  7920  miles,  the  sur- 


38 


CONCLUDING    REMARKS. 


face  in  round  numbers  may  be  called  200  millions  of 
square  miles. 

Millions. 


The  surface  .     . 
Whereof  the  sea  oc- 
cupies .     .     . 
The  land    .     .     . 


200 


|,160 
h    40 


Of  that  40,  America  is  14, 
about  j ;  Asia  1 1 ,  about 
-^;  Africa  10,^;  Europe 
not  5,  or  -|.. 

Annual  motion  of  ;he  njdes. 


mm. 

3ec 

7 

0 

19 

2 

22 

2 

14 

5 

16 

9 

Places  of  the  nodes  for  1750. 

sign.   deg.    min.    sec. 

Mercury  ....     I  15  20  43 

Venus     2  14  26  18 

Mars 1  17  38  38 

Jupiter 3  7  55  32 

Saturn    3  21  32  22 

Georgium  Sidus  3  12  33  31 

Inclination  of  the  Orbits. 

Mercury 7      0     0 

Venus .  3    23   35 

Mars 151      0 

Jupiter 1    18    56 

Saturn 2    19    50 

Georgium  Sidus     0  46  20 

I  shall  conclude  this  general  survey  of  the  solar  system 
in  the  words  of  that  excellent  mathematician,  Mr.  Mac- 
laurin.  "  The  view  of  nature,  which  is  the  immediate  ob- 
ject of  sense,  is  very  imperfect,  and  of  small  extent ;  but  by 
the  assistance  of  art,  and  the  aid  of  reason,  becomes  en- 
larged, till  it  loses  itself  in  infinity.  As  magnitude  of 
every  sort,  abstractedly  considered,  is  capable  of  being  in- 
creased to  infinity,  and  is  also  divisible  without  end  ;  so  we 
find,  that  in  nature  the  limits  of  the  greatest  and  least  di- 
mensions of  things  are  actually  placed  at  an  immense  dis- 
tance from  each  other. 

"  We  can  perceive  no  bounds  of  the  vast  expanse,  in 
which  natural  causes  operate,  nor  fix  any  limit,  or  termi- 
nation, to  the  universe.  The  objects  we  commonly  call 
great,  vanish,  when  we  contemplate  the  vast  body  of  the 
earth.  The  terraqueous  globe  itself  is  lost  in  the  solar 
system  ;  even  the  sun  dwindles  into  a  star ;  Saturn's  vast 
orbit,  and  all  the  orbits  of  the  comets,  crowd  into  a  point, 
when  viewed  from  numberless  places  between  the  earth 
and  the  nearest  fixed  stars.    Other  sun's  kindle  to  illumi- 


EXPLANATION    OF    THE    SEASONS,    &C.  39 

nate  other  systems,  where  our  sun's  rays  are  unperceived; 
but  these  also  are  swallowed  up  in  the  vast  expanse.  When 
we  have  risen  so  high,  as  to  leave  all  definite  measures  far 
behind  us,  we  find  ourselves  no  nearer  to  a  final  term  or 
limit. 

"  Our  views  of  nature,  however  imperfect,  serve  to  re- 
present to  us,  in  a  most  sensible  manner,  that  mighty  pow- 
er which  prevails  throughout,  acting  with  a  force  and  effi- 
cacy that  suffers  no  diminution  from  the  greatest  distances 
of  space  or  intervals  of  time  ;  and  to  prove  that  all  things 
are  ordered  by  infinite  wisdom,  and  perfect  goodness. 
Scenes  which  should  excite  and  animate  us  to  correspond 
with  the  general  harmony  of  nature. " 


LECTURE  XXXVIII, 


EXPLANATION  OF  THE  SEASONS,  AND  OTHER  PHENO- 
MENA. 


1  AM  now  going  to  consider  the  earth  as  a  planet, 
having  already  given  you  an  outline  of  the  solar  system, 
of  which  the  sun  is  the  centre,  with  the  seven  planetary 
globes  revolving  in  their  respective  orbits  around  him. 
The  earth  we  inhabit  is  one  of  those  seven  revolving  pla- 
nets, and  completes  its  revolution  in  365*  days,  5  hours, 
49  minutes,  which  constitutes  our  year  ;  for  it  is  by  this 
progression,  or  annual  motion  of  the  earth,  that  our  year 
is  measured.  But  besides  this,  in  the  space  of  24  hours,  it 
makes  one  complete  rotation  on  its  axis,  by  which  motion 
day  and  night  are  alternately  occasioned  all  over  the  world. 
To  explain  the  phenomena  on  these  principles,  and  to  re- 
move objections  and  difficulties,  will  be  the  subject  of  this 
lecture ;  and  first  of  ail,  it  will  be  necessary  to  prove  to 
you  the  globular  form  of  our  earth. 


F     40     J 


OF    THE    SHAPE    OR    FIGURE    OF    THE    EARTH. 

I  have  already  observed,  that  the  appearance  of  tne  hea- 
venly bodies  is  not  the  same  to  the  inhabitants  of  all  the 
various  parts  of  the  earth  ;  that  the  sun,  the  moon,  and 
stars,  rise  and  set  in  Greenland  in  a  manner  very  different 
from  what  they  do  in  the  East  Indies,  and  in  both  places 
very  different  from  what  they  do  in  England :  and  as  it 
was  natural  to  attribute  the  cause  of  this  change  in  the 
apparent  face  of  the  heavens,  to  the  figure  of  the  earth, 
for  appearances  must  ever  answer  to  the  form  and  struc- 
ture of  the  things ;  the  nature  of  this  figure  was,  there- 
fore, one  of  the  first  objects  of  inquiry  among  philoso- 
phers and  astronomers. 

Some  sages  of  antiquity  concluded,  that  the  earth  must 
necessarily  be  of  a  spherical  figure,  because  that  figure 
was,  on  many  accounts,  the  most  convenient  for  the  earth, 
as  a  habitable  world :  they  also  argued,  that  this  figure 
was  the  most  natural,  because  any  body  exposed  to  for- 
ces, which  tend  to  one  common  centre,  as  is  the  case  with 
the  earth,  would  necessarily  assume  a  round  figure.  The 
assent,  however,  of  the  modern  astronomers  to  this  truth, 
was  not  determined  by  speculative  reasoning  ;  but  on  evi- 
dence, derived  from  facts  and  actual  observation.  From 
these  I  shall  select  those  arguments,  that  I  think  will  have 
the  greatest  weight. 

It  is  known,  from  the  laws  of  optics  and  perspective, 
that  if  any  body,  in  all  situations,  and  under  all  circum- 
stances, project  a  circular  shadow,  that  body  must  be  a 
globe. 

It  is  also  known,  that  eclipses  of  the  moon  are  caused 
by  the  shadow  of  the  earth. 

And  we  find,  that  whether  the  shadow  be  projected  to- 
wards the  east  or  the  west,  the  north  or  the  south,  under 
every  circumstance  it  is  circular ;  the  body,  therefore, 
that  casts  the  shadow,  which  is  the  earth,  must  be  of  a 
globular  figure. 

You  will  obtain  another  convincing  proof  of  the  glo- 
bular shape  of  the  earth,  by  inquiring  in  what  manner  a 
person  standing  upon  the  coast  of  the  sea,  and  waiting 


FIGURE    OF    THE    EARTH.  41 

for  a  vessel  which  he  knows  is  to  arrive,  sees  that  vessel. 
We  shall  find,  that  he  first  of  all,  and  at  the  greatest  dis- 
tance, sees  the  top  of  the  mast  rising  out  of  the  water  ; 
and  the  appearance  is,  as  if  the  ship  were  swallowed  up  in 
the  water.  As  he  continues  to  observe  the  object,  more 
and  more  of  the  mast  appears ;  at  length  he  begins  to  see 
the  top  of  the  deck,  and  by  degrees  the  whole  body  of 
the  vessel.  On  the  other  hand,  if  the  ship  be  departing 
from  us,  we  first  lose  sight  of  the  hull,  at  a  greater  dis- 
tance the  main-sails  disappear,  and  at  a  still  greater  the  top* 
sail.  But  if  the  surface  of  the  sea  were  a  plane,  the  body 
of  the  ship,  being  the  largest  part  of  it,  would  be  seen 
first,  and  from  the  greatest  distance,  and  the  masts  would 
not  be  visible  till  it  came  nearer. 

To  render  this,  if  possible,  still  clearer,  let  us  consider 
two  ships  meeting  at  sea,  the  top-masts  of  each  are  the 
parts  first  discovered  by  both,  the  hull,  &c.  being  con- 
cealed by  the  convexity  of  the  globe  which  rises  between 
them.  The  ships  may,  in  this  instance,  resemble  two 
men,  who  approach  each  other  on  the  opposite  sides  of  a 
hill ;  their  heads  will  be  first  seen,  and  gradually,  as  they 
approach,  the  body  will  come  entirely  into  view.  From 
hence  is  derived  a  rational  method  of  estimating  the  dis- 
tance of  a  ship,  which  is  in  use  among  sea-faring  people* 
namely,  by  observing,  how  low  they  can  bring  her  down ; 
that  is  to  say,  the  man  at  the  mast-head  fixes  his  eyes  on  the 
vessel  in  sight,  and  slowly  descends  by  the  shrouds,  till 
she  becomes  no  longer  visible.  The  less  the  distance,  the 
lower  he  must  descend  before  she  disappears.  If  obser- 
vations of  this  kind  be  made  with  a  telescope,  the  effect 
will  be  still  more  remarkable ;  as  the  distance  increases 
or  diminishes,  the  ship  in  sight  will  appear  to  become 
more  and  more  immersed,  or  to  rise  gradually  out  of 
the  water. 

This  truth  is  fully  evinced  by  the  following  consider- 
ation ;  that  ships  have  sailed  round  the  earth,  have  gone 
to  the  westward,  and  have  come  home  from  the  east- 
ward ;  or,  in  other  words,  the  ships  have  kept  the  same 
course,  and  yet  returned  from  the  opposite  side  into  the 
harbour  whence  they  first  sailed.  Now  we  are  certain  that 
this  could  not  be  the  case,  if  the  earth  were  a  plane ;  for 

VOL.  IV.  G 


4-2  FIGURE    OF    THE    EARTH. 

then  a  person  who  should  set  out  from  any  one  point, 
and  go  on  straight  forward,  without  stopping,  would  be 
continually  going  farther  from  the  point  from  which  he 
be  set  out. 

Plate  2.  Jig.  1  and  2,  are  illustrations  of  the  forego- 
ing principles.  Fig.  1  shows,  that  if  the  earth  were  a 
plane,  the  whole  of  a  ship  would  be  seen  at  once,  how- 
ever distant  from  the  spectator,  and  that,  whether  he  be 
placed  at  the  top  or  bottom  of  a  hill.  From  Jig.  2,  it 
appears,  that  the  rotundity  of  the  earth,  represented  by 
the  circle  ABC,  conceals  the  lower  part  of  the  ship  d, 
while  the  top-mast  is  still  visible ;  and  that  it  is  not  till 
the  ship  comes  to  e,  that  the  whole  of  it  is  visible. 

The  following  remarks  evince  the  same  truth.  Ob- 
serve any  star  near  the  northern  part  of  the  horizon, 
and  if  you  travel  to  the  south,  it  will  seem  to  dip  far- 
ther and  farther  downwards,  till  proceeding  on,  it  will 
descend  entirely  out  of  sight.  In  the  mean  time,  the 
stars  to  the  southward  of  our  traveller  will  seem  to  rise 
higher  and  higher.  The  contrary  appearances  would 
happen,  if  he  went  to  the  northward.  This  proves,  that 
the  earth  is  not  a  plane  surface,  but  a  curve  in  the  di- 
rection south  and  north.  By  an  observation  nearly  si- 
milar to  this,  the  traveller  may  prove  the  curvature  of 
the  earth,  in  an  east  and  west  direction. 

The  globular  figure  of  the  earth  may  be  also  inferred 
from  the  operation  of  levelling,  or  the  art  of  conveying 
water  from  one  place  to  another  :  for  in  this  process,  it 
is  found  necessary  to  allow  for  the  difference  between  the 
true  and  apparent  level ;  or,  in  other  words,  for  the  fi- 
gure of  the  earth.  For  the  true  level  is  not  a  straight 
line,  but  a  curve  which  falls  below  the  straight  line 
about  eight  inches  in  a  mile,  four  times  eight  in  two 
miles,  nine  times  eight  in  three  miles,  sixteen  times 
eight  in  four  miles,  nearly  increasing  as  the  square  of 
the  distance. 

What  the  earth  loses  of  its  sphericity  by  mountains 
and  vallies,  is  very  inconsiderable ;  the  highest  emi- 
nence bearing  so  little  proportion  to  its  bulk,  as  to  be 
scarcely  equivalent  to  the  minutest  protuberance  on  the 
surface  of  a  lemon. 


DIURNAL    MOTION    OF    THE    EARTH.  fS 

It  is  proper,  however,  to  acquaint  you,  that  though 
we  call  our  earth  a  globe,  and  that  when  speaking  in  ge- 
neral terms,  it  may  be  considered  as  such  ;  yet  in  the 
strictness  of  truth,  it  must  be  observed,  that  it  is  not 
exactly  and  perfectly  a  sphere,  but  is  a  spheroid,  flatten < 
ed  a  little  towards  the  poles,  and  swelling  at  the  equa- 
tor ;  the  equatorial  diameter  being  about  thirty-four 
miles  longer  than  the  diameter  from  pole  to  pole. 


•  OF    THE    DIURNAL    MOTION    OF    THE    EARTH. 

Though  it  is  this  motion  which  gives  us  the  grateful 
vicissitude  of  day  and  night,  adjusted  to  the  times  of 
labour  and  rest ;  yet  most  people  find  some  difficulty 
in  conceiving  that  the  earth  moves ;  the  more  so,  be- 
cause, in  order  to  allow  it,  they  must  give  up,  in  a  great 
measure,  the  evidence  of  their  external  senses,  of  which 
the  impressions  are  exceedingly  strong  and  lively.  It 
will,  therefore,  be  necessary  to  prove  to  you,  that  you 
can  by  no  means  infer,  that  the  earth  is  at  rest,  because 
it  appears  to  be  so,  and  to  convince  you,  by  a  variety 
of  facts,  that  reason  W2is  given  to  correct  the  fallacies 
of  the  senses. 

To  this  end  we  shall  here  point  out  some  instances, 
where  apparent  motion  is  produced  in  a  body  at  rest, 
by  the  real  motion  of  the  spectator.  Let  us  suppose  a 
man  in  a  ship  to  be  carried  along  by  a  brisk  gale,  in  a 
direction  parallel  to  a  shore,  at  no  great  distance  from 
him  ;  while  he  keeps  his  eye  on  the  deck,  the  mast,  the 
sails,  or  any  thing  within  the  ship  ;  that  is  to  say,  whil  : 
he  sees  nothing  but  some  part  of  the  vessel  on  board  6f 
which  he  is,  and  consequently  every  part  of  which 
moves  with  him,  he  will  not  perceive  that  the  ship  moves 
at  all.  Let  him,  after  this,  look  to  the  shore,  and  h^ 
will  see  the  houses,  trees,  and  hills,  run  from  him  in  a 
direction  contrary  to  the  motion  of  the  vessel ;  and  sup- 
posing him  to  have  received  no  previous  information  on 
this  subject,  he  might  naturally  conclude,  that  the  ap- 
parent motion  of  these  bodies  was  real. 


1*  Or    TH£    Dl U RNAL    MOTION 

In  a  similar  situation  to  this,  we  may  conceive  the 
inhabitants  of  the  earth,  who,  in  early  times,  knowing 
nothing  of  the  true  structure  or  laws  of  the  universe, 
saw  the  sun,  the  stars,  and  the  planets,  rise  and  set,  and 
perform  an  apparent  revolution  about  the  earth.  They 
had  no  idea  of  the  motion  of  the  earth,  and  therefore 
all  this  appearance  seemed  reality.  But  ask  is  highly 
reasonable  to  suppose,  that  as  soon  as  the  slightest  hint 
.should  be  given  to  the  man,  of  the  motion  of  the  ves- 
sel, he  would  begin  to  form  a  new  opinion,  and  conceive 
it  to  be  more  rational,  that  so  small  a  thing  as  the  ship 
should  move,  rather  than  all  that  part  of  the  earth  which 
was  open  to  his  view  ;  so,  in  the  same  manner,  no 
sooner  was  an  idea  formed  of  the  vast  extent  and  great- 
ness of  the  universe,  with  respect  to  this  earth,  than 
mankind  began  to  conceive  it  would  be  more  rational 
that  the  earth  should  move,  than  the  whole  fabric  of 
the  heavens. 

By  another  familiar  instance,  it  will  be  easy  to  show 
you,  that  as  the  eye  does  not  perceive  its  own  motion, 
it  always  judges  from  appearances.  Go  into  a  com- 
mon windmill,  and  desire  the  miller  to  turn  the  mill 
round,  while  you  are  sitting  within  it  with  your  eyes 
fixed  upon  the  upright  post  in  the  centre  thereof;  this 
post,  though  at  rest,  will  appear  to  you  to  turn  round 
with  considerable  velocity,  the  real  motion  of  the  mill 
being  the  cause  of  the  apparent  motion  of  the  swivel- 
post. 

Sea  faring  people  are  furnished  with  various  instances 
to  illustrate  this  subjest ;  those  who  are  busy  in  the  hold 
of  a  ship  at  anchor,  cannot  by  any  perception  determine 
whether  the  ship  has  swung  round  or  not  by  the  turn  of 
the  tide.  When  a  ship  first  gets  under  way  with  a  light 
breeze,  she  may  be  going  at  a  good  rate  before  those 
who  are  between  decks  can  perceive  it.  Having  thus 
obviated  the  objections  which  arise  from  the  testimony 
of  the  senses,  we  may  now  proceed  to  consider  the  ar- 
guments which  tend  more  directly  to  prove  the  motion 
of  the  earth. 

All  the  celestial  motions  will,  on  this  supposition,  be 
incomparably  more  simple  and  moderate. 


OF    THE    EARTH.  45 

This  opinion  is  much  more  agreeable  to  our  notions 
of  final  causes,  and  our  knowledge  of  the  economy  of 
nature ;  for,  if  the  earth  be  at  rest,  and  the  stars,  &c. 
move  round  it  once  in  twenty-four  hours,  their  veloci- 
ty must  be  immense  ;  and  it  is  certainly  more  agreeable 
to  reason,  that  one  single  body,  and  that  one  of  the 
smallest,  should  revolve  on  its  own  axis  in  twenty-four 
hours,  than  that  the  whole  universe  should  be  carried 
round  it  in  the  same  time,  with  inconceivable  velocity. 

The  rotation  of  the  earth  round  its  axis  is  analogous 
to  what  is  observed  in  the  sun,  and  most  of  the  planets  ; 
it  being  highly  probable,  that  the  earth,  which  is  itself 
one  of  the  planets,  should  have  the  same  motion  as  they 
have,  for  producing  the  same  effect ;  and  it  would  be 
as  absurd  in  us  to  contend  for  the  motion  of  the  whole 
heavens  round  us  in  twenty-four  hours,  rather  than  al- 
low a  diurnal  motion  to  our  globe,  as  it  would  be  for 
the  inhabitants  of  Jupiter  to  insist,  that  our  globe,  and 
the  whole  heavens,  must  revolve  round  him  in  ten  hours, 
that  all  its  parts  might  succeesively  enjoy  the  light, 
rather  than  grant  a  diurnal  motion  to  their  habitation. 

All  the  phenomena  relative  to  this  subject  are  as  easi- 
ly solved  on  the  supposition  of  the  earth's  diurnal  mo- 
tion, as  on  the  contrary  hypothesis. 

Besides  the  foregoing  considerations,  there  are  seve- 
ral arguments  to  be  deduced  from  the  higher  parts  of 
astronomy,  which  demonstrably  prove  the  diurnal  mo- 
tion of  the  earth. 

OF    THE    PHENOMENA    OCCASIONED   BY    THE    EARTH'S 
DIURNAL    ROTATION. 

As  the  earth  is  of  a  spherical  figure,  that  part  which 
at  any  time  comes  under  the  confined  view  of  an  ob- 
server, will  seem  to  be  extended  like  a  plane  ;  and  the 
heavens  will  appear  as  a  concave  spherical  superficies 
divided  by  the  aforesaid  plane  into  two  parts,  one  of 
which  is  concealed  from  us  by  the  opacity  of  the  earth. 

Now  the  earth,  by  its  revolution  round  its  axis,  carries 
the  spectator  and  the  aforesaid  plane  from  west  to  east ; 
therefore  all  these  bodies  which  could  not  be  seen,  be- 


4(i  PHENOMENA    OCCASIONED    BY 

cause  they  were  below  the  plane  of  the  horizon,  will  be- 
come visible,  or  rise  above  it,  when  by  the  rotation  of 
the  earth,  the  horizon  sinks  as  it  were  below  them.  On 
the  other  hand,  the  opposite  part  of  the  plane,  towards 
the  west,  rising  above  the  stars  on  that  side,  will  hide 
them  from  the  spectator,  and  they  will  appear  to  set, 
or  go  below  the  horizon. 

As  the  earth,  together  with  the  horizon  of  a  specta- 
tor, continues  moving  to  the  east,  and  about  the  same 
axis,  all  such  bodies  as  are  separated  from  the  earth, 
and  which  do  not  partake  of  that  motion,  will  seem  to 
move  uniformly  in  the  same  time,  but  in  an  opposite 
direction,  that  is  from  east  to  west ;  excepting  the  ce- 
lestial poles,  which  will  appear  to  be  at  rest.  Therefore, 
when  we  say,  that  the  whole  concave  sphere  of  the  hea- 
vens appear  to  turn  round  upon  the  axis  of  the  world, 
while  the  earth  is  performing  one  rotation  round  its 
own  axis,  we  must  be  understood  to  except  the  two 
poles  of  the  world,  for  these  two  do  not  partake  of  this 
apparent  motion. 

It  is,  therefore,  on  account  of  the  revolution  of  the 
earth  round  its  axis,  that  the  spectator  imagines  the 
whole  stary  firmament  and  every  point  of  the  heaven, 
excepting  the  two  celestial  poles,  to  revolve  about  the 
earth  from  east  to  west  every  twenty-four  hours,  each 
point  describing  a  greater  or  less  circle,  as  it  is  more  or 
less  remote  from  one  of  the  celestial  poles. 

Although  every  place  on  the  surface  of  the  terraqueous 
globe  is  illuminated  by  all  the  stars  which  is  above  the 
horizon  of  that  place ;  yet  when  the  sun  is  above  the 
horizon,  his  light  is  so  strong,  that  it  quite  extinguishes 
the  faint  light  of  the  stars,  and  produces  day.  When 
the  sun  goes  below  the  horizon,  or  more  properly, 
when  our  horizon  gets  above  the  sun,  the  stars  give 
their  light,  and  we  are  in  that  state  which  is  called 
night. 

Now  as  the  earth  is  an  opake  spherical  body,  at  a 
great  distance  from  the  sun,  one  half  of  it  will  always  be 
illumined  thereby,  while  the  other  half  will  remain  in 
darkness. 


THE    EARTH'S    DIURNAL    ROTATION.  47 

The  circle  which  distinguishes  the  illuminated  face 
of  the  earth  from  the  dark  side,  and  is  the  boundary 
between  light  and  darkness,  is  generally  called  the  ter- 
minator. A  line  drawn  from  the  centre  of  the  sun  to 
the  centre  of  the  earth,  is  perpendicular  to  the  plane  of 
this  circle. 

When  any  point  in  the  globe  first  gets  into  the  en- 
lightened hemisphere,  the  sun  is  just  risen  to  that  part ; 
when  it  gets  half- way,  or  to  its  greatest  distance  from  the 
terminator,  it  is  then  noon  ;  and  when  it  leaves  the  en- 
lightened hemisphere,  it  is  then  sun  set ;  but  it  still  enjoys 
some  light  from  the  sun,  which  is  reflected  by  the  atmo- 
sphere, till  it  gets  eighteen  degrees  beyond  the  termina- 
tor y  this  glimmering  light  is  called  twilight. 

OF    THE    CORRESPONDENCE    OF    THE    CELESTIAL    AND 
TERRESTRIAL    CIRCLES. 

As  the  earth  daily  revolves  on  its  axis,  every  point  on 
its  surface  is  successively  presented  to  all  the  points  in  the 
heavens,  describing  circles  whose  planes  are  perpendicu- 
lar to  its  axis,  and  their  centres  therein ;  whence  it  fol- 
lows, that  those  planes  are  parallel  to  each  other,  and 
may  be  considered  as  the  elements  of  a  sphere. 

Therefore  all  the  stars  must  seem  to  turn  every  day  uni- 
formly about  the  earth's  axis,  and  in  parallel  circles,  as 
though  they  were  placed  in  the  concavity  of  a  sphere  to 
which  the  earth  is  concentric. 

The  two  points  at  the  extremities  of  the  earth's  axis 
whereon  it  turns,  are  the  only  points  of  its  surface  that 
no  not  change  their  places ;  every  other  point  describes 
a  circle  greater  as  it  is  farther  distant  from  those  fixed 
points  or  poles. 

In  the  heavens,  therefore,  there  must  be  two  points, 
PC^,  plate  2,  fig.  3,  that  appear  fixed,  wherein  a  star 
can  have  no  apparent  motion  ;  these  points  are  determin- 
ed in  the  heavens,  by  prolonging  the  axis  of  the  earth  ; 
these  are  the  poles  of  a  great  circle  of  the  celestial  sphere, 
formed  in  the  heavens  by  continuing  the  plane  of  the  ter- 
restrial equator,  and  all  the  stars  will  appear  to  turn  round 
these  two  poles. 


*8  CORRESPONDENCE   OF  THE    CELESTIAL 

Thus  the  axis,  p  q,  of  the  earth,  p  e  q  z,  plate  2,  y%.  3, 
are  the  poles  of  a  great  circle,  ETZ,  of  the  celestial  sphere, 
formed  in  the  heavens  by  continuing  the  plane  of  the  ter- 
restrial equator  e  z  z,  and  all  the  stars  will  appear  to  turn 
round  the  two  poles  P,  Q^,  or  rather  round  the  axis  P  Q. 
Let  P  represent  the  north  pole,  Q^the  ,south  pole. 

If,  through  the  centre  of  the  earth  C,  and  any  point,  m, 
on  its  surface,  a  right  line,  C  m,  be  supposed  prolonged 
to  the  heavens,  the  extremity,  M,  of  that  right  line  will, 
by  the  earth's  diurnal  rotation,  describe  the  celestial  pa- 
rallel L  M  M  L,  answering  to  the  terrestrial  parallel, 
1  m  m  1,  of  the  point  m.  And  if  C  M  be  supposed  to  be 
prolonged  on  the  other  side  to  the  heavens  in  T,  then  T 
will  describe  in  the  heavens  a  parallel  TTVV,  equal  to 
the  parallel  L  M  M  L,  answering  to,  and  having  the  same 
declination  with  the  terrestrial  parallel  1 1  u  u. 

Hence  it  follows,  1 .  That  the  plane  of  the  celestial  pa- 
rallel LMML,  and  that  of  the  correspondent  parallel 
1  m  m  1,  are  similar  elements  of  a  cone,  whose  axis  is  the 
same  as  that  of  the  earth,  and  whose  vertex,  C,  is  at  the 
earth's  centre.  Therefore,  the  plane  of  a  celestial  parallel) 
cannot  be  the  same  with  its  corresponding  terrestrial  paral- 
lel, only  the  lane,  E  Z  Z,  of  the  celestial  equator  is  the  same 
with  the  plane,  e  z  z,  of  the  terrestrial  equator. 

2.  When,  in  the  earth's  diurnal  rotation,  a  star  passes 
through  the  observer's  zenith,  the  parallel  of  that  star 
corresponds  with  the  observer's  terrestrial  parallel ;  that 
is  to  say,  the  celestial  parallel  is  as  far  distant  from  the 
celestial  equator,  as  the  terrestrial  parallel  is  from  the  ter- 
restrial equator  :  for  then  the  line  of  the  observer's  zenith 
is  a  right  line  drawn  from  the  earth's  centre  through  the 
observer's  eye,  terminating  at  the  star,  and  is  the  line  that 
describes  the  star's  parallel  in  the  heavens. 

If,  therefore,  the  arc  M  Z,  the  distance  of  the  star,  M, 
from  the  celestial  equator  be  measured,  it  gives  also  the 
measure  of  the  arc  m  z,  the  observer's  distance  from  the 
terrestrial  equator.  Therefore,  the  arc  of  the  observers 
distance  from  the  terrestrial  equator,  is  equal  to  the  decli- 
nation of  the  star  that  passes  through  his  zenith.  So  that 
if  it  be  a  fixed  star,  and  the  observer  change  his  place, 
the  different  declinations  of  the  stars  that  pass  through 


ANb    TERRESTRIAL    CIRCLES.  49 

his  zenith  at  the  various  places  he  comes  to,  will  show 
how  much  he  approaches  to,  or  recedes  from  the  equa- 
tor. 

It  follows,  lastly,  that  anyplace  on  the  terrestrial  sphere 
may  be  represented  by  its  corresponding  zenith-point  in  the 
heavens.  For  the  celestial  parallel  passing  through  that 
point,,  represents  the  terrestrial  parallel  of  the  place, 
the  declination  of  the  celestial  parallel  measuring  the  dis- 
tance of  that  place  from  the  equator  ;  also  the  great  cir- 
cle of  the  celestial  sphere  described  from  that  zenith  as  a 
pole,  denotes  the  plane  of  the  horizon  of  that  place,  and 
the  particular  phenomena  of  a  place  on  the  earth  may  be 
explained,  by  denoting  that  place  merely  by  its  zenith  in 
the  heavens.* 


OF  THE    ANNUAL    MOTION    OF    THE    EARTH. 

It  is  owing  to  the  industry  of  modern  astronomers,  that 
the  annual  motion  of  the  earth  has  been  fully  evinced  ;  for 
though  this  motion  was  known  to,  and  adopted  by  many 
among  the  ancient  philosophers,  yet  they  were  not  able 
to  give  their  opinions  that  degree  of  probability,  which  is 
attainable  from  modern  discoveries,  much  less  the  evi- 
dence arising  from  those  demonstrative  proofs,  of  which 
we  are  now  in  possession.  I  shall,  therefore,  enumerate 
some  of  the  reasons  which  induce  astronomers  to  believe, 
that  the  earth  moves  round  the  sun,  and  then  explain  fur- 
ther the  nature  of  this  motion,  which  is  calculated  to  af- 
ford us  the  useful  and  delightful  variety  of  the  seasons, 
the  mutual  allay  of  immoderate  heat  and  cold,  and  the 
successive  growth,  maturity,  and  decay  of  vegetation. 

On  the  supposition  of  the  earth's  motion,  the  celestial 
motions  become  incomparably  more  simple,  and  free  from 
those  looped  contoitions  which  must  be  supposed  in  the 
other  case,  and  which  are  not  only  extremely  improbable, 
but  incompatible  with  what  we  know  of  motion. 


*  The  whole  of  what  is  said,  under  this  head,  may  be  clearly  illustrated 
By  the  ancillary  sphere. 

VOL.  IV.  H 


CO  gun's  apparent  motion  from 

This  opinion  is  also  more  reasonable,  on  account  of  the 
extreme  minuteness  of  the  earth,  when  compared  with  the 
immense  bulk  of  the  sun,  Jupiter,  and  Saturn  ;  and  there 
are  no  known  laws  of  motion,  according  to  which  so  great 
a  body  as  the  sun  can  revolve  about  so  small  a  one  as  the 
earth. 

The  sun  is  the  fountain  of  light  and  heat,  which  it  darts 
through  the  whole  system  ;  it  ought,  therefore,  to  be  in 
the  centre,  that  its  influence  may  be  regularly  diffused 
through  the  whole  heavens,  and  communicated  in  just 
gradations  to  the  whole  system. 

When  we  consider  the  sun  as  the  centre  of  the  system, 
we  find  all  the  bodies  moving  round  it,  agreeably  to  the 
universal  laws  of  gravity  ;  but  upon  any  other  considera- 
tion we  are  left  in  the  dark. 

The  motion  of  the  earth  round  the  sun  accords  with 
that  general  harmony,  and  universal  law,  which  all  the 
other  moving  bodies  in  the  system  observe,  namely,  that 
the  squares  of  the  periodic  times  are  as  the  cubes  of  the  dis- 
tances ;  but  if  the  sun  move  round  the  earth,  that  law 
is  destroyed,  and  the  general  order  and  symmetry  of  na- 
ture interrupted. 

The  annual  motion  of  the  earth  is  incontestibly  proved 
by  observation,  a  motion  having  been  discovered  in  all  the 
fixed  stars,  which  arises  from  a  combination  of  the  motion 
of  light  with  the  motion  of  the  earth  in  its  orbit. 
•  It  will  be  clearly  shown  in  its  place,  that  Venus  and 
Mercury  move  round  the  sun  in  orbits  that  are  between 
it  and  the  earth  ;  that  the  orbit  of  the  earth  is  situate  be- 
tween that  of  Venus  and  Mars ;  and  that  the  orbits  of 
Mars,  Jupiter,  &c.  are  exterior  to,  and  include  the  other 
three. 

OF    THE    APPARENT    MOTION    OF    THE    SUN,    ARISING 
'     FROM  THE   EARTH'S  ANNUAL   MOTION   ROUND  IT. 

As  when  a  person  sails  along  the  sea-coast,  the  shore, 
the  villages,  and  other  remarkable  places  on  land,  appear 
to  change  their  situation,  and  to  pass  by  him ;  so  it  is  in 
the  heavens.  To  a  spectator  upon  the  earth,  as  it  moves 
along  its  orbit,  or  sails  as  it  were  through  celelestial  space, 


THE    EARTH'S    ANNUAL    MOTION.  51 

the  sun,  the  planets,  and  the  fixed  stars,  appear  to  change 
their  places. 

Apparent  change  of  place  is  of  two  sorts ;  the  one  is, 
that  of  bodies  at  rest,  the  change  of  whose  place  depends 
solely  on  that  of  the  spectator  ;  the  other  is,  that  of  bodies 
in  motion,  whose  apparent  change  of  place  depends  as 
well  on  their  own  motion,  as  on  that  of  the  spectator. 

I  shall  first  consider  that  apparent  change  which  takes 
place  in  those  which  are  at  rest,  and  which  is  owing  wholly 
to  the  motion  of  the  earth ;  and  show  that  the  sun,  when 
seen  from  the  earth,  will  appear  to  move  in  the  same  man- 
ner, whether  it  revolves  round  the  earth,  or  the  earth  round 
the  sun. 

Let  us  suppose  the  earth  at  rest,  without  any  motion  of 
its  own,  and  let  the  sun  be  supposed  to  revolve  round  it 
in  the  orbit  A  B  CD,  plate  4,  fig,  1 ,  and  let  E  F  G  H  be 
a  circle  in  the  concave  sphere  of  the  starry  heavens  ;  as  the 
sun  moves  in  the  order  of  the  letters,  ABCD,  in  its  orbit, 
it  will  appear  to  a  spectator  on  the  earth  to  have  described 
the  circle  EFGH.  When  the  sun  is  at  A,  it  will  appear 
as  if  it  were  among  the  fixed  stars  that  are  at  E  ;  when  it 
is  at  B,  it  will  appear  among  the  fixed  stars  at  F;  when 
at  G,  among  those  at  H  ;  and  when  it  is  at  D,  it  will  appear 
among  the  fixed  stars  at  G.  Indeed,  the  fixed  stars  and 
the  sun  are  not  seen  at  the  same  time  ;  but  I  have  shown, 
that  we  may  tell  in  what  part  of  the  heavens  the  sun  is,  or 
what  fixed  stars  it  is  near,  by  knowing  those  which  are 
opposite  to  it,  or  come  to  the  south  at  midnight.  There- 
fore,  if  we  find  that  any  set  of  stars,  as  those  at  G  for  in- 
stance, come  to  the  south  at  midnight,  we  may  be  sure 
that  they  are  opposite  to  the  sun;  and,  consequently,  if 
we  could  see  the  stars  in  that  part  of  the  heavens  where 
the  sun  is,  we  should  find  them  to  be  those  at  F. 

Secondly,  let  us  suppose  that  S  is  the  sun,  that  it  has  no 
motion  of  its  own,  that  it  rests  within  the  orbit  ABCD, 
in  which  we  shall  now  suppose  the  earth  to  move,  in  the 
order  of  the  letters  ABCD.  Upon  this  supposition,  when 
the  earth  is  at  A,  the  sun  will  appear  in  that  part  of  the 
heavens  where  the  stars,  H,  are  ;  when  the  earth  is  at  B, 
the  sun  will  appear  in  that  part  of  the  heavens  where  the 


52  THE    EARTHS    ANNUAL    MOTION. 

stars,  G,  are ;  when  the  earth  is  at  C,  the  sun  will  appear 
in  that  part  of  the  heavens  where  the  stars,  E,  are ;  and 
as  the  earth  revolves  round  the  sun,  in  the  orbit  A  BCD, 
the  sun  will  appear  to  a  spectator  on  the  earth  to  describe 
the  circle  E  F  G  H. 

Thus,  whether  the  earth  be  at  rest,  and  the  sun  revolves 
in  the  orbit  A  B  C  D  ;  or  the  sun  be  at  rest,  and  the 
earth  revolves  in  the  same  orbit,  a  spectator  on  the 
earth  will  see  the  sun  describe  the  same  circle,  EFGH, 
in  the  concave  sphere  of  the  heavens.* 

Hence,  if  the  plane  of  the  earth's  orbit  be  imagined 
to  be  extended  to  the  heavens,  it  would  cut  the  starry- 
firmament  in  that  very  circle  in  which  a  spectator  in 
the  sun  would  see  the  earth  revolve  every  year :  while 
an  inhabitant  of  the  earth  would  observe  the  sun  to  go 
through  the  same  circle,  and  in  the  same  space  of  time 
that  the  solar  spectator  would  see  the  earth  describe  it. 
The  inhabitants  of  all  the  other  planets  will  observe 
just  such  motions  in  the  sun  as  we  do,  and  for  the  very 
same  reasons  ;  and  the  sun  will  be  seen  from  every 
planet  to  describe  the  same  circle,  and  in  the  same  space 
of  time,  that  a  spectator  in  the  sun  would  observe  the 
planet  to  do  it.  For  example,  an  inhabitant  of  Jupiter 
would  think  that  the  sun  revolved  round  him,  describ- 
ing a  circle  in  the  heavens  in  the  space  of  twelve  years  : 
this  circle  would  not  be  the  same  with  our  ecliptic,  nor 
would  the  sun  appear  to  pass  through  the  same  stars 
which  he  does  to  us.  On  the  same  account,  the  sun, 
seen  from  Saturn,  would  appear  to  move  in  another  cir- 
cle, distinct  from  either  of  the  former,  and  would  not 
seem  to  finish  his  period  in  less  time  than  thirty  years. 
Now,  as  it  is  impossible  that  the  sun  can  have  all  these 
motions  really  in  itself,  we  may  safely  affirm,  that  none 
of  them  are  real,  but  that  they  are  all  apparent,  and 
arise  from  the  motions  of  the  respective  planets. 


*  This  is  plea?ingl\  illustrated  by  the  armillary  sphere,  where  the 
sphere  may  be  moved  independent' of  the  earth,  and  the  earth  of  the 
sphere. 


THE    EARTH'S   ANNUAL    MOTION.  5$ 

One  phenomenon  arising  from  the  annual  motion  of 
the  earth,  which  has  already  been  slightly  touched  up. 
on,  may  now  be  more  fully  explained  ;  for  as  from 
this  motion,  the  sun  appears  to  move  from  west  to 
east  in  the  heavens,  if  a  star  rise  or  set  along  with  the 
sun  at  any  time,  it  will  in  the  course  of  a  few  days, 
rise  or  set  before  it,  because  the  sun's  apparent  place 
in  the  heavens  will  be  removed  to  the  eastward  of  that 
Star.  Hence  those  stars  which,  at  one  time  of  the  year, 
set  with  the  sun,  and  therefore  do  not  appear  at  all, 
shall,  at  another  time  of  the  year,  rise  when  the  sun 
sets,  and  shines  all  the  night.  And  as  any  one  star 
shifts  its  place  with  respect  to  the  sun,  and  in  conse- 
quence of  that  with  respect  to  the  hour  of  the  night,  so 
do  all  the  rest.  Hence  it  is,  that  all  those  stars,  which 
at  one  time  of  the  year  appear  on  any  one  side  of  the 
pole-star  in  the  evening,  shall,  in  half  a  year,  appear 
on  the  contrary  side  thereof. 

From  what  has  been  said  it  follows  in  general, 
J.  That,  in  whatever  regards  the  sun's  place,  only  with 
respect  to  the  point  in  the  heavens  in  which  it  appears,  it 
may  be  supposed  to  move  in  an  infinitely  great  circle,  called 
the  ecliptic,  whose  centre  is  the  observer's  eye, 

2.  That  the  earth's  true  place  in  his  orbit  being  known, 
from  observation  or  calculation  ;  six  signs  added  to  or  sub- 
tracted from  it,  gives  the  sun's  true  place  in  the  ecliptic. 
Therefore  the  theory  of  the  sun's  motion  seen  from  the 
earth,  is  the  same  with  that  of  the  earth's  motion  seen 
from  the  sun. 

3.  To  an  inhabitant  of  the  earth,  the  plane  of  the  eclip- 
tic is  that,  whereto  the  annual  motions  of  the  planets  in 
their  orbits  must  be  naturally  compared.  In  the  same  man- 
ner as  the  plane  of  the  equator  is  that  whereto  the  po- 
sition of  the  parallels,  which  the  stars  appear  to  de^ 
scribe  in  consequence  of  the  earth's  diurnal  revolution, 
must  also  be  compared. 

In  order  to  combine  the  sun's  annual  motion  with  its 
diurnal,  you  are  to  observe,  that  if  the  plane  of  the 
ecliptic  coincided  with  that  of  the  equator,  the  sun 
would,  by  its    diurnal   revolution,  seem   to  describe 


54  THTE    EARTH'S   ANNUAL    MOTION. 

every  day  the  same  circle,  viz.  the  equator ;  and  there- 
fore  could  have  no  declination.  For,  in  describing  the 
ecliptic  by  its  annual  revolution,  the  sun  would  then 
successively  answer  to  all  the  stars  in  the  equator,  con- 
sequently, its  diurnal  revolution  would  be  made  in  the 
same  circle  as  that  of  the  stars  ;  but  I  have  already  ob- 
served to  you,  that  the  sun  appears  to  describe  daily 
different  parallels  ;  it  is  evident,  therefore,  that  the  plane 
of  the  ecliptic  does  not  coincide  with  that  of  the  equa- 
tor, but  is  inclined  thereto. 

If  so,  the  sun  must,  in  consequence  of  its  annual  mo- 
tion,  describe  a  great  circle  NBTLN, plate  2, Jig.  4, 
representing  the  ecliptic,  and  bisecting  the  equator  E  B 
Z  L  E.  The  sun  must  therefore  appear  sometimes  to- 
wards one  pole,  and  sometimes  towards  the  other  pole. 

1 .  Let  us  suppose  that  it  is  in  B,  one  of  the  intersec- 
tions of  the  equator  and  the  ecliptic,  its  diurnal  revolu- 
tion must  describe  the  equator,  and  have  no  declina- 
tion ;  as  the  sun  gradually  advances  in  the  ecliptic  from 
B  towards  A,  it  appears  gradually  to  recede  from  the 
equator  with  an  increasing  northern  declination,  and  to 
describe  smaller  and  smaller  parallels,  till  it  arrive  at 
A,  where  it  appears  to  describe  the  parallel  A I  V  A. 

2.  The  sun  being  arrived  at  L,  three  signs  or  90 
degrees  from  B,  and  three  months  after  his  departure 
therefrom,  it  is  then  in  that  point  of  the  ecliptic  the 
most  distant  from  the  equator  at  its  greatest  northern 
declination,  and  describes  the  smallest  parallel  O  T. 

3.  In  the  three  following  months  the  sun  going  from 
T  to  L,  draws  nearer  the  equator,  its  northern  decli- 
nation diminishes,  its  parallels  augment ;  so  that  when 
arrived  in  L,  the  other  intersection  of  the  equator  and 
the  ecliptic,  it  then  has  no  declination,  and  on  that  day 
again  describes  the  celestial  equator. 

4.  The  sun  after  that,  passing  from  L  to  N,  enters 
the  south  part  of  the  heavens,  its  southern  declination 
then  increases,  and  its  parallels  diminish,  till  being- 
come  to  N,  three  signs  from  the  point  L,  its  southern 
declination  is  then  the  greatest,  and  it  describes  its  least 
parallel  ND, 


THE    SEASONS    OF    THE    YEAR.  55 

5.  The  sun,  continuing  its  course  from  N  to  B,  again 
draws  nearer  the  equator,  and  its  southern  declination 
diminishes ;  so  that  when  returned  to  B,  a  year  after 
its  departure  from  that  point,  it  is  again  in  the  equator 
and  without  declination,  and  then  begins  a  new  course 
attended  with  the  same  phenomena. 

It  is  evident,  therefore,  that  the  sun  moving  continually 
in  the  ecliptic,  the  parallels  it  every  day  describes  cannot  be 
circles,  but  a  kind  of  spiral,  such  as  the  curves  made  by 
a  thread  wound  about  a  sphere.  For  after  a  diurnal 
revolution,  the  sun  does  not  come  to  the  same*  point 
from  whence  it  departed,  but  according  as  it  approach- 
ed to,  or  receded  from  the  equator,  is  either  a  little  be- 
low or  above  that  point. 

The  angle  formed  by  the  plane  of  the  equator  and 
ecliptic,  is  called  the  obliquity  of  the  ecliptic. 

The  obliquity  of  the  ecliptic  is  equal  to  the  sun's 
greatest  declination,  namely,  when  in  the  tropic,  and  is 
about  23 \  degrees ;  consequently,  the  axis  of  the  earth 
must  be  inclined  to  the  ecliptic  in  an  angle  of  66x  de- 
grees. The  consideration  of  this  obliquity  brings  us 
to  an  explanation — 


OF    THE    SEASONS    OF    THE    YEAR. 


It  is  our  business  under  the  present  head  to  account 
for  the  phenomena  of  the  seasons,  those  greatful  vicis- 
situdes on  which  so  much  both  of  the  business  and  hap- 
piness of  man  depends. 

Before  I  explain  the  causes  of  those  changes  that  are 
termed  the  seasons  of  the  year,  it  wall  be  necessary  to 
premise  a  few  considerations  :  First,  that  on  account  of 
the  immense  distance  of  the  sun  from  the  earth,  the 
rays  which  proceed  from  it  may  be  considered  as  pa- 
rallel to  each  other.  Secondly,  that  only  one-half  of 
the  globe  can  be  illuminated  by  parallel  rays,  and  there- 
fore only  one-half  the  earth  will  be  enlightened  by  the 
sun  at  one  time.  Thirdly,  that  we  may  call  the  line, 
which  divides  light  from  darkness,  the  terminator. 


$6  THE    bEASONS    OF    THE    YEAR. 

In  the  diagram,  plate  5,  Jig.  1,  S  represents  the  sun, 
from  which  we  suppose  parallel  rays  to  flow  in  all  direc- 
tions ;  A,  B,  C,  represent  three  different  positions  of 
the  globe  of  the  earth,  the  bright  part  being  that  which 
is  illuminated  by  the  rays  proceeding  from  the  sun ; 
the  dark  part,  the  portion  of  the  globe  which  is  in  ob- 
scurity at  these  different  situations  ;  N  the  north,  S  the 
south  pole  of  the  globe,  T  T  the  terminator,  or  boun- 
dary of  light  and  darkness. 

At  C,  the  poles  coincide  with  the  terminator. 

At  A,  the  north  pole  is  altogether  in  the  illuminated 
hemisphere,  and  the  south  pole  in  the  dark  hemis- 
phere. 

It  is  evident,  that  it  is  day  in  any  given  place  on  the 
globe,  so  long  as  that  place  continues  in  the  enlightened 
hemisphere ;  but  when,  by  the  diurnal  rotation  of  the 
the  earth  on  its  axis,  it  is  carried  into  the  dark  hemis- 
phere, it  becomes  night  to  that  place. 

The  length  of  the  day  and  night  depends  on  the  position 
of  the  terminator,  with  respect  to  the  axis  of  the  earth. 

If  the  poles  of  the  earth  be  situate  in  the  terminator, 
as  at  C,  every  parallel  will  be  divided  into  two  equal 
parts  ;  and  as  the  uniform  motion  of  the  earth  causes 
any  given  place  to  describe  equal  parts  of  its  parallel 
in  equal  times,  the  day  and  night  would  be  equal  on 
every  parallel  of  latitude,  that  is,  all  over  the  globe,  ex- 
cept at  the  poles,  where  the  sun  would  neither  rise  nor 
set,  but  still  continue  in  the  horizon. 

But  if,  as  at  A  and  B,  the  axis  be  not  placed  in  the 
plane  of  the  terminator,  the  terminator  will  divide  the 
equator  into  two  equal  parts,  but  all  the  circles  parallel 
to  it  into  unequal  parts  ;  those  circles  that  are  situate 
towards  the  enlightened  pole,  will  have  a  greater  part 
of  their  circumference  in  the  enlightened  than  in  the 
dark  hemisphere  ;  while  similar  parallels  towards  the 
other  pole  will  have  the  greater  part  of  their  circumfer- 
ence in  the  dark  hemisphere.  Whence  it  follows,  that 
the  first-mentioned  parallels  will  enjoy  longer  days  than 
nights ;  and  the  contrary  will  happen  to  the  latter, 
where  the  days  will  be  the  shortest,  and  the  nights  the 


THE    SEASONS    OF    THE    YEAR,  57 

longest ;  while  at  the  equator,  the  days  and  nights  con- 
tinue  equal.  All  this  is  evident  from  the  bare  inspec- 
tion of  the  figures  ;  it  is  also  observeable,  that  the  dis- 
proportion is  greatest  in  the  greatest  latitude  ;  and  that 
those  places,  whose  distance  from  the  pole  is  less  than 
that  of  the  pole  from  the  terminator,  must  enjoy  either  a 
constant  day,  or  a  constant  night,  because  they  are  ne- 
ver carried  into  the  opposite  hemisphere  by  the  diurnal 
rotation  of  the  earth.  In  this  position  of  the  axis,  the 
inhabitants  on  one  side  of  the  equator  may  be  said  to  en- 
joy summer,  and  those  on  the  other  side,  winter,  with 
respect  to  each  other. 

From  what  has  been  said,  it  is  plain,  that  the  vicissi- 
tudes in  the  days  and  nights  are  occasioned  by  the  posi- 
tion of  the  terminater,  or  boundary  of  light  and  dark- 
ness, with  the  axis  of  the  earth  ;  or,  in  other  words,  by 
the  different  aspect  of  the  earth  with  respect  to  the  sun. 

We  have  now  only  to  show  what  causes  the  changes 
of  position  in  the  terminator,  which  are,  1 .  The  inclina- 
tion of  the  earth's  axis  to  the  plane  of  the  ecliptic,  or 
orbit  in  which  it  moves.  2.  That  through  the  whole  of 
its  annual  course,  the  axis  of  the  earth  preserves  its  po- 
sition, or  continues  parallel  to  itself;  that  is,  if  a  line  be 
conceived  as  drawn  parallel  to  the  axis  while  the  earth 
is  in  any  one  point  of  its  orbit,  the  axis  will  in  every 
other  position  of  the  earth  be  parallel  to  the  said  line. 

If  the  axis  of  the  earth  were  perpendicular  to  the 
plane  of  its  orbit,  the  equator  and  the  orbit,  or  ecliptic, 
would  coincide  ;  and  as  the  sun  is  always  in  the  plane  of 
the  ecliptic,  it  would  in  this  case  be  always  over  the  equa- 
tor, and  the  two  poles  would  be  in  the  terminator,  and 
there  would  be  no  diversity  in  the  days  and  nights,  and 
but  one  season  in  the  year ;  but  as  this  is  not  the  case, 
we  may  fairly  infer,  that  the  axis  of  the  earth  is  not  per- 
pendicular to  the  plane  of  its  orbit. 

But,  if  the  earth's  axis  be  inclined  to  the  plane  of  the 
ecliptic,  when  the  earth  is  in  the  situation  represented  at 
A,  plate  5,  Jig,  1,  the  pole  N  will  be  towards  the  sun, 
and  the  pole  S  will  be  turned  from  it;  but  just  the  con- 
trary will  happen,  when  the  earth,  by  going  half-round 
the  sun,  has  arrived  at  the  opposite  point  in  its  orbit. 

VOL.  IV.  1 


58  THE    SEASONS    OF    THE    YEAR. 

Hence  the  sun  will  not  be  always  in  the  equator,  but  at 
one  time  of  the  year  it  will  appear  nearer  to  one  of  the 
poles,  and  at  the  opposite  season,  it  will  appear  nearer  to 
the  other.  To  this  circumstance  the  change  of  seasons 
is  owing  ;  for  when  the  sun  leaves  the  equator  and  ap- 
proaches to  one  of  the  poles,  it  will  be  summer  on  that 
side  of  the  equator,  and  when  the  sun  departs  from 
thence  and  approaches  to  the  other  pole,  it  will  be  win- 
ter. Thus,  from  the  inclination  of  the  axis,  each  part 
of  the  earth  enjoys  the  benefit  of  summer  in  its  turn  -y 
for  it  is  evident,  from  what  has  been  said  already,  that 
when  it  is  winter  towards  one  of  the  poles,  on  one  side 
the  equator,  it  is  summer  towards  the  other  pole,  or  on 
the  other  side  of  the  equator. 

A  better  notion  of  the  effects  of  the  inclination  of  the 
earth's  axis  will  be  obtained  by  observing  plate  5,  Jig.  2. 
in  which  the  ellipsis  represents  the  earth's  orbit,  seen  at 
a  distance  ;  the  eye  being  supposed  to  be  elevated  a  lit- 
tle above  the  plane  of  it.  The  earth  is  here  represented 
in  the  first  point  of  each  of  the  twelve  signs,  as  marked 
in  the  figure,  with  the  twelve  months  annexed  ;  e  is  the 
north  pole,  and  e  d  the  axis  of  the  ecliptic,  always  per- 
pendicular to  the  plane  of  the  orbit ;  P  the  north  pole  of 
the  world ;  P  m  its  axis,  about  which  the  earth's  daily 
motion  is  made  from  west  to  east.  P  C  E  shows  the 
angle  of  its  inclination,  which  preserves  its  parallelism 
through  every  part  of  its  orbit. 

When  the  earth  is  in  the  first  point  of  Aries,  the  sun 
then  appears  in  the  opposite  point  of  the  ecliptic  at  Libra, 
about  the  twenty-second  of  September,  N.  S.  and  when 
the  earth  is  in  Libra,  the  sun  will  then  appear  in  Aries 
about  the  nineteenth  of  March ;  at  which  times  of  the 
year  the  edge  of  the  enlightend  hemisphere  is  parallel  to 
the  solstitial  colure,  and  passes  through  the  two  poles  of  the 
world,  dividing  every  parallel  to  the  equator  into  two 
equal  parts  ;  whence  the  diurnal  parallel  of  every  inha- 
bitant on  the  surface  of  the  earth  will,  at  either  of  these 
seasons,  be  half  in  the  illuminated,  and  half  in  the 
obscure  part  of  the  earth  ;  consequently,  the  day  and 
night  will  be  equal  in  all  places. 


THE    SEASONS    OF    THE    YEAR.  39 

Conceive  the  earth  to  have  moved  from  Libra  to  Ca- 
pricorn, its  line  of  direction  keeping  its  parallelism  will 
now  coincide  with  the  solstitial  colure,  and  the  edge  of 
the  disk  will  be  perpendicular  thereto,  and  pass  through 
e,  the  pole  of  the  ecliptic.  In  this  situation  of  the  earth, 
all  places  within  the  northern  polar  circle  are  illuminated 
throughout  the  whole  diurnal  revolution,  at  which  time 
their  inhabitants  see  the  sun  longer  than  twenty-four 
hours  ;  but  those  which  lie  under  the  polar  circle  touch 
the  edge  of  the  disk,  and  therefore  their  inhabitants  only 
see  the  sun  skim  round  their  horizon  as  at  its  first  ap- 
pearance. Every  other  parallel  intersects  the  edge  of 
the  disk,  and  as  the  illuminated  part  of  each  is  much 
greater  than  the  obscure  part,  the  days  are,  consequent- 
ly, at  the  season  of  the  summer  solstice,  which  happens 
about  the  twenty-first  of  June,  longer  than  the  nights. 
While  the  earth  is  moving  from  Libra  through  Capri- 
corn to  Aries,  the  north  pole,  P,  being  in  the  illuminated 
hemisphere,  will  have  six  months  continual  day  ;  but 
while  the  earth  passes  from  Aries  through  Cancer  to  Li- 
bra, the  north  pole  will  be  in  the  obscure  parts,  and  have 
continual  night,  the  south  pole  of  the  globe  at  the  same 
dme  enjoying  continual  day.  When  the  earth  is  at 
Cancer,  the  sun  appears  at  Capricorn.  At  this  season 
the  nights  will  as  much  exceed  the  days,  as  the  days 
exceeded  the  nights  when  the  earth  was  in  the  opposite 
point  of  its  orbit ;  for  the  nocturnal  arcs,  or  obscure 
part  of  their  paths,  are  here  equal  to  the  illuminated 
parts  when  the  earth  was  at  Capricorn  ;  and  the  illumi- 
nated part  is  here  no  more  than  the  obscure  part  was  in 
that  place. 

By  considering  the  three  globes,  A,  B,  C, plate  5.  Jig, 
1,  you  may  gain  a  clear  idea  of  the  daily  apparent  change 
in  the  sun's  declination ;  there  is  a  line  drawn  from  the 
centre  of  the  sun  to  the  centre  of  each  globe ;  it  is  broad- 
er than  the  other  lines.  This  line  may  be  called  the  cen- 
tral solar  ray.  About  the  twenty-first  of  December, 
when  the  earth  is  in  Cancer,  this  ray  will  terminate,  or 
fall  upon  the  southern  tropic,  as  at  D  ;  or  the  tropic  of 
Capricorn,  as  at  B  ;  and,  consequently,  by  the  earth's 
rotation  round  its  axis,  the  inhabitants  of  every  part  cf 


60  THE  SEASONS  OF  THE  YEAH. 

this  circle  will  successively  have  the  sun  in  their  zenith  ; 
or,  in  other  words,  he  will  be  vertical  to  them  that  day 
at  noon,  as  the  sun  appears  that  day  to  be  carried  round 
in  the  tropic  of  Capricorn. 

About  the  twentieth  of  March,  the  earth  is  at  Libra, 
and  the  sun  will  then  appear  in  Aries  ;  the  central  solar 
ray  terminates  upon  the  surface  of  the  earth,  in  the  equa- 
tor, as  at  C  ;  and  therefore  the  sun  appears  to  be  carried 
round  in  the  celestial  equator,  and  is  successively  vertical 
to  those  who  live  under  that  circle. 

About  the  twenty-first  of  June,  when  the  earth  is  in 
Capricorn,  the  central  solar  ray  terminates  on  the  sur- 
face of  the  earth,  in  the  northern  tropic,  as  at  A ;  and 
for  that  day  the  sun  appears  to  be  carried  round  in  the 
tropic  of  Cancer,  and  is  vertical  to  those  who  live  under 
that  circle.  About  the  twenty-second  of  September, 
the  earth  is  in  Aries,  and  the  sun  in  Libra,  and  the  cen- 
tral solar  ray  again  terminates  at  the  equator  ;  conse- 
quently, the  sun  again  appears  in  the  celestial  equator, 
and  is  vertical  to  those  who  live  under  it. 

We  have  seen,  that  as  the  sun  moves  in  the  ecliptic, 
from  the  vernal  equinox  to  the  tropic  of  Cancer,  it  gets 
to  the  north  of  the  equator,  or  its  declination  towards 
our  pole  increases.  Therefore,  from  the  vernal  equinox, 
when  the  days  and  nights  are  equal,  till  the  sun  comes 
to  the  tropic  of  Cancer,  our  days  lengthen,  and  our 
nights  shorten ;  but  when  the  sun  comes  to  the  tropic 
of  Cancer,  it  is  then  in  its  utmost  northern  limit,  and 
returns  in  the  ecliptic  to  the  equator  again.  During  this 
return  of  the  sun,  its  declination  towards  our  pole  de- 
creases, and  consequently  the  days  decrease,  and  the 
nights  increase,  till  the  sun  is  arrived  in  the  equator 
again,  and  is  in  the  autumnal  equinoctial  point,  when 
the  days  and  nights  will  again  be  equal.  As  the  sun 
moves  from  thence  towards  the  tropic  of  Capricorn,  it 
gets  to  the  south  of  the  equator  ;  or  its  decimation  to- 
wards the  south  pole  increases.  Therefore,  at  that  time 
of  the  year,  our  days  shorten,  and  our  nights  lengthen, 
till  the  sun  arrives  at  the  tropic  of  Capricorn  ;  bur  when 
the  sun  is  arrived  there,  it  is  then  at  its  utmost  southern 
limit,  and  returns  in   the  ecliptic  to  the  equator  again. 


THE    SEASONS    OF    THE    YEAR.  61 

During  this  return,  its  distance  from  our  pole  lessens, 
and  consequently  the  days  will  lengthen,  as  the  nights 
will  shorten,  till  they  become  equal,  when  the  sun  is 
come  round  to  the  vernal  equinoctial  point. 

Our  summer  is  nearly  eight  days  longer  than  the  summer. 

By  summer  is  meant  here  the  time  that  passes  between 
the  vernal  and  autumnal  equinoxes  ;  by  winter,  the  time 
between  the  autumnal  and  vernal  equinoxes.  The  eclip- 
tic is  divided  into  six  northern,  and  six  southern  signs, 
and  intersects  %the  equator  at  the  first  of  Aries,  and  the 
first  of  Libra.  In  our  summer,  the  sun's  apparent  mo- 
tion is  through  the  six  northern,  and  in  our  winter 
through  the  six  southern  signs  ;  yet  the  sun  is  1 86  days, 
11  hours,  ,51  minutes,  in  passing  through  the  six  first; 
and  only  178  days,  17  hours,  58  minutes,  in  passing 
through  the  six  last.  The  difference,  7  days,  17  hours, 
53  minutes,  is  the  length  of  time  by  which  our  summer 
exceeds  the  winter. 

In  plate  6,  Jig.  1 ,  A  B  C  D  represents  the  earth's  or- 
bit ;  S  the  sun  in  one  of  its  foci ;  when  the  earth  is  at 
B,  the  sun  appears  at  H,  in  the  first  point  of  Aries ;  and 
while  the  earth  moves  from  B,  through  C  to  D,  the  sun 
appears  to  run  through  the  six  northern  signs,  from  t 
through  a  to  -=  at  F.  When  the  earth  is  at  D,  the  sun 
appears  at  F,  in  the  first  point  of  Libra  ;  and  as  the 
earth  moves  from  D,  through  A  to  B,  the  sun  appears 
to  move  through  the  six  southern  signs,  from  =&  through 
v?  to  Aries  at  H. 

Hence  the  line  F  H,  drawn  from  the  first  point  of 
Aries,  through  the  sun  at  S,  to  the  first  point  of  ^,  di- 
vides the  ecliptic  into  two  equal  parts  ;  but  the  same 
line  divides  the  earth's  elliptical  orbit  into  two  unequal 
parts.  The  greater  part,  B  C  D,  is  that  which  the  earth 
describes  in  the  summer,  while  the  sun  appears  in  the 
northern  signs.  The  lesser  part  is  DAB,  which  the 
earth  describes  in  winter,  while  the  sun  appears  in  the 
southern  signs.  C,  the  earth's  aphelion,  where  it  moves 
slowest,  is  in  the  greater  part  ;  A,  its  perihelion,  is  in  the 
lesser  part,  where  the  sun  moves  fastest. 

There  are,  therefore,  two  reasons  why  our  summer 
is  longer  than  our  winter  \  first,  because  the  sun  conti- 


62  THE    SEASONS    OF    THE    YEAR. 

nues  in  the  northern  signs,  while  the  earth  is  describing 
the  greater  part  of  its  orbit ;  and  secondly,  because  the 
sun's  apparent  motion  is  slower  while  it  appears  in  the 
northern  signs,  than  whilst  it  appears  in  the  southern 
ones. 

The  sun's  apparent  diameter  is  greater  in  our  winter 
than  in  summer,  because  the  earth  is  nearer  to  the  sun 
when  at  A,  in  the  winter,  than  it  is  when  at  C,  in  the 
summer.  The  sun's  apparent  diameter,  in  the  middle  of 
winter,  is  32  minutes,  47  seconds ;  in  the  middle  of 
summer,  31  minutes,  40  seconds. 

But  if  the  earth  is  farther  from  the  sun  in  summer 
than  in  winter,  it  may  be  asked,  why  our  winters  are  so 
much  colder  than  our  summers.  To  this  it  may  be  an- 
swered, rhat  our  summer  is  hotter  than  the  winter,  first, 
on  account  of  the  greater  height  to  which  the  sun  rises 
above  our  horizon  in  the  summer ;  secondly,  the  greater 
length  of  the  days.  The  sun  is  much  higher  at  noon  in 
summer  than  in  winter,  and  consequently,  as  its  rays  in 
summer  are  less  oblique  than  in  winter,  more  of  them 
will  fall  upon  the  surface  of  the  earth.  In  the  summer, 
the  days  are  very  long,  and  the  nights  very  short ;  there- 
fore the  earth  and  air  are  heated  by  the  sun  in  the  day- 
time, more  than  they  are  cooled  in  night ;  and,  upon 
this  account,  the  heat  will  keep  increasing  in  the  sum- 
mer, and  for  the  same  reason  will  decrease  in  winter, 
when  the  nights  lengthen. 

I  should  exceed  the  limits  of  a  lecture,  were  I  to  in- 
quire into  the  several  concurring  causes  of  the  tempera- 
tures that  obtain  in  various  climates;  it  may  be  sufficient, 
therefore,  to  observe  what  a  remarkable  provision  is  made 
in  the  world,  and  the  several  parts  of  it,  to  keep  up  a 
perpetual  change  in  the  degrees  of  heat  and  cold.  These 
two  are  antagonists,  or,  as  Lord  Bacon  calls  them,  "  the 
very  hands  of  nature  with  which  she  chiefly  worketh," 
the  one  expanding,  the  other  contracting  bodies,  so  as  to 
maintain  an  oscillatory  motion  in  all  their  parts  ;  and  so 
serviceable  are  these  changes  in  the  natural  world,  that 
they  are  promoted  every  year,  every  hour,  every  mo- 
ment. From  the  oblique  position  of  the  ecliptic,  the 
earth  continually  presents  a  different  face  to  the  sun,  and 


THE    SEASONS    OF    THE    YEAR.  63 

never  receives  his  rays  two  days  together  in  the  same 
direction.  In  the  day  and  night,  the  differences  are  so 
obvious,  that  they  need  not  be  mentioned,  though  they 
are  most  remarkable  in  those  climates,  where  the  sun 
at  his  setting  makes  the  greatest  angle  with  the^  horizon. 
Every  hour  of  the  day,  the  heat  varies  with  the  sun's 
altitude,  is  altered  by  the  interposition  of  clouds,  and 
the  action  of  winds  ;  and  there  is  little  room  to  doubt, 
but  what  the  various  changes  that  thus  take  place,  con- 
cur in  producing  many  of  the  smaller  and  greater  phe- 
nomena of  nature. 

Be  this  however  as  it  may,  it  is  certain,  that  the  va- 
rious irregularities  and  intemperatures  of  the  elements, 
which  seem  to  destroy  nature  in  one  season,  serve  to 
revive  it  another  :  the  immoderate  heat  of  summer, 
and  the  excessive  cold  of  winter,  prepare  the  beauties 
of  the  spring,  and  the  rich  fruits  of  autumn.  These 
vicissitudes,  which  seem  to  superficial  minds  the  effects 
of  a  fortuitous  concourse  of  irregular  causes,  are  re- 
gulated according  to  weight  and  measure  by  that  Sove- 
reign Wisdom,  who  weighs  the  earth  as  a  grain  of  sand, 
the  s^a  as  a  drop  of  water. 

Our  observations  on  the  seasons  cannot  be  better  con- 
cluded than  in  the  words  of  the  excellent  Hooker.  A 
long  and  uninterrupted  enjoyment  of  blessings  is  apt  to 
extinguish  in  us  that  gratitude  towards  the  Author  of 
them,  which  it  ought  to  cherish  and  invigorate  ;  the 
course  of  nature  often  glides  on  unobserved  when  there 
are  no  variations  therein  ;  and  the  sun  himself  shineth 
unnoticed,  because  he  shineth  every  day.  Since  the 
time  that  God  did  first  proclaim  the  edicts  of  his  law, 
says  Hooker,  heaven  and  earth  have  hearkened  unto  his 
voice,  and  their  labour  has  been  to  do  his  will.  But  if 
nature  should  intermit  her  course,  and  leave  altogether, 
though  it  were  but  for  a  while,  the  observation  of  her 
laws  ;  if  those  principles  and  mother-elements,  whereof 
ail  things  in  this  world  are  made,  should  loose  the  qua- 
lities they  now  possess  ;  if  the  frame  of  that  heavenly 
arc  erected  over  our  heads  should  loosen  and  dissolve 
itself ;  if  the  celestial  globes  should  forget  their  wonted 
motions,  and  by  irregular  volubility  turn  themselves 


64  PHENOMENA    OF    THE    PLANETS. 

any  way  as  it  might  happen  ;  if  the  prince  of  the  lights 
of  heaven,  which  now  as  a  giant  doth  run  his  unweari- 
ed course,  should  as  it  were,  through  a  languishing 
faintness,  begin  to  stand  and  to  rest  himself;  if  the 
moon  should  wander  from  her  beaten  way,  the  times 
and  seasons  of  the  year  would  blend  themselves  toge- 
ther by  disorder  and  confused  mixture,  the  winds  breathe 
out  their  last  gasp,  the  clouds  yield  no  rain,  the  earth  be 
defeated  of  heavenly  influence,  and  her  fruits  pine  away 
as  children  at  the  withered  breasts  of  their  mother,  no 
longer  able  to  yield  them  relief;  what  would  become 
of  man  himself,  whom  all  those  things  do  now  serve  P 
And  how  would  he  look  back  on  those  benefits,  for 
which,  when  they  were  daily  poured  upon  him  in  bound- 
less profusion,  he  forgot  to  be  thankful  ? 


LECTURE  XXXIX. 


AN  EXPLANATION  OF  THE  PHENOMENA  OF  THE    PLA- 
NETS, ACCORDING  TO  THE  COPERNICAN  SYSTEM. 


I  SHALL  here  define  again  some  words  which  I 
have  already  explained,  and  recal  your  attention  to  some 
circumstances  which  I  mentioned  in  a  former  lecture. 
These  repetitions  will  not,  I  hope,  be  a  subject  of  com- 
plaint, as  they  will  render  this  lecture  more  perfect,  and 
answer  the  beneficial  purpose  of  grounding  yeu  more 
firmly  in  the  science  we  are  now  treating  of. 

The  line  that  a  planet  describes  round  the  sun  is 
called  its  orbit ;  the  motion  of  all  the  planets  in  their 
orbits  is  from  west  by  the  south  to  the  east ;  this  is  called 
their  annual  motion. 


PHENOMENA    OF    THE    PLANETS.  65 

The  orbits  of  the  planets  are  not  all  in  the  same 
plane,  but  in  planes  inclined  to  each  other,  or  intersect- 
ing each  other  at  different  angles.  The  orbit  of  the 
earth  is  taken  as  a  standard,  from  whence  their  respec- 
tive inclinations  are  computed. 

The  planes  of  the  several  orbits  of  the  planets  pro- 
duced to  the  fixed  stars,  mark  the  several  circles  which 
each  planet  would  appear  to  describe  in  the  sphere  of 
the  heavens  to  a  spectator  placed  in  the  sun  ;  these 
circles  may  be  called  the  heliocentric  orbits  of  the  planets. 

The  heliocentric  orbit  of  the  earth  is  the  ecliptic  : 
to  a  spectator  in  the  sun,  the  earth  would  appear  to 
go  round  the  sun  in  the  ecliptic  eastward  in  twelve 
months. 

We  may  suppose  as  many  great  circles  as  we  please 
to  be  described  upon  the  sphere  of  the  heavens,  inter- 
secting one  another  at  the  poles  of  the  ecliptic,  and  cut- 
ting it  at  right  angles  ;  these  are  termed  secondaries  of 
the  ecliptic,  and  circles  of  latitude.  The  latitude  of  a  pla- 
net or  star  is  its  distance  from  the  ecliptic,  measured  in 
degrees,  &c.  upon  a  circle  of  latitude  passing  through 
the  star  or  planet. 

The  latitude  a  planet  would  appear  to  have,  when 
viewed  from  the  sun,  is  its  heliocentric  latitude ;  that 
which  it  appears  to  have  to  an  inhabitant  of  the  earth,  is 
called  its  geocentric  latitude. 

By  the  place  of  a  planet  is  meant  the  place  of  its  cen- 
tre ;  its  geocentric  place  is  that  where  it  appears  to  art 
inhabitant  of  the  earth. 

The  two  points  where  the  ecliptic  is  cut  by  the  helio- 
centric orbit  of  a  planet,  are  the  nodes  of  the  planet. 
The  ascending  node  &,  is  the  point  where  the  ecliptic  is 
cut  by  the  planet,  before  it  deviates  northward  there- 
from;  the  descending  node,  ?S,  is  the  point  where  the 
planet  cuts  the  ecliptic  before  it  deviates  southward. 

When  any  planet  has  passed  its  ascending  node,  it 
deviates  more  and  more  northward  till  it  has  got  ninety 
degrees  from  the  node,  then  it  is  at  its  utmost  heliocen- 
tric northern  latitude,  or  northern  limit ;  from  thence 
it  continually  approaches  the  ecliptic,  till  it  comes  to  ?5* 
VOL.  IV.  k 


66  PHENOMENA    OF    THE    PLANETS. 

after  passing  which  it  deviates  more  and  more  south- 
ward, till  it  is  ninety  degrees  from  this  node,  when  it  is 
at  its  southern  limit,  or  utmost  southern  heliocentric  la- 
titude, which  from  thence  continually  decreases  till  the 
planet  returns  again  to  the  ascending  node.  A  planet 
seen  from  the  earth,  appears  in  the  ecliptic  only  when  it 
is  in  one  of  its  nodes. 

The  equator  cuts  the  ecliptic  in  two  opposite  points  ; 
when  the  sun  appears  in  one  of  these  points,  it  is  our 
vernal;  when  in  the  other,  it  is  our  autumnal  equinox. 
The  point  of  the  vernal  equinox  is  counted  the  first  point 
of  the  ecliptic,  because  spring  begins  the  astronomical 
year  ;   this  point  is  marked  Lf  Aries. 

The  lo?igitude  of  a  celestial  object  is  the  number  of 
degrees,  &c.  on  the  ecliptic,  reckoning  from  r  eastward, 
to  the  point  where  a  circle  of  latitude  drawn  through 
the  object  cuts  the  ecliptic.  The  longitude  and  latitude 
of  an  object  being  given,  its  place  in  the  sphere  of  the 
heavens  is  known  ;  and  its  place  is  usually  expressed, 
by  saying  it  is  in  such  a  degree  and  minute  of  such  a 
sign,  and  in  such  latitude. 

A  planet  is  said  to  be  in  conjunction  with  the  sun, 
when  its  geocentric  place  is  very  near  the  geocentric 
place  of  the  sun  ;  that  is,  when  the  sun  is  between  our 
earth  and  the  planet,  or  when  the  planet  is  between  the 
earth  and  the  sun. 

A  planet  is  said  to  be  in  opposition,  when  its  geocen- 
tric place  is  opposite  to  the  geocentric  place  of  the  sun ; 
that  is,  when  the  earth  is  between  the  sun  and  the  planet. 

An  exact  or  central  conjunction  or  opposition  can 
happen  only  when  a  planet  is  in  one  of  its  nodes ;  it  is, 
however,  usual  to  term  it  a  conjunction  or  opposition, 
when  the  same  secondary  of  the  ecliptic  passes  through 
the  sun  and  any  planet,  though  the  planet  has  latitude. 

When  the  geocentric  place  of  a  planet  is  a  quarter  of 
a  circle  distant  from  the  geocentric  place  of  the  sun,  the 
planet  is  said  to  be  in  quadrature. 

A  planet  is  said  to  be  direct,  when  its  geocentric  mo- 
tion is  eastward ;  retrograde,  when  westward ;  stationary, 
when  its  geocentric  place  continues  the  same  for  some 
time. 


MERCURY    AND    VENUS.  67 

The  distance  an  inferior  planet,  seen  from  the  earth, 
appears  to  be  from  the  sun,  is  cslled  its  elongatian. 


OF    THE   CONJUNCTIONS    AND    ELONGATIONS    OF    THE 
INFERIOR    PLANETS,    MERCURY    AND    VENUS. 

There  are  two  different  situations,  in  which  an  infe- 
rior planet  will  appear  in  conjunction  with  the  sun;  one, 
when  the  planet  is  between  the  sun  and  the  earth  ;  the 
other,  when  the  sun  is  between  the  earth  and  the  planet. 

Let  A,  plate  6,  Jig.  2,  be  the  earth  in  its  orbit,  E  the 
place  of  Venus  in  E  H  G  her  orbit,  S  the  sun,  F  V  P  Q 
R  T  D  an  arc  in  the  starry  heavens. 

In  the  situation  of  things  represented  in  this  diagram, 
the  sun  and  Venus  will  appear  in  the  same  point  of  the 
heavens,  and  so  be  in  conjunction.  If  Venus  be  at  G, 
there  will  also  be  a  conjunction.  When  the  planet  is  at 
E,  nearer  to  the  earth  than  the  sun,  it  is  called  its  infe* 
rior  conjunction;  but  when  the  planet  is  at  G,  farther 
from  the  earth  than  the  sun,  it  is  termed  the  superior  con- 
junction of  the  planet. 

When  the  planet  is  either  at  E  or  G,  it  has  no  elon- 
gation ;  but  as  the  planet  moves  from  E  to  y,  its  elon- 
gation increases  ;  for,  when  it  is  at  y,  it  appears  in  the 
line  A  y  P,  while  the  sun  appears  in  the  line  A  S  Qj  so 
that  P  A  Q^will  be  the  angular  measure  of  its  elongation 
or  distance  from  the  sun.  When  the  planet  arrives  at  x, 
it  appears  in  the  line  A  x  V,  which  is  a  tangent  to  its  or- 
bit, and  then  V  A  QJs  the  angular  measure  of  its  elon- 
gation ;  which  is  the  greatest  that  can  be  on  that  side 
the  sun,  for,  after  this,  the  elongation  decreases.  When 
the  planet  is  at  K,  its  elongation  is  P  A  Qj  when  at  G, 
it  is  nothing,  because  it  is  then  in  its  superior  conjunc- 
tion; as  the  planet  moves  on  from  G,  its  elongation 
again  increases ;  for,  when  it  comes  to  C,  it  appears  in 
the  line  A  C  R,  and  its  elongation  is  R  A  Q^  When  the 
planet  comes  to  H,  a  line  drawn  from  the  earth  through 
the  planet  is  a  tangent  to  the  orbit,  and  the  elongation  is 
T  A  Q,  the  greatest  it  can  have  when  it  is  on  the  other 


68  RETROGRADE,  &C.    MOTIONS 

side  of  the  sun ;  for,  after  this,  the  elongation  again  de- 
creases. 

Hence  it  is  clear,  that  the  inferior  planets  can  never 
appear  far  from  the  sun,  but  must  always  accompany  it 
in  its  apparent  motion  through  the  ecliptic.  When  we 
see  either  Venus  or  Mercury,  it  is  either  in  the  evening, 
soon  after  the  sun  has  set  -,  or  in  the  morning,  a  little 
before  the  sun  rises.  Venus  is  indeed  bright  enough 
sometimes  to  be  seen  in  the  day-time,  but  then  she  is  ne- 
ver far  from  the  sun.  The  greatest  elongation  of  Ve- 
nus is  about  47,  and  of  Mercury  about  28  degrees. 

If  the  earth  be  at  A,  plate  6,  fig.  2,  when  Venus  ap- 
pears in  any  part  of  the  arc  ExG,  she  is  westward  from 
the  sun,  and  therefore  rises  before  him  in  the  morning, 
and  is  called  the  morning  star.  When  she  appears  any 
where  in  the  arc  G  H  E,  she  is  eastward  from  the  sun, 
and  therefore  sets  after  him  ;  is  seen  in  the  evening, 
and  is  called  the  evening  star. 

i 

FALSITY    OF    THE    PTOLEMAIC    SYSTEM. 

From  the  apparent  motion  of  the  inferior  planets,  we 
derive  an  argument  to  show  the  falsity  of  the  Ptolemaic 
system.  If  the  earth  were  within  the  orbit  of  Venus, 
as  this  system  supposes,  she  might  be  sometimes  on  one 
side  the  earth  whilst  the  sun  was  on  the  opposite  side ; 
in  other  words,  Venus  might  be  sometimes  in  opposition: 
but  Venus  is  never  seen  in  opposition,  therefore  the 
earth  is  not  within  the  orbit  of  Venus,  and  consequently 
the  Ptolemaic  system  is  not  true.  The  same  reasoning 
applies  to  Mercury. 

OF  THE  RETROGRADE,  AND  DIRECT  MOTIONS,  AND 
STATIONARY  SITUATIONS  OF  MERCURY  AND 
VENUS. 

It  is  easy,  on  the  Copernican  system,  to  explain  why 
the  inferior  planets  appear  to  move  sometimes  in  one  di- 
rection, sometimes  in  a  contrary  one,  and  at  other  times 


OF    MERCURY    AND    VENUS.  69 

to  be  stationary  ;  for  it  is  the  natural  result  of  the  re- 
spective situations  and  motions  of  the  earth  and  these 
planets.  But  on  the  Ptolemaic  system,  it  is  inexplica- 
ble without  calling  in  the  aid  of  a  very  complicated  hy- 
pothesis. 

When  the  inferior  planets  are  passing  from  their  great- 
est elongation,  on  one  side  of  the  sun,  through  their  su- 
perior conjunction,  to  their  greatest  elongation  on  the 
other  side,  their  motion,  as  viewed  from  the  earth,  is 
direct.  In  order  to  explain  this  proposition,  we  shall  first 
suppose  the  earth  to  be  at  rest  at  A,  plate  6,  Jig.  2,  and 
correct  this  supposition  afterwards,  by  showing  that  the 
apparent  motion  of  Venus  or  Mercury,  E,  seen  from  the 
earth,  is  the  same  in  this  respect,  whether  the  earth  move 
in  its  orbit,  or  rest  at  A. 

The  proposition  to  be  explained  is  this ;  that  as  Ve- 
nus, for  instance,  moves  from  x,  its  greatest  elongation 
on  one  side  of  the  sun,  through  G  its  superior  conjunc- 
tion, to  H  its  greatest  elongation  on  the  other  side,  it 
will  appear  to  a  spectator  upon  the  earth  to  move  from 
west  to  east  according  to  the  order  of  the  signs  ;  that  is, 
its  geocentric  motion  will  be  direct. 

The  planets  move  round  the  sun  from  west  to  east, 
and  consequently  if  there  were  a  spectator  at  the  sun, 
they  would  appear  to  him  to  move  through  the  zodiac, 
according  to  the  order  of  the  signs  ;  or,  in  other  words, 
the  heliocentric  motion  of  Venus  is  direct.  Now,  if  the 
sun  and  the  earth  A,  be  both  on  the  same  side  of  the 
planet,  a  spectator  at  the  earth  is  in  the  same  situation 
with  respect  to  the  planet  and  its  motion,  as  if  he  had 
been  at  the  sun  ;  for,  whilst  the  planet  is  moving  from 
x,  through  G  to  H,  a  spectator  either  at  A  or  B  is  on 
the  concave  side  of  the  planet's  orbit ;  and  consequently 
the  planet  will  appear  to  move  in  the  same  manner  from 
either  ;  but  the  apparent  motion  of  the  planet,  when 
seen  from  the  sun,  is  direct,  and  consequently  its  motion, 
when  seen  from  the  earth,  will  also  be  direct. 

When  Venus  is  at  x,  it  appears  to  a  spectator  on  the 
earth,  at  A,  to  be  in  the  line  A  x  V,  or  is  seen  among 
the  stars  at  V ;  when  Venus  has  moved  to  K,  it  is  seen 
among  the  fixed  stars  at  P ;  when  it  has  moved  to  G,  it 


70  RETROGRADE,  &C.    MOTIONS 

is  in  its  superior  conjunction  ;  when  it  has  moved  to  C, 
it  appears  among  the  fixed  stars  at  R  ;  and  when  it  is 
come  to  K,  it  appears  among  the  fixed  stars  at  T.  Thus, 
while  Venus  has  moved  in  its  orbit  from  x,  its  greatest 
elongation  on  one  side  of  the  sun,  through  G,  its  supe- 
rior conjunction  to  H,  its  greatest  elongation  on  the 
other  side,  it  appears  to  have  described  the  arc  VPO 
R  T,  in  the  concave  sphere  of  the  heavens ;  but  the 
letters,  x  K  G  C  H  lie  from  west  to  east,  because  they 
lie  in  the  same  direction  that  the  planet  moves  round 
the  sun  :  and  the  letters  V  P  O  R  T  lie  in  the  same  di- 
rection  with  x  K  G  C  H.  Therefore,  as  the  planet 
seems  to  a  spectator  on  the  earth  to  describe  the  arc 
VPOR  T,  its  apparent  motion  seen  from  the  earth  is 
direct,  or  from  west  to  east. 

Ag  the  inferior  planets  move  from  their  greatest 
elongation  on  one  side  of  the  sun,  through  their  infe- 
rior conjunction,  to  their  greatest  elongation  on  the 
other  side,  their  geocentric  motion  is  retrograde. 

While  Venus,  for  instance,  is  moving  from  its  great- 
est elongation  H,  through  its  inferior  conjunction  E,  to 
its  other  greatest  elongation  x,  it  appears  to  a  spectator 
upon  the  earth  at  A,  to  move  backwards  or  from  east 
to  west,  contrary  to  the  order  of  the  signs. 

A  spectator  at  the  sun  is  on  the  concave  side  of  the 
planet's  orbit.  But  while  Venus  is  moving  from  its 
greatest  elongation  H,  on  one  side,  through  E  its  infe- 
rior conjunction,  to  x,  its  greatest  elongation  on  the 
other  side,  a  spectator  upon  the  earth  is  on  the  convex 
side  of  its  orbit. 

Therefore,  if  a  spectator  at  the  sun,  S,  would  see  the 
planet  move  one  way,  a  spectator  at  the  earth,  A,  would 
see  it  move  the  contrary  way  ;  or  the  geocentric  mo- 
tion would  be  contrary  to  its  heliocentric  motion,  and 
therefore  retrograde  ;  for,  as  seen  from  the  sun,  its 
motion  is  always  direct. 

That  two  spectators,  one  at  the  earth,  the  other  at  the 
sun,  being  on  contrary  sides  of  the  arc  HE.v,  would 
see  the  planet  apparently  move  contrary  ways,  may  be 
rendered  more  plain  by  the  following  familiar  considera- 
tion.    If  two  men  stand  with  their  faces  towards  each 


OF    MERCURY    AND     VENUS.  71 

other,  and  a  ball  be  rolled  along  upon  the  ground,  this 
ball  will  move  from  the  right  hand  of  one  of  the  men 
towards  his  left,  and  from  the  left  hand  of  the  other  to- 
wards his  right.  In  like  mariner,  if  one  man  be  at  the 
earth  A,  and  the  other  at  the  sun  S,  then,  while  the 
planet  is  describing  the  arc  H  E  x,  wThich  is  between 
them,  it  will  appear  to  move  from  the  right  hand  of  the 
man  at  S  towards  his  left,  and  from  the  left  hand  of  the 
man  at  A  towards  his  right. 

While  the  motion  of  Venus  is  direct,  or  describing  the 
arc  x  G  H,  it  appears  to  move  from  V  to  T,  among  the 
fixed  stars.  But,  after  it  has  been  carried  in  its  orbit 
from  H  to  z,  it  appears  in  the  line  A  z  R,  and  is  seen 
among  the  fixed  stars  at  R.  When  it  comes  to  E,  it 
appears  at  Q  ;  and  when  at  y,  its  apparent  place  in  the 
heavens  is  at  P.  Thus,  as  the  planet  passes  from  its 
greatest  elongation  H,  on  one  side  of  the  sun,  through 
its  inferior  conjunction  E,  to  its  greatest  elongation  *, 
on  the  other  side,  it  apparently  goes  back  from  T  to  V. 

Venus  is  stationary,  or  has  no  apparent  motion  for 
some  time,  when  it  is  at  its  greatest  elongation  ;  that  is, 
when  it  is  at  H  or  x,  and  its  apparent  place  is  either  at 
Tor  V. 

When  either  of  the  inferior  planets,  Venus  for  in- 
stance, is  at  its  greatest  elongation  H  or  *,  a  line  drawn 
from  the  earth  through  the  planet,  as  A  H  T  or  A  x  V, 
is  a  tangent  to  the  orbit.  Now,  though  a  line  touches 
a  circle  but  in  one  point,  yet  some  part  of  the  circle 
greater  than  a  point  is  so  near  to  the  tangent,  as  not  to 
be  distinguished  from  it.  Thus  the  arc  at  H,  so  near- 
ly coincides  with  the  tangent  A  H  T,  that  a  spectator's 
eye  placed  at  A,  could  not  distinguish  the  tangent  from 
this  part  of  the  curve.  Consequently,  while  the  planet 
is  describing  this  arc,  no  other  change  will  be  made  in 
its  geocentric  place,  than  if  it  were  to  move  in  the  tan- 
gent. 

But  the  geocentric  place  of  the  planet  would  not  be 
altered,  if  the  planet  were  to  move  in  the  tangent.  For, 
if  it  were  to  move  from  T  towards  A,  or  from  A  to  V, 
the  apparent  place  of  it  in  the  heavens  would  in  one 
case  be  at  T,  in  the  other  case  at  V.     Therefore,  while 


72  RETROGRADE,  &C.  MOTIONS 

the  planet  is  at  its  greatest  elongation,  and  is  describing 
a  small  arc  in  its  orbit,  that  nearly  coincides  with  the 
tangent,  its  geocentric  place  does  not  alter,  but  it  ap- 
pears to  continue  for  some  time  in  the  same  part  of  the 
heavens,  or  is  stationary. 

We  have  hitherto  supposed  the  earth  to  be  at  rest, 
and  upon  that  supposision  have  explained  the  progress 
and  regress,  the  conjunctions  and  stations  of  the  infe- 
rior planets.  If  this  supposition  were  true,  V  T,  or  the 
arc  which  the  planet  at  any  time  describes  in  its  pro- 
gress, and  T  V,  the  arc  which  it  describes  in  its  regress, 
would  always  be  in  the  same  part  of  the  heavens.  The 
planet,  when  in  conjunction,  would  always  appear  at 
O j  among  the  same  fixed  stars;  and  at  its  greatest  elon- 
gation, or  when  it  is  stationary,  it  would  always  appear 
among  the  same  fixed  stars,  T,  on  one  side  of  the  sun, 
and  at  V  on  the  other  side. 

But  this  supposition  is  not  true ;  for  the  earth  re- 
volves in  its  orbit,  ABO,  round  the  sun.  Now,  if  the 
earth  be  at  A,  at  the  time  of  either  conjunction,  the 
planet  at  this  conjunction  would  appear  among  the  fix- 
ed stars  at  Q,  and  the  arcs  of  the  greatest  elongation, 
Q  V  and  Q  T,  would  be  on  each  side  of  those  stars. 
But  if  the  earth  be  at  B,  at  the  time  of  either  of  the 
conjunctions,  then  at  the  time  of  this  conjunction  the 
planet  will  appear  in  the  line  B  S  T,  and  be  seen  among 
the  fixed  stars  at  T,  and  the  arcs  of  the  greatest  elon- 
gation will  be  on  each  side  of  these  stars  ;  that  is, 
the  conjunctions  and  elongations  will  happen  in  a  differ- 
ent part  of  the  heavens,  when  the  earth  is  at  B,  from 
what  they  happen  in  when  the  earth  is  at  A.  In  other 
respects,  the  foregoing  phenomena  will  be  much  the 
same,  notwithstanding  the  motion  of  the  earth,  only  the 
planet  will  be  more  direct  in  the  farthest  part  of  the 
orbit,  and  less  retrograde  in  the  nearest. 

The  direct  and  retrograde  motion  is  sometimes  swift- 
er, sometimes  slower.  The  direct  motion  is  swiftest, 
when  the  sun  is  between  us  and  the  planet ;  the  retro- 
grade swiftest  when  the  planet  is  between  us  and  the 
sun. 


OF    MERCURY    AND    VENUS.  73 

When  an  inferior  planet,  viewed  from  a  superior, 
moves  apparently  retrograde,  the  superior  planet  has 
also  a  retrograde  motion. 

When  a  superior  planet,  viewed  from  an  inferior,  ap- 
pears stationary,  the  inferior  planet  viewed  at  the  same 
time  from  the  superior  is  also  stationary. 


OTHER    APPEARANCES    OF    THE    INFERIOR    PLANETS. 

The  inferior  planets,  it  is  plain  from  what  I  have 
shown  you,  always  appear  very  near  the  sun.  But,  from 
the  motion  of  the  earth,  the  sun  appears  in  different 
parts  of  the  heavens  at  different  times  of  the  year  ;  con- 
sequently, as  the  inferior  planets  are  always  very  near 
the  sun,  they  will  appear  in  different  parts  of  the  hea- 
vens at  different  times  of  the  year ;  and  their  conjunc- 
tions, &c.  will  happen  sometimes  in  one  part  of  the  hea- 
vens, sometimes  in  another. 

Venus,  seen  from  the  earth,  appears  to  vibrate  in  an 
arc,  half  of  which  is  on  one  side  of  the  sun's  apparent 
place,  half  on  the  other. 

Venus  has  been  seen  sometimes  moving  across  the 
sun's  disk  in  the  form  of  a  round  black  spot,  with  an 
apparent  diameter  of  about  59  seconds. 

It  has  been  found  by  observation,  that  the  orbit  of 
Venus  is  an  ellipse,  having  the  sun  in  one  focus. 

The  upper  apsis  of  the  orbit  is  called  the  aphelion,  the 
lower  apsis  is  called  the  perihelion,  of  Venus.  The  line 
of  the  apsides  has  a  slow  motion  eastward,  at  the  rate  of 
2  degrees,  44  minutes,  46  seconds  in  a  century.  The 
nodes  of  Venus  move  westward  about  31  seconds  in  a 
year. 

Venus  moves  in  her  orbit,  so  as  to  describe  round  the 
sun  areas  proportional  to  the  times. 


OF  THE    PHASES    OF    VENUS. 

That  the  planets  are  opake  bodies,  and  shine  only  by 
the  light  ihey  receive  from  the  sun,  is  plain,  because 

VOL.  iV.  L 


74  THE    PHASES    OF    VENUS. 

they  are  not  visible  in  such  parts  of  their  orbits  as  are 
between  the  sun  and  the  earth,  that  is,  when  their  illu- 
minated side  is  turned  from  us. 

The  line  in  the  pianet's  body,  which  distinguishes  the 
lucid  from  the  obscure  part,  appears  sometimes  straight, 
sometimes  crooked.  The  convex  part  of  the  curve  is 
sometimes  towards  the  splendid  part,  and  the  concave 
side  towards  that  which  is  obscure ;  and  vice  versa,  ac- 
cording to  the  situation  of  the  planet  with  respect  to  the 
eye  and  the  sun. 

The  inferior  planets  going  round  the  sun  in  less  or- 
bits than  our  earth  does,  will  sometimes  have  more, 
sometimes  less  of  their  illuminated  side  towards  us ;  and 
as  it  is  the  illuminated  part  only  which  is  visible  to  us, 
Mercury  and  Venus  will,  through  a  good  telescope,  ex- 
hibit the  several  appearances  of  the  moon,  from  a  fine 
thin  crescent  to  the  enlightened  hemisphere. 

If  we  view  Venus  through  a  telescope,  when  she  fol- 
lows the  sun's  rays  on  the  eastern  side,  and  appears 
above  the  horizon  after  sun-set,  we  shall  see  her  appear 
nearly  round,  and  but  small ;  she  is  at  that  time  beyond 
the  sun,  and  presents  to  us  an  enlightened  hemisphere. 
As  she  departs  from  the  sun,  towards  the  east,  she  aug- 
ments in  her  apparent  size  ;  and,  on  viewing  her  through 
a  telescope,  is  seen  to  alter  her  figure,  abating  of  her  ap- 
parent roundness,  and  appearing  successively  like  the 
moon  in  the  different  stages  of  her  decrease.  At  length, 
when  she  is  at  her  greatest  elongation,  she  is  like  the 
moon  in  her  first  quarter,  and  appears  as  she  does  when 
from  a  full  she  has  decreased  to  a  half  moon. 

After  this,  as  she  approaches,  in  appearance,  to  the 
sun,  she  appears  concave  in  her  illuminated  part,  as  the 
moon  when  she  forms  a  crescent  ;  thus  she  continues, 
till  she  is  hid  entirely  in  the  sun's  rays,  and  presents  to 
us  her  whole  dark  hemisphere,  as  the  moon  does  in  her 
conjunction,  no  part  of  the  planet  being  then  visible. 

When  she  departs  out  of  the  sun's  rays  on  the  west- 
ern side,  we  see  her  in  the  morning,  just  before  day- 
break. It  is  in  this  situation  that  Venus  is  called  the 
morning  star,  as  in  the  other  she  is  called  the  evening 
star.     She  at  this  time  appears  very  beautiful,  like  a  fine 


THE    PHASES    OF    VENUS.  75 

thin  crescent :  just  a  verge  of  silver  light  is  seen  on  her 
ed^e.  From  this  period  she  grows  more  and  more  en- 
lightened every  day,  till  she  is  arrived  at  her  greatest  di- 
gression or  elongation,  when  she  again  appears  as  a  half 
moon,  or  as  the  moon  in  her  first  quarter  :  from  this 
time,  if  continued  to  be  viewed  with  a  telescope,  she  is 
found  to  be  more  and  more  enlightened,  though  she  is 
all  the  while  decreasing  in  magnitude,  and  thus  conti- 
nues growing  smaller  and  rounder,  till  she  is  again  hid 
or  lost  in  the  sun's  rays. 

Plate  8,  fig.  1,  represents  the  orbits  of  Venus  and  the 
earth,  with  the  sun  in  the  centre  of  them.  The  planet 
Venus  is  drawn  in  eight  different  situations,  with  its  il- 
luminated hemisphere  towards  the  sun.  If  we  suppose 
the  earth  to  be  at  T,  when  Venus  is  at  A,  her  dark  he- 
misphere is  towards  the  earth,  and  she  is  therefore  invi- 
sible, except  the  conjunction  happens  in  her  node,  for 
then  she  appears  like  a  dark  spot  upon  the  disk  of  the 
sun.  When  Venus  is  at  B,  a  little  of  her  enlightened 
side  is  turned  towards  the  earth,  and  therefore  she  ap- 
pears sharp-horned  ;  when  she  is  at  C.  half  her  enlight- 
ened hemisphere  is  turned  towards  the  earth,  and  she 
appears  like  a  half  moon  ;  at  D,  more  than  half  her  en- 
lightened hemisphere  is  towards  us,  and  she  appears  like 
the  moon  about  three  days  before  it  is  full  ;  at  E,  the 
whole  enlightened  hemisphere  is  towards  the  earth  ;  Ve- 
nus is  then  either  behind  the  sun,  or  so  very  near  him, 
that  she  can  hardly  be  seen  ;  but  if  she  could,  she  would 
appear  round  like  the  full  moon.  At  F,  she  is  like  the 
moon  three  days  after  the  full ;  at  G,  like  a  half  moon 
again ;  at  H,  like  a  crescent,  with  the  points  of  the 
horns  turned  the  contrary  way  to  what  they  were  at  B. 
All  this  is  equally  applicable  to  Mercury. 

Plate  8,  fig.  2,  exhibits  the  different  appearances  of 
Venus,  corresponding  to  her  several  situations  in  the 
foregoing  figure  ;  thus,  when  Venus  is  at  A,  fig.  1,  she 
is  quite  dark,  as  at  A,  fig.  2  ;  when  she  is  at  B,  fig.  1, 
she  appears  as  at  B,/g.  2,  &c. 

The  inferior  planets  do  not  shine  brightest  when  they 
are  full ;  thus,  Venus  does  not  appear  brightest  in  her 
superior  conjunction,  though  her  illuminated  hemisphere 


76  OF    MERCURY. 

be  then  turned  towards  us.  Her  splendour  is  more  di- 
minished by  her  being  at  a  greater  distance  from  us, 
than  the  conspicuous  part  of  her  illuminated  disk  is  in- 
creased. Dr.  Halley  has  shown  that  Venus  is  brightest 
when  her  elongation  from  the  sun  is  about  40  degrees. 
Mercury  is  in  his  greatest  brightness,  when  very  near  his 
utmost  elongation. 


OF    MERCURY. 

The  planet  Mercury  resembles  Venus  in  all  the  cir- 
cumstances of  her  apparent  motion,  and  we  make  similar 
inferences  with  respect  to  the  real  motions.  His  orbit  is 
an  ellipse,  having  the  sun  in  one  focus.  The  apsides 
move  eastward  1  degree,  51  minutes,  20  seconds  in  a 
century ;  the  nodes  move  westward  45  seconds  in  a 
year  ;  and  areas  are  described  proportional  to  the  times. 


OF    THE    SUPERIOR    PLANETS. 

The  superior  planets  exhibit  phenomena  consider- 
ably different  from  those  exhibited  by  Mercury  and 
Venus. 

They  come  to  our  meridian  both  at  noon  and  mid- 
night ;  when  they  come  to  our  meridian  at  noon,  and 
are  in  the  ecliptic,  they  are  never  seen  crossing  the  sun's 
disk. 

They  are  always  retrograde  when  in  opposition,  and 
direct  when  in  conjunction. 

I  have  already  observed  to  you,  that  the  greatest  elon- 
gation of  either  of  the  inferior  planets  is  less  than  90 
degrees,  or  a  quarter  of  a  circle ;  so  that  they  are  never 
far  from  the  sun,  but  canstantly  attend  it.  But  the  supe- 
rior planets  do  not  always  accompany  the  sun,  as  the  in- 
ferior ones  do  :  they  are  indeed  sometimes  in  conjunction 
with  it,  but  then  they  are  also  sometimes  in  opposition 
to,  or  180°  from  it. 

To  be  more  particular,  let  S,  plate  6,  Jig.  3,  be  the 
sun ;  A  B  C  D,  the  orbit  of  any  superior  planet,  Mars, 


DIRECT,  &C.  MOTIONS  OF  THE  SUPERIOR  PLANETS.  77 

for  instance  ;  E  F  G,  the  earth's  orbit.  If  the  earth  be 
at  E,  the  sun  at  S,  and  the  planet  at  D,  the  sun  and  the 
planet  will  be  both  on  the  same  side  of  the  earth;  and, 
consequently,  the  planet  will  appear  in  conjunction  with 
the  sun.  But,  as  the  orbit  of  the  earth  is  between  the 
sun  and  the  orbit  of  the  superior  planet,  it  is  possible 
for  the  earth  to  be  between  the  sun  and  the  planet,  and 
consequently  for  the  planet  and  the  sun  to  be  on  oppo- 
site sides  of  the  earth,  or  the  .planet  to  be  in  opposi- 
tion ;  thus  if,  when  the  earth  is  at  E,  Mars  be  at  A,  he 
is  then  in  opposition  to  the  sun. 

A  superior  planet  is  in  quadrature  with  the  sun,  when 
its  geocentric  place  is  90°  from  the  geocentric  place  of 
the  sun  ;  thus,  if  the  earth  be  at  E,  and  Mars  at  B  or 
C,  he  is  in  quadrature  with  the  sun  ;  for  the  lines  AE, 
E  B,  form  a  right  angle,  as  do  also  the  lines  E  A,  EC. 


OF  THE  DIRECT  AND  RETROGRADE  MOTIONS,  AND 
STATIONARY  SITUATIONS  OF  THE  SUPERIOR  PLA- 
NETS. 

As  the  earth  goes  round  the  sun  in  less  time,  and  in 
a  less  orbit  than  any  of  the  superior  planets,  it  will  not 
be  amiss  to  suppose  a  superior  planet  to  stand  still  in 
some  part  of  its  orbit,  while  the  earth  goes  once  round 
the  sun  in  hers,  and  consider  the  appearances  the  pla- 
nets would  then  have,  which  are  these  :  1.  While  the 
earth  is  in  her  most  distant  semicircle,  the  apparent 
motion  of  the  planet  would  be  direct.  2.  While  the 
earth  is  in  her  nearest  semicircle,  the  planet  would  be 
retrograde.  3.  While  the  earth  is  near  the  points  of 
contact  of  a  line  drawn  from  the  planet,  so  as  to  be  a 
tangent  to  the  earth's  orbit,  the  planet  would  be  sta- 
tionary. 

To  illustrate  this,  let  A  B  C  D  E  F  G  H,  plate  7,  Jig. 
1,  be  the  orbit  of  the  earth  ;  S,  the  sun  ;  P  Q  O  V,  the 
orbit  of  Mars ;  L  M  N  R  T,  an  arc  of  the  ecliptic.  Let 
us  suppose  the  planet  Mars  to  continue  at  P,  while  the 
earth  goes  round  in  her  orbit,  according  to  the  order 
of  the  letters,  A  B  C,  &c.     A  B  C  D  E  F  G  H  may  be 


78  DIRECT,  &C.    MOTIONS 

considered  as  so  many  stations,  from  whence  an  inha- 
bitant of  the  earth  would  view  Mars  at  different  times  of 
the  year ;  and  if  straight  lines  be  drawn  from  each  of 
these  stations,  through  Mars  atP,  and  continued  to  the 
ecliptic,  they  will  point  out  the  apparent  place  of  Mars 
at  these  different  stations. 

Thus,  supposing  the  earth  at  A,  the  planet  will  be 
seen  among  the  stars  at  L  ;  when  the  earth  has  arrived 
at  B,  the  planet  will  appear  at  M;  and  in  the  same 
manner  when  at  C,  D,  and  E,  it  will  be  seen  among  the 
stars  at  N,  R,  T;  therefore,  while  the  earth  moves  over 
the  large  part  of  the  orbit,  A  B  C  D  E,  the  planet  will 
have  an  apparent  motion  from  L  to  T,  and- this  motion 
is  from  west  to  east,  or  the  same  way  with  the  earth ; 
and  the  planet  is  said  to  move  direct,  or  according  to 
the  order  of  the  signs.  When  the  earth  is  near  to  A 
and  E,  the  point  of  contact  of  the  tangent  to  the  earth's 
orbit,  the  planet  will  appear  stationary  for  a  short  space 
of  time. 

When  the  earth  moves  from  E  to  H,  the  planet 
seems  to  return  from  T  N;  and  while  it  moves  from 
H  to  A,  it  will  be  retrograde  to  L,  where  it  will  again 
be  stationary  :  and,  since  the  part  of  the  orbit  which 
the  earth  describes  in  passing  from  A  to  E,  is  much 
greater  than  the  part  E  H  P,  though  the  space,  T  L, 
which  the  planet  describes  in  direct  and  retrograde  mo- 
tion is  the  same,  the  direct  motion  from  L  to  T  must 
be  much  slower  than  the  retrograde  motion  from  T  to  L. 

When  the  earth  is  at  C,  a  line  drawn  from  C  through 
S  and  P  to  the  ecliptic,  shows  that  Mars  is  then  in  con- 
junction with  the  sun.  But  when  the  earth  is  at  H,  a 
line  drawn  from  H  through  P,  and  continued  to  the 
ecliptic,  would  terminate  in  a  point  opposite  to  S  ; 
therefore,  in  this  situation  Mars  would  be  in  opposition  to 
the  sun.  Thus  it  appears,  that  the  motion  of  Mars  is 
direct  when  in  conjunction,  and  retrograde  when  in  op- 
position. 

The  retrograde  motions  of  the  superior  planets  hap- 
pen oftener  the  slower  their  motions  are  ;  as  the  retro- 
grade motions  of  the  inferior  planets  happen  oftener, 
the  swifter  their  angular  motions  j  because  the  retro- 


OF    THE    SUPERIOR    PLANETS.  79 

grade  motions  of  the  superior  planets  depend  upon  the 
motions  of  the  earth  ;  but  those  of  the  inferior,  on  their 
own  angular  motion.  A  superior  one  is  retrograde 
once  in  each  revolution  of  the  earth ;  an  inferior  one 
in  every  revolution  of  its  own. 


OTHER    PHENOMENA    OF    THE    SUPERIOR    PLANETS. 

The  superior  planets  are  sometimes  nearer  the  earth 
than  at  other  times;  they  also  appear  larger  or  smaller, 
according  to  their  different  distances  from  us.  Thus, 
suppose  the  earth  to  be  at  C ;  if  Mars  be  at  V,  he  is 
the  whole  diameter  of  the  earth's  orbit  nearer  to  us, 
than  if  he  were  at  P,  and  consequently  his  di^k  must 
appear  larger  at  V,  than  it  would  at  P.  In  other 
places,  the  distances  of  Mars  from  the  earth  are  in- 
termediate. 

The  superior  planets  going  round  the  sun  in  larger 
orbits  than  the  earth,  turn  much  the  greater  part  of 
their  enlightened  hemisphere  towards  it,  and  therefore 
appear  round  like  the  full  moon ;  except  Mars,  who 
sometimes  appears  like  the  moon  at  a  little  distance 
from  the  full. 

They  also  move  in  an  ellipse,  having  the  sun  in 
the  centre ;  the  areas  described  are  proportional  to  the 
times. 

They  are  sometimes  nearer  to,  sometimes  farther 
from  the  earth,  and  their  apparent  diameter  is  found  to 
vary  according  to  their  distance. 


OF    THE    SECONDARY    PLANETS,    OR    SATELLITES. 

As  the  moon  turns  round  the  earth,  enlightening  our 
nights,  by  reflecting  the  light  she  receives  from  the  sun, 
so  do  other  satellites  enlighten  the  planets  to  which  they 
belong  ;  and  as  it  keeps  company  with  the  earth  in  its 
annual  revolution  round  the  sun,  so  do  they  severally 
accompany  the  planets  to  which  they  belong  in  their  se- 
veral courses  round  that  luminary.     Jupiter  has  four 


80  THE    SECONDARY    PLANETS, 

satellites,  Saturn  seven,  the  Georgium  Sidus,  from  the 
discoveries  of  Dr.  Herscbel,  six. 

The  existence  of  all  the  satellites,  except  the  moon, 
would  have  been  unknown  to  us  without  the  use  of  the 
telescope. 

The  satellites  are  distinguished,  according  to  their 
places,  into  first,  second,  &c.  the  first  being  that  which 
is  nearest  the  planet. 

The  satellites  revolve  round  their  primaries  in  elliptic 
orbits,  the  primary  planets  being  in  one  of  the  foci. 

The  orbits  of  all  Jupiter's  satellites  are  nearly,  but  not 
exactly  in  the  same  plane,  which  produced,  makes  an 
angle  with  the  plane  of  Jupiter's  orbit  of  about  three 
degrees.     The  second  deviates  a  little  from  the  rest. 

The  planes  of  the  orbits  of  the  secondary  planets  pro- 
duced, intersect  the  heliocentric  orbits  of  their  primaries 
in  two  opposite  points,  which  are  called  their  nodes. 
The  planes  of  the  orbits  of  the  satellites  of  Jupiter  and 
Saturn  produced,  intersect  the  ecliptic  in  two  opposite 
points :  these  points  of  intersection,  to  distinguish  them 
from  the  other,  may  be  called  the  geocentric  nodes  of 
the  satellites.  The  orbits  of  Jupiter  and  Saturn  are  so 
small  in  comparison  of  the  sphere  of  the  fixed  stars,  that 
the  places  of  their  satellites'  nodes  are  not  sensibly  altered 
by  their  primaries  being  in  different  parts  of  their  orbits. 

The  orbits  of  all  Saturn's  satellites,  except  the  5th, 
which  deviates  from  the  rest  several  degrees,  are  nearly 
in  the  same  plane.  They  are  nearly  parallel  to  the  plane 
of  the  equator.  The  orbit  of  Saturn's  5th  satellite  makes 
an  angle  with  the  orbit  of  his  primary  of  1 3°  8'. 

A  satellite  in  one  of  its  nodes,  seen  from  its  primary, 
appears  in  the  orbit  of  its  primary  :  in  all  other  parts  of 
its  orbit,  it  has  latitude  seen  from  its  primary. 

Every  circle,  whose  plane  produced  passes  through 
the  eye,  appears  a  straight  line  :  every  circle  viewed  ob- 
liquely will  appear  an  ellipsis,  more,  or  less  wide,  according 
as  the  eye  is  more  or  less  elevated  above  the  plane  of  the 
circle.  The  orbit  of  a  satellite  seen  from  the  earth, 
when  its  primary  heliocentric  place  is  in  his  satellites  true 
node,  and  the  earth  in  its  geocentric  node,  appears  a 
straight  line :  when  the  primary  is  in  any  other  part  of 
his  orbit,  the  satellite's  orbit  will  appear  an  ellipsis,  whose 


OR    SATELLITES.  81 

shortest  axis  increases,  the  farther  the  primary  is  from 
the  node  of  the  satellite. 

The  earth's  orbit  is  so  small  in  comparison  of  the 
orbits  of  Jupiter  and  Saturn,  that  whatever  part  of 
her  orbit  the  earth  is  in,  when  these  planets  are  in  their 
satellites'  true  nodes,  their  satellites  will  appear  to  de- 
scribe lines  very  near  to  straight  ones. 

When  a  satellite  is  in  its  superior  semicircle,  viz.  that 
which  is  farthest  from  the  earth,  its  geocentric  motion 
is  direct ;  when  in  its  inferior  semicircle,  or  that  near- 
est our  earth,  its  geocentric  motion  is  retrograde.  Both 
these  motions  seem  quickest,  when  the  satellites  is  near- 
est the  centre  of  the  primary,  and  slower  when  they 
are  more  distant  ;  at  the  greatest  distance,  they  appear 
stationary  for  a  short  time. 

Since  the  distance  of  Jupiter  and  Saturn  from  our 
earth  is  very  great,  and  our  eye  is  never  elevated  much 
above  the  planes  of  their  orbits,  every  satellite  of  Ju- 
piter or  Saturn  seen  from  our  earth,  will  appear  always 
near  its  primary,  and  to  have  an  oscillatory  motion, 
sometimes  in  a  straight  line,  sometimes  in  an  elliptic 
curve,  going  from  its  primary,  and  returning  to  it  again, 
first  on  one  side,  and  then  on  the  other.  The  satellites 
of  Jupiter  or  Saturn  will  sometimes  be  hid  from  us  by 
their  primary,  sometimes  pass  between  us  and  their  pri- 
mary, and  sometimes  a  satellite  will  pass  between  us  and 
another  satellite. 

The  satellites  and  their  primaries  mutually  eclipse 
each  other  ;  but  there. are  three  cases  in  which  the  satel- 
lites disappear  to  us. 

The  one  is  when  the  satellite  is  directly  behind  the 
body  of  its  primary,  with  respect  to  the  earth  ;  this  is 
called  an  occupation  of  the  satellite. 

Another  is  when  it  is  directly  behind  its  primary, 
with  respect  to  the  sun,  and  so  falls  into  its  shadow,  and 
suffers  an  eclipse,  as  the  moon  when  the  earth  is  inter- 
posed between  it  and  the  sun. 

The  last  is,  when  it  is  interposed  between  the  earth 
and  its  primary  ;  for  then  it  cannot  be  distinguished 
from  the  primary  itself. 
VOL.  IV.  M 


82  THE    SECONDARY    PLANETS, 

It  is  not  often  that  a  satellite  can  be  discovered  upon 
the  disk  of  Jupiter,  even  by  the  best  telescopes,  except- 
ing its  first  entrance,  when  by  reason  of  its  being  more 
directly  illuminated  by  the  rays  of  the  sun  than  the 
planet  itself,  it  appears  like  a  lucid  spot  upon  it ;  some- 
times, however,  a  satellite  is  seen  passing  over  the  disk 
like  a  dark  spot ;  this  has  been  attributed  to  spots  on 
the  surface  of  the  satellite,  and  that  the  more  probably, 
as  the  satellite  has  been  known  to  pass  over  the  disk  at 
one  time  as  a  dark  spot,  and  at  another  time  to  be  so  lu- 
minous as  only  to  be  distinguished  from  the  planet  at  its 
ingress  and  egress.  The  beginnings  and  endings  of 
these  eclipses  are  easily  seen  by  a  telescope,  when  the 
planet  is  in  a  proper  situation ;  but  when  it  is  m  con- 
junction with  the  sun,  the  brightness  of  that  luminary 
renders  both  the  planet  and  the  satellite  invisible. 

By  observing  the  eclipses  of  Jupiter's  satellites,  it  was 
discovered  that  light  was  not  propagated  instantaneous- 
ly, though  it  moves  with  an  incredible  velocity  ;  so  that 
light  passes  from  the  sun  to  us  in  the  space  of  about  8 
minutes  of  time,  at  the  rate  of  more  than  100,000  miles 
in  a  second. 

The  orbits  of  all  the  satellites  of  Saturn,  except  the 
fifth,  are  nearly  in  the  same  plane,  which  plane  makes 
an  angle  with  that  of  Saturn's  orbit  of  about  31°  ;  this 
inclination  is  so  great,  that  they  cannot  pass  either  across 
Saturn  or  behind  it,  with  respect  to  the  earth,  except 
when  they  are  very  near  their  nodes  ;  so  that  their  eclip- 
ses are  not  near  so  frequent  as  those  of  Jupiter.  An 
occultation  of  the  fourth  behind  the  body  of  Saturn  has 
been  observed,  and  Cassini  once  saw  a  star  covered  by 
the  fourth  satellite,  so  that  for  13  minutes  they  appear- 
ed as  one. 

They  are  so  minute  as  not  to  be  visible,  unless  the 
air  be  exceedingly  clear  ;  Cassini  observed  the  fifth 
satellite  to  diminish  in  size,  as  it  went  through  the 
eastern  part  of  the  orbit,  until  it  became  quite  invisible  ; 
while  in  the  western  part,  it  increased  in  brightness, 
until  it  arrived  at  its  greatest  splendour.  In  1 705  it  was 
visible  in  all  parts  of  its  orbit,  though  the  same  tele- 
scopes were  often  used  before  to  discover  it  without 
success. 


OR    SATELLITES. 


83 


The  Georgium  Sidus  is  attended  by  two  satellites.* 
The  following  table  shows  the  general  affections  of 
them. 

SECONDARY  PLANETS. 


JUPITER'S     *)  J»  or  Innerm' 

FOUR  > „ 

SATELLITES. j  £ 


(~    C    discovered 
|  thn   1789,     by 

SATURN'S  Z(Df'^cU 

SEVEN  *{   ^ 

SATELLITES-  I  5 

j  6. ::::::::::::::.:::::::::: 
\j 


Distance 

Synodical  Re- 

from its 

volutions 

Primary. 

round  its 

Primary. 

g 

X 

|S 

CD 

English 

'< 

2 

p 

miles. 

j- 



269,10.5 
428,312 
683,071 

1 
j 

7 

1 8 

13 
3 

28 
17 
5<5 

36 

54 
36 

1,201,386 

It 

18 

5 

7 

126,000 

22 

40 

46 

162,000 

1 

8 

53 

c 

195,671 

1 

21 

18 

2? 

250,631 

2 

17 

44 

22 

350,099 

4 

12 

25 

12 

811,610 

1£ 

22 

M 

38 

2,365,222 

rs 

7 

47 

— 

Proportion 
of  bulk 
with    re- 
spect to  the 
Earth. 


i 

About ± 


Ring  of  Saturn  21,000  miles  broad,  and  21,000  miles  distant  from  his 
Rodv  on  all  sides.     Thickness  of  the  Ring-  unknown. 


GEORGIAN'S 
SATELLITES 


discovered" 
11  Jan.  1787, 
bv  Dr.  Hersc. 


About 

[289,1181 
387,5051 


I  8ilr 

13  11 


n 


*  The  following  discoveries  respecting  this  pi  un-t  and  its  satellites  by 
Dr.  Herschel,  it  may  be  proper  tq  acquaint  the  reader  with,  in  addition  to 
what  I  have  given  in  the  note  to  page  30,  and  the  author's  annexed  table. 
The  reader  must  observe,  that  they  are  the  two  old  satellites  u  that  move 
in  a  retrograde  direction.  Whether  the  motion  of  the  four  new  ones  be 
direct  or  retrograde,  appears  not  yet  determined." 

The  two  old  satellites  were  formerly  found  to  revolve,  the  first  in  8  days, 
17  hours,  1  min.  17  sec.  at  the  distance  of  33"  from  its  primary  ;  the  second 
in  13  d.  11  h.  5  m.  1.5  sec.  at  the  distance  of  44.23".  "  Tiie  new  satellites 
revolve  as  follows:  the  periodical  times  being  inferred  from  their  greatest 
elongations  ;  the  interior  satellite  in  5  d.  21  h.  25  m.  at  the  distance  of  25.5" ; 
a  satellite  intermediate  between  the  two  old  ones  in  10  d.  23  h.  4  m.  at  the 
distance  of  38.57"  ;  the  nearest  exterior  satellite  at  about  double  the  distance 
of  the  farthest  old  one,  and  consequently  its  periodical  lime  38  id.  1  h.  49  m. 
and  the  most  distant  satellite  full  tour  times  as  far  from  its  primary  as  the 
old  second  satellite,  whence  it  will  take  at  least 107  d.  16  h.  40  m.  to  com- 
plete its  revolution.  The  disk  of  the  Georgium  Sidus  he  finds  to  be  flat- 
tened, and  therefore  it. must  revolve  with  considerable  rapidity  on  its  axis. 
From  the  very  faint  light  of  the  satellites,  they  are  observed  to  disappear 
in  those  parts  of  their  orbits,  which  bring  them  apparently  nearer  to  the 


C      84     ] 


OF    THE    MOON'S    MOTION, 


You  have  seen,  that  four  of  the  primary  planets  are 
attended  in  their  revolutions  by  secondary  planets  ;  we, 
as  one  of  these,  are  attended  by  the  moon,  she  enligh- 
tens our  nights,  by  reflecting  the  light  she  receives  from 
the  sun,  as  the  other  satellites  enlightens  the  planets  to 
which  they  severally  belong. 

If  you  imagine  the  plane  of  the  moon's  orbit  to 
be  extended  to  the  sphere  of  the  heavens,  it  would 
mark  therein  a  great  circle,  which  may  be  called  the 
moon's  apparent  orbit ;  because  the  moon  appears  to 
the  inhabitants  of  the  earth  to  move  in  that  circle, 
through  the  twelve  signs  of  the  zodiac,  in  a  periodical 
month.  This  position  is  illustrated  by  plate  9,  Jig,  3  ; 
let  E  F  G  H  I  be  the  orbit  of  the  earth,  S  the  sun,  abed 
the  orbit  of  the  moon,  when  the  earth  is  at  E  :  let  A  B 
C  D  be  a  great  circle  in  the  sphere  of  the  heavens,  in 
the  same  plane  with  the  moon's  orbit.  The  moon,  by 
going  round  her  orbit  according  to  the  order  of  the  let- 
ters, appears  to  an  inhabitant  of  the  earth  to  go  round  in 
the  great  circle  A  B  C  D,  according  to  the  order  of 
those  letters  :  for  when  the  moon  is  at  a,  seen  from  the 
earth  at  E,  she  appears  at  A  ;  when  the  moon  is  got  to 
b,  she  appears  at  B  ;  when  to  c,  she  will  appear  at  C  ; 
when  arrived  at  d,  she  will  apear  at  D.  It  is  true,  when 
the  moon  is  at  b,  the  visual  line  drawn  from  E,  through 
the  moon  terminates  in  L  ;  as  it  does  in  M,  when  the 
moon  is  at  d  ;  but  the  lines,  L  M  and  D  B,  being  pa- 
rallel, and  not  farther  distant  from  each  other  than  the 
distance  of  the  earth's  orbit,  are  as  to  sense  coincident, 
their  distance  measured  in  the  sphere  of  the  heavens 
being  insensible  ;  for  the  same  reason,  though  the  earth 
moves  from  E  to  F,  in  the  time  that  the  moon  goes 
round  her  orbit,  so  that  at  the  end  of  a  periodical 
month  the  moon  will  be  at  a,  and  is  seen  from  the 


planet.  Tins  dors  not  ai  ise  from  an  atmosphere,  for  the  effect  is  the  same 
whether  the  satellite  be  within  or  beyond  the  planet." 

His  observations  were  made  with  reflecting  telescopes  of  7,10,  and  20 
feei  in  length,  and  the  powers  from  lib  to  2400.— K.  Kdit* 

f  For  particulars  of  four  others,  recently  discovered,  bee  the  note  in  the 
preceding-  page. — E.  Edit. 


THE    MOON'S    MOTION.  8,5 

earth  at  F,  in  the  line  F  N ;  the  moon  will  notwith- 
standing, appear  at  A,  the  lines,  FN  and  E  A,  being 
parallel,  and  as  to  sense  coincident :  in  like  manner, 
in  whatever  part  of  her  orbit  the  earth  is,  as  at  H  or  I, 
the  moon,  by  going  round  in  her  orbit,  will  appear  to 
an  inhabitant  of  the  earth  to  go  round  in  the  great  circle 
ABCD. 

The  plane  of  the  moon's  orbit  produced  till  it  cuts 
the  -plane  of  the  ecliptic,  makes  an  angle  with  it  of 
about  5°  :  this  angle  is  sometimes  more,  sometimes  less 
than  5°.  The  points  where  the  moon's  orbit  produced 
cuts  the  ecliptic,  are  called  the  moon's  nodes  ;  her  as- 
cending Q  the  dragon's  head,  and  her  descending  node 
£5  the  dragon's  tail.  The  moon's  nodes  have  a  slow 
motion  of  19°  22'  in  a  year,  which  carries  them  round 
the  ecliptic,  contrary  to  the  order  of  the  signs,  in  19 
years. 

The  line  of  the  moon's  nodes  is  a  line  drawn  from 
one  node  to  the  other. 

The  extremities  of  the  line  of  the  nodes  are  not  al- 
ways directed  to  the  same  point  of  the  ecliptic,  but  con- 
tinually shift  their  places  from  east  to  west,  or  contra- 
ry to  the  order  of  the  signs,  performing  an  entire  revo- 
lution about  the  earth,  in  the  space  of  something  less 
than  nineteen  years. 

The  moon  appears  in  the  ecliptic  only  when  she  is  in 
one  of  her  nodes  :  in  all  other  parts  of  her  orbit  she  is 
either  in  north  or  south  latitude,  sometimes  nearer  to, 
sometimes  farther  removed  from  the  ecliptic,  accord- 
ing as  she  happens  to  be  more  or  less  distant  from  the 
nodes. 

When  the  place,  in  which  the  moon  appears  to  an  in- 
habitant of  the  earth,  is  the  same  with  the  sun's  place, 
she  is  said  to  be  in  conjunction.  When  the  moon's  place 
is  opposite  to  the  sun's  place,  she  is  said  to  be  in  opposi- 
tion. When  she  is  a  quarter  of  a  circle  distant  from  the 
sun,  she  is  said  to  be  in  quadrature.  Both  the  conjunc- 
tion and  opposition  of  the  moon  are  termed  syzygies. 

The  common  lunar  month,  or  the  time  that  passes 
between  any  new  moon  and  the  next  that  follows  it  is 
called  a  synodical  month,  or  a  lunation.  This  month  con- 
tains 29  days,  12  hours,  44  minutes,  3  seconds. 


86  THE    MOON'S    MOTION. 

The  moon's  motion  in  her  orbit  is  considered  either 
absolutely,  or  with  relation  to  the  sun.  The  moon's 
motion  in  her  orbit,  which  is  also  her  motion  in  longi- 
tude, is  sometimes  swifter,  sometimes  slower  ;  her  mean 
motion  is  13  degrees,  10  minutes,  35  seconds,  in  a  day, 
which  carries  her  round  the  zodiac  in  27  days,  7  hours, 
43  minutes.  The  time  wherein  the  moon  is  carried 
round  the  zodiac,  called  a  periodical  month,  is  the  time 
in  which  the  moon  performs  one  entire  revolution  about 
the  earth,  from  any  point  in  the  zodiac  to  the  same 
again ;  and  contains  27  days,  7  hours,  43  minutes. 

The  moon's  motion  considered  with  relation  to  the 
sun  is  called  her  elongation  from  the  sun.  '  The  moon's 
motion  from  the  sun  is  the  excess  of  the  velocity  of  the 
moon's  motion,  above  the  velocity  of  the  sun's  appa- 
rent motion  in  the  ecliptic  ;  this  excess  is  sometimes 
more,  sometimes  less.  The  moon's  mean  motion  from 
the  sun  is  1 2  degrees,  1 1  minutes,  26  seconds  in  a  day, 
which  carries  the  moon  from  one  conjunction  with  the 
sun  to  another  in  29  days,  12  hours,  44  minutes,  3  se- 
conds. The  time  between  any  conjunction  and  the  con- 
junction immediately  following,  as  before  observed,  is 
called  a  synodical  month,  or  a  lunation,  wherein  the 
moon  appears  in  all  her  phases. 

If  the  earth  had  no  revolution  round  the  sun,  or  the 
sun  no  apparent  motion  in  the  ecliptic,  the  periodical 
and  synodical  months  would  be  the  same  ;  but  as  this 
is  not  the  case,  the  moon  takes  up  a  longer  time  to  pass 
from  one  conjunction  to  the  next,  than  to  describe  its 
whole  orbit ;  or  the  time  between  one  new  moon  and 
the  next,  is  longer  than  the  moon's  periodical  time. 

The  moon  going  round  our  earth  in  an  orbit,  whose 
ssmidiameter  is  less  than  the  nearest  distance  of  any 
planet,  may  come  between  our  eye  and  any  planet  or 
star  that  is  near  the  ecliptic.  The  time  when  the  moon 
appears  to  touch  a  planet  or  star,  is  called  its  appulse, 
which  being  instantaneous,  serves  to  determine  the  lon- 
gitude of  aifferent  places  where  it  is  observed. 

The  moon  revolves  round  the  earth  from  west  to 
east,  and  the  sun  apparently  revolves  round  the  earth 
the  same  way.  Now  at  the  new  moon,  or  when  the 
sun  and  moon  are  in  conjunction,  they  both  set  out 


the  moon's  motion.  87 

from  the  same  place,  to  move  the  same  way  round  the 
earth  ;  but  the  moon  moves  much  faster  than  the  sun, 
and  consequently  will  overtake  it ;  and  when  the  moon 
does  overtake  it,  it  will  be  a  new  moon  again.  If 
the  sun  had  no  apparent  motion  in  the  ecliptic,  the  moon 
would  come  up  to  it  or  be  in  conjunction  again,  after 
it  had  gone  once  round  in  its  orbit ;  but  as  the  sun 
moves  forward  in  the  ecliptic,  whilst  the  moon  is  going 
round,  the  moon  must  move  a  little  more  than  once 
round,  before  it  comes  even  with  the  sun,  or  before  it 
comes  to  conjunction.  Hence  it  is,  that  the  time  be- 
tween one  conjunction,  and  the  next  in  succession,  is 
something  more  than  the  time  the  moon  takes  up  to  go 
once  round  its  orbit ;  or  a  synodical  month  is  longer 
than  a  periodical  one. 

In  plate  8,  fig.  3,  let  S  be  the  sun,  C  F  a  part  of  the 
earth's  orbit,  M  D  a  diameter  of  the  moon's  orbit  when 
the  earth  is  at  A,  and  m  d  another  diameter  parallel  to 
the  former,  when  the  earth  is  at  B.  While  the  earth 
is  at  A,  if  the  moon  be  at  D,  she  will  be  in  conjunction  ; 
and  if  the  earth  were  to  continue  at  A,  when  the  moon 
had  gone  once  round  its  orbit,  from  D  through  M,  so  as 
to  return  to  D  again,  it  would  again  be  in  conjunction. 
Therefore,  upon  the  supposition  that  the  earth  has  no 
motion  in  its  orbit,  the  periodical  and  synodical  months 
would  be  equal  to  one  another.  But  as  the  earth  does 
not  continue  at  A,  it  will  move  forward  in  its  orbit,  du- 
ring the  revolution  of  the  moon  from  A  to  B  :  and  as 
the  moon's  orbit  moves  with  it,  the  diameter,  M  D,  will 
then  be  in  the  position  m  d  ;  therefore,  when  the  moon 
has  described  it's  orbit,  it  will  be  at  d,  in  this  diame- 
ter m  d  ;  but  if  the  moon  be  at  d,  and  the  sun  at  S,  the 
moon  will  not  be  in  conjunction,  consequently,  the  pe- 
riodical month  is  completed  before  the  synodical.  The 
moon,  in  order  to  come  to  conjunction,  when  the  earth 
is  at  B,  must  be  at  e,  in  the  diameter  e  f ;  or  besides 
going  once  round  in  its  orbit,  it  must  also  describe  the 
arc  d  e.  The  synodical  month  is,  therefore,  longer  than 
the  periodical,  by  the  time  the  moon  takes  up  to  describe 
the  arc  d  e. 

This  may  be  also  explained  in  another  manner,  by 
considering  the  motion  of  the  sun  ;  a  view  of  the  sub- 


88  the  moon's  motion. 

ject,  that  may  render  it  more  easy  to  some  young  minds 
than  the  foregoing.  Thus,  let  us  suppose  the  earth  at 
rest  at  E,  plate  &>Jig.  4,  M  the  moon  in  conjunction 
with  the  sun  at  S,  while  the  moon  describes  her  orbit, 
ABC,  about  the  earth  at  E,  let  the  sun  advance  by 
his  apparent  annual  motion  from  S  to  D.  It  is  plain, 
that  the  moon  will  not  come  in  conjunction  with  the 
sun  again,  till,  besides  describing  her  orbit,  she  hath 
described,  over  and  above^  that,  the  arc  M  F,  corre- 
sponding to  the  arc  S  D. 

As  the  moon  goes  round  the  earth  in  a  much  smaller 
orbit  than  that  in  which  the  earth  revolves  round  the 
sun,  sometimes  more,  sometimes  less,  and  sometimes  no 
part  of  her  enlightened  half  will  be  towards  us  ;  hence 
she  is  incessantly  varying  her  appearance  ;  sometimes 
she  looks  full  upon  us,  and  her  vissage  is  all  lustre  ; 
sometimes  she  shows  only  half  her  enlightened  face  ; 
soon  she  appears  as  a  radiant  crescent  ;  in  a  little  time 
all  her  brightness  vanishes,  and  she  becomes  a  beamless 
orb. 

The  full  moon,  or  opposition,  is  that  state  in  which 
her  whole  disk  is  enligntened,  and  we  see  it  all  bright, 
and  of  a  circular  figure.  The  new  moon  is  when  she 
is  in  conjunction  with  the  sun  ;  in  this  state,  the  whole 
surface  turned  towards  us  is  dark,  and  she  is  therefore 
invisible  to  us. 

The  first  quarter  of  the  moon  she  appears  in  the  form 
of  a  semicircle,  whose  circumference  is  turned  towards 
the  west.  At  the  last  quarter,  she  appears  again  under 
the  form  of  a  semicircle,  but  with  the  circumference 
turned  towards  the  east. 

The  moon  is  generally  invisible  a  day  or  two  before 
and  after  conjunction,  and  the  obscure  light,  visible  in 
the  moon  a  little  before  and  after  conjunction,  is  reflect- 
ed upon  her  from  the  earth. 

These  phases  may  be  illustrated  in  a  very  pleasing 
manner,  by  exposing  an  ivory  ball  to  the  sun,  in  a  va- 
riety of  positions,  by  which  it  may  present  a  greater  or 
smaller  part  of  its  illuminated  surface  to  the  observer. 
If  it  be  held  nearly  in  opposition,  so  that  the  eye  of  the 
observer  may  be  almost  immediately  between  it  and  the 
sun,  the  greatest  part  of  the  enlightened  side  will  be 


THE    MOON'S    MOTION.  89 

seen  ;  but  if  it  be  moved  in  a  circular  orbit,  towards  the 
sun,  the  visible  enlightened  part  will  gradually  decrease, 
and  at  last  disappear,  when  the  ball  is  held  directly  to- 
wards the  sun.  Or,  to  apply  the  experiment  more  im- 
mediately to  our  purpose  ;  if  the  ball,  at  any  time  when 
the  sun  and  moon  are  both  visible,  be  held  directly  be- 
tween the  eye  of  the  observer  and  the  moon,  that  part 
of  the  ball,  on  which  the  sun  shines,  will  appear  exactly 
of  the  same  figure  as  the  moon  itself. 

The  phases  of  the  moon,  like  those  of  Venus,  may 
also  be  illustrated  by  a  diagram  ;  thus  m  plate  9,  Jig  1, 
let  S  be  the  sun,  T  the  earth,  ABCDEFGH  the 
orbit  of  the  moon.  The  first  observation  to  be  deduc- 
ed from  this  figure,  is,  that  the  half  of  the  earth  and 
moon,  which  is  towards  the  sun,  is  wholly  enlightened 
by  it  ;  and  the  other  half,  which  is  turned  from  it,  is 
totally  dark.  When  the  moon  is  in  conjunction  with 
the  sun  at  A,  her  enlightened  hemisphere  is  turned  to. 
wards  the  sun,  and  the  dark  one  towards  the  earth  ;  in 
which  case,  we  cannot  see  her,  and  it  is  said  to  be  new 
moon.  When  the  moon  has  removed  from  A  to  B,  a 
small  portion  of  her  enlightened  hemisphere  will  be 
turned  towards  the  earth  ;  which  portion  will  appear  of 
the  form  represented  at  B,  plate  9,  Jig.  2,  a  figure 
which  exhibits  the  phases  as  they  appear  to  us. 

As  the  moon  proceeds  in  her  orbit  according  to  the 
order  of  the  letters,  more  and  more  of  her  enlightened 
parts  is  turned  towards  the  earth .  When  she  arrives  at 
C,  in  which  position  she  is  said  to  be  in  quadrature, 
one  half  of  that  part  towards  the  earth  is  enlightened, 
appearing,  as  at  C,  among  the  phases  ;  this  appearance 
is  called  a  half  moon.  When  she  come  to  D,  the  great- 
est part  of  that  half  which  is  towards  us  is  enlightened  ; 
the  moon  is  then  said  to  be  gibbous,  and  of  that  figure 
which  is  seen  at  D,  in  j£g.  2. 

When  the  moon  comes  to  E,  she  is  in  opposition  to 
the  sun,  and  consequently  turns  all  her  illuminated  sur- 
face towards  the  earth,  and  shines  with  a  full  face,  for 
which  reason  she  is  called  a  full  moon.     As  she  passes 

through  the  other  half  of  her  orbit,  from  E  by  F,  G, 
VOL.  IV.  N 


90  THE 

and  H  to  A,  she  again,  puts  on  the  same  phases  as  be- 
fore, but  in  a  contrary  order  or  position. 

As  the  moon,  by  reflected  light  from  the  sun,  illumi- 
nates the  earth,  so  the  earth  does  more  than  repay  her 
kindness,  in  enlightening  the  surface  of  the  moon,  by 
the  sun's  reflex  light,  which  she  diffuses  more  abun- 
dantly upon  the  moon,  than  the  moon  does  upon  us ; 
for  the  surface  of  the  earth  is  considerably  greater  than 
that  of  the  moon,  and,  consequently,  if  both  bodies 
reflect  light  in  proportion  to  their  size,  the  earth  will 
reflect  much  more  light  upon  the  moon  than  it  receives 
from  it. 

In  the  new  moon,  the  illuminated  side  of  the  earth  is 
fully  turned  towards  the  moon,  and  the  Lunarians  will 
have  a  full  earth,  as  we,  in  a  similar  position,  have  a  full 
moon.  And  from  thence  arises  that  dim  light  which 
is  observed  in  the  old  and  new  moons,  whereby,  be- 
sides the  bright  and  shining  horns,  we  can  perceive  the 
rest  of  her  body  behind  them,  though  but  dark  and  ob- 
scure. Now,  when  the  moon  comes  to  be  in  opposi- 
tion to  the  sun,  the  earth,  seen  from  the  moon,  will  ap- 
pear in  conjunction  with  him,  and  its  dark  side  will  be 
turned  towards  the  moon,  in  which  position  the  earth 
will  be  invisible  to  the  Lunarians  ;  after  this,  the  earth 
wiil  appear  to  them  as  a  crescent.  In  a  word,  the  earth 
exhibits  the  same  appearance  to  the  inhabitants  of  the 
moon,  that  the  moon  does  to  us. 

The  moon  turns  about  upon  its  own  axis  in  the  same 
time  that  it  moves  round  the  earth  ;  it  is  on  this  account 
that  she  always  presents  nearly  the  same  face  to  us  :  for 
by  this  motion  round  her  axis,  she  turns  just  so  much 
of  her  surface  constantly  towards  us,  as  by  her  motion 
about  the  earth  would  be  turned  from  us.  This  motion 
about  her  axis  is  equable  and  uniform,  but  that  about 
the  earth  is  unequal  and  irregular,  as  being  performed 
in  an  ellipsis  ;  consequently,  the  same  precise  part  of  the 
moon's  surface  can  not  be  shown  constantly  to  the 
earth  :  this  is  confirmed  by  a  telescope,  by  which  we 
often  observe  a  little  segment  on  the  eastern  and  west- 
ern limb  appear  and  disappear  by  turns,  as  if  her  body 
Iibrated  backwards  and  forwards ;  this  phenomenon  is 


OF    ECLIPSES.  91 

called  the  moon's  libration.  The  lunar  motions  are  subject 
to  several  other  irregularities,  which  are  fully  discussed  in 
the  larger  works  on  astronomy.* 


LECTURE  XL. 


OF    ECLIPSES. 


1  HOSE  phenomena,  that  are  termed  eclipses,  were 
in  former  ages  beheld  with  terror  and  amazement,  and 
looked  upon  as  prodigies  that  portended  calamity  and  mi- 
sery to  mankind.  These  fears,  and  the  erroneous  opinions 
which  produced  them,  had  their  source  in  the  hierogly- 
phical  language  of  the  first  inhabitants  of  the  earth.  We 
do  not,  however,  imagine,  that  even  the  most  ancient  of 
these  knew  any  more  of  the  laws  and  motions  of  the  hea- 
venly bodies,  than  what  could  be  discovered  from  imme- 
diate sight ;  or  that  they  knew  enough  of  the  lunar  sys- 
tem to  calculate  an  eclipse,  or  even  that  they  ever  at- 
tempted it. 

The  word  eclipse  is  derived  from  the  Greek,  and  sig- 
nifies dereliction,  a  fainting  away,  or  swooning.  Now, 
as  the  moon  falls  into  the  shadow  of  the  earth,  and  is  de- 
prived of  the  sun's  enlivening  rays,  at  the  time  of  her 
greatest  brightness,  and  even  appears  pale  and  languid 
before  her  obscuration,  lunar  eclipses  were  called  luntz 
labores,  the  struggles  or  labours  of  the  moon ;  to  relieve 

*  See  a  Complete  System  of  Astronomy,  by  the  Rev.  5.  Vince ,  4to. 

E.  j&»aT. 


92  OF    ECLIPSES. 

her  from  these  imagined  distresses,  supertition  adopted 
methods  as  impotent  as  they  were  absurd. 

When  the  moon,  by  passing  between  us  and  the  sun, 
deprived  the  earth  of  its  light  and  heat,  the  sun  was 
thought  to  turn  away  his  face,  as  if  in  abhorrence  of  the 
crimes  of  mankind,  and  to  threaten  everlasting  night  and 
destruction  to  the  world.  But  thanks  to  the  advance- 
ment of  science,  which,  while  it  has  delivered  us  from 
the  foolish  fears  and  idle  apprehensions  of  the  ancients, 
leaves  us  in  possession  of  their  representative  knowledge, 
enables  us  to  explain  the  appearances  on  which  it  was 
founded,  and  points  out  the  perversion  and  abuse  of  it. 

Any  opake  body  that  is  exposed  to  the  light  of  the 
sun,  will  cast  a  shadow  behind  it.  This  shadow  is  a  space 
deprived  of  light,  into  which,  if  another  come,  it  cannot 
be  seen  for  want  of  light ;  the  body,  thus  falling  within 
the  shadow,  is  said  to  be  eclipsed. 

The  earth  and  moon,  being  opake  bodies,  and  deriving 
their  light  from  the  sun,  do  each  of  them  cast  a  shadow 
behind,  or  towards  the  hemisphere  opposed  to  the  sun. 
Now,  when  either  the  moon  or  the  earth  passes  through 
the  other's  shadow,  it  is  thereby  deprived  of  illumination 
from  the  sun,  and  becomes  invisible  to  a  spectator  on  the 
body  from  whence  the  shadow  comes ;  and  such  a  spec- 
tator will  observe  an  eclipse  of  the  body  which  is  passing 
through  the  shadow  ;  while  a  spectator  on  the  body  which 
passes  through  the  shadow,  will  observe  an  eclipse  of  the 
sun,  being  deprived  of  his  light. 

Hence  there  must  be  three  bodies  concerned  in  an 
eclipse;  1.  The  luminous  body;  2.  The  opake  body, 
that  casts  the  shadow ;  and  3.  The  body  involved  in  the 
shadow. 

OF    ECLIPSES    OF    THE    MOON. 

As  the  earth  is  an  opake  body,  enlightened  by  the  sun, 
it  will  cast  a  shadow  towards  those  parts  that  are  oppo- 
site to  the  sun,  and  the  axis  of  this  shadow  will  alway  be 
in  the  .plane  of  the  ecliptic,  because  both  the  sun  and  the 
earth  are  always  there.  % 


ECLIPSES    OF    THE    MOON.  93 

The  sun  and  the  earth  are  both  spherical  bodies;  if  they 
were,  therefore,  of  an  equal  size,  the  shadow  of  the  earth 
would  be  cylindrical,  as  in  plate  8,  fig.  5,  and  would  con- 
tinue of  the  same  breadth  at  all  distances  from  the  earth, 
and  would  consequently  extend  to  an  infinite  distance,  so 
that  Mars,  Jupiter,  or  Saturn,  might  be  eclipsed  by  it ; 
but  as  these  planets  are  never  eclipsed  by  the  earth,  this 
is  not  the  shape  of  the  shadow,  and  consequently  the 
earth  is  not  equal  in  size  to  the  sun. 

If  the  sun  were  less  than  the  earth,  the  shadow  would 
be  wider,  the  farther  it  was  from  the  earth,  see  plate  8, 
fig.  6,  and  would  therefore  reach  to  the  orbits  of  Jupiter 
and  Saturn,  and  eclipse  any  of  these  planets  when  the 
earth  came  between  the  sun  and  them ;  but  the  earth 
never  eclipses  them,  therefore  this  is  not  the  shape  of  its 
shadow,  and  consequently  the  sun  is  not  less  than  the 
earth. 

As  we  have  proved,  that  the  earth  is  neither  larger  than, 
nor  equal  to  the  sun,  we  may  fairly  conclude  that  it  is 
less ;  and  that  the  shadow  of  the  earth  is  a  cone,  which 
ends  in  a  point  at  some  distance  from  the  earth,  see  plate 
8,  fig.  7.  ^ 

The  axis  of  the  earth's  shadow  falls  always  upon  that 
point  of  the  ecliptic  that  is  opposite  to  the  sun's  geocentric 
place  ;  thus,  if  the  sun  be  in  the  first  point  of  Aries,  the 
axis  of  the  earth's  shadow  will  terminate  in  the  first  point 
of  Libra.  It  is  clear,  therefore,  that  there  can  be  no  eclipse 
of  the  moon  but  when  the  earth  is  interposed  between  it  and 
sun,  that  is,  at  the  time  of  its  opposition,  or  when  it  is  full ; 
for  unless  it  be  opposite  to  the  sun,  it  can  never  be  in  the 
earth's  shadow :  and  if  the  moon  did  always  move  in 
the  plane  of  the  ecliptic,  she  would  every  full  moon  pass 
through  the  body  of  the  shadow,  and  there  would  be  a 
total  eclipse  of  the  moon. 

We  have  already  observed,  that  the  moon's  orbit  is  in. 
dined  to  the  plane  of  the  ecliptic,  and  only  coincides  with 
it  in  two  places,  which  are  termed  the  nodes.  It  may, 
therefore,  be  full  moon*  without  her  being  in  the  plane 

*  A  planet  may  be  in  opposition  to,  or  conjunction  with,  the  sun,  without 
being  in  a  right  line  that  passes  through  the  sun  and  the  earth.  Astronomers 
term  it.  in  conjunction  with  the  sun,  if  it  be  in  the  same  part  of  the  zodiac; 
in  opposition,  if  it  be  in  a  part  of  the  zodiac,  180  degrees  from  the  sun. 


94  ECLIPSES    OF    THE    MOON. 

of  the  ecliptic ;  she  may  be  either  in  the  north  or  the 
south  side  of  it ;  in  either  of  these  cases,  she  will  not  enter 
into  the  shadow,  but  be  above  it  in  the  one,  and  below 
it  in  the  other. 

To  illustrate  this,  let  H  G,  plate  10,  Jig.  1,  represent 
the  orbit  of  the  moon,  E  F  the  plane  of  the  ecliptic,  in 
which  the  centre  of  the  earth's  shadow  always  moves,  and 
N  the  node  of  the  moon's  orbit ;  A  B  C  D  four  places  of 
the  shadow  of  the  earth  in  the  ecliptic.  When  the  shadow 
is  at  A,  and  the  moon  at  I,  there  will  be  no  eclipse  ;  when 
the  full  moon  is  nearer  the  node,  as  at  K,  only  part  of 
her  globe  passes  through  the  shadow,  and  that  part  be- 
coming dark,  it  is  called  a  partial  eclipse;  and  it  is  said 
to  be  of  so  many  digits  as  there  are  twelfth  parts  of  the 
moon's  diameter  darkened.  When  the  full  moon  is  at 
M,  she  enters  into  the  shadow  C,  and  passing  through  it 
becomes  wholly  darkened  at  L,  and  leaves  the  shadow  at 
O  ;  as  the  whole  body  of  the  moon  is  here  immersed  in 
the  shade,  this  is  called  a  total  eclipse.  But  when  the 
moon's  centre  passes  through  that  of  the  shadow,  which 
can  only  happen  when  she  is  in  the  node  at  N,  it  is  called 
a  total  and  central  eclipse.  There  will  always  be  such 
eclipses,  when  the  centre  of  the  moon,  and  the  axis  of  the 
shadow,  meet  in  the  nodes. 

The  duration  of  a  central  eclipse  is  so  long,  as  to  let 
the  moon  go  the  length  of  three  of  its  diameters  totally 
eclipsed,  which  stay  in  the  earth's  shadow  is  computed  to 
be  about  four  hours ;  whereof  the  moon  takes  one  hour 
from  its  beginning  to  enter  the  shadow,  till  quite  im- 
mersed therein ;  two  hours  more  she  continues  totally 
dark  ;  and  the  fourth  hour  is  taken  up  from  her  first  be- 
ginning to  come  out  of  the  shadow,  till  she  is  quite  out 
of  it. 

From  the  magnitude  of  the  sun,  the  size  of  the  earth, 
their  distance  from  each  other,  the  refraction  of  the  at- 
mosphere, and  the  distance  of  the  moon  from  the  earth, 
it  has  been  calculated,  that  the  shadow  of  the  earth  ter- 
minates in  a  point,  which  does  not  reach  so  far  as  the 
moon's  orbit.  The  moon  is  not,  therefore,  eclipsed  by 
the  shadow  of  the  earth  alone.  The  atmosphere?  by  re- 
fracting some  of  the  rays  of  the  sun,  and  reflecting  others. 


ECLIPSES    OF    THE    MOON.  95 

casts  a  shadow,  though  not  so  dark  a  one  as  that  which 
arises  from  an  opake  body  ;  when,  therefore,  we  say  that 
the  moon  is  eclipsed,  by  passing  into  the  shadow  of  the 
earth,  it  is  to  be  understood  of  the  shadow  of  the  earth, 
together  with  its  atmosphere.  Hence  it  is  that  the  moon 
is  visible  in  eclipses,  the  shadow  cast  by  the  atmosphere 
not  being  so  dark  as  that  cast  by  the  earth.  The  cone  of 
this  shadow  is  larger  than  the  cone  of  the  earth's  shadow, 
the  base  thereof  broader,  the  axis  longer.  There  have 
been  eclipses  of  the  moon,  in  which  the  moon  has  entirely 
disappeared  :  Hevelius  mentions  one  of  this  kind  which 
happened  in  August  1647,  when  he  was  not  able  to  distin- 
guish the  place  of  the  moon,  even  with  a  good  telescope, 
although  the  sky  was  sufficiently  clear  for  him  to  see  the 
stars  of  the  fifth  magnitude. 

All  opake  bodies,  when  illuminated  by  the  rays  of  the 
sun,  cast  a  shadow  from  them,  which  is  encompassed 
by  a  penumbra  or  thinner  shadow,  every  where  sur- 
rounding the  former,  and  growing  larger  and  larger  as  we 
recede  from  the  body  ;  in  other  words,  the  penumbra  is 
all  that  space  surrounding  the  shadow,  into  which  the  rays 
of  light  can  only  come  from  some  part  of  that  half  of  the 
globe  of  the  sun,  which  is  turned  towards  the  planet,  all 
the  rest  being  intercepted  by  the  intervening  body. 

Let  S,  plate  10,  fig.  2,  be  the  sun,  E  the  planet,  then 
the  penumbral  cone  is  F  G  H.  The  nearer  any  part  of  the 
penumbra  is  to  the  shadow,  the  less  light  it  receives  from 
the  sun  ;  but  the  farther  it  is,  the  more  it  is  enlightened  ; 
thus,  the  parts  of  the  penumbra  near  M  are  illuminated 
only  by  those  rays  of  light,  which  come  from  that  part 
of  the  sun  near  to  I,  all  the  rest  being  intercepted  by  the 
planet  E :  in  like  manner,  the  parts  about  N  can  only 
receive  the  light  that  comes  from  the  part  of  the  sun  near 
to  L,  whereas  the  parts  of  the  penumbra  at  P  and  O  are 
enlightened  in  a  much  greater  degree  ;  for  the  planet  in- 
tercepts from  P  only  those  rays  which  come  from  the  sun 
near  L,  and  hides  from  Q^only  a  smali  part  of  the  sun 
near  L 

The  moon  passes  through  the  penumbra  before  she 
enters  into  the  shadow  of  the  atmosphere ;  this  causes 


96  ECLIPSES    OF    THE    MOON. 

her  gradually  to  lose  her  light,  which  is  not  sensible  at 
first,  but  as  she  goes  into  the  darker  part  of  the  penum- 
bra, she  grows  paler ;  the  penumbra,  where  it  is  conti- 
guous to  the  shadow,  is  so  dark,  that  it  is  difficult  to  dis- 
tinguish one  from  the  other.  If  the  atmosphere  be  serene, 
every  eclipse  of  the  moon  is  visible  at  the  same  instant  to 
all  the  inhabitants  of  that  side  of  the  earth  to  which  she 
is  opposite. 

If  we  imagine  a  plane  parallel  to  the  base  of  the  earth's 
conical  shadow,  to  pass  through  the  shadow  at  the  dis- 
tance of  the  moon's  centre  from  the  earth,  there  will  be 
projected  upon  the  plane  the  circle  of  the  earth's  shadow, 
surrounded  with  the  circle  of  the  penumbra :  the  centre 
of  those  circles  is  always  in  the  plane  of  the  ecliptic.  The 
circle  of  the  earth's  shadow,  when  the  earth  is  at  the  same 
distance  from  the  sun,  is  greater  the  nearer  the  moon  is 
to  the  earth.  The  circle  of  the  earth's  shadow  is  greater, 
wrhen  the  moon  is  at  the  same  distance  from  the  earth, 
the  farther  the  earth  is  from  the  sun.  The  apparent  semi- 
diameter  of  the  moon  in  her  syzygies  is  about  15  minutes: 
the  semidiameter  of  the  circle  of  the  earth's  shadow  is 
about  three  times  as  great  as  the  semidiameter  of  the 
moon. 

If  the  moon  in  opposition  be  in  the  node,  the  eclipse 
of  the  moon  will  be  total  and  central ;  if  very  near  the 
node,  total,  but  not  central ;  if  so  far  from  the  node,  that 
only  pan  falls  into  the  shadow,  the  eclipse  is  partial;  if  so 
far  from  the  node,  that  the  distance  of  her  centre  from  the 
centre  of  the  circle  of  the  earth's  shadow  is  greater  than 
the  semidiamer  of  the  shadow  added  to  the  semidiameter 
of  the  moon,  she  will  not  be  eclipsed  at  all.  The  moon 
passes  through  the  penumbra  before  she  falls  into  the  sha- 
dow ;  this  makes  her  gradually  lose  her  light,  and  grow 
paler,  a  little  before  she  begins  to  be  eclipsed. 

The  moon  is  sometimes  in  the  middle  of  a  total  eclipse 
invisible  in  some  places,  and  not  in  others,  because  of  the 
different  constitution  of  the  air ;  but  generally  she  ap- 
pears of  a  dusky  red  colour,  especially  towards  the  edges, 
being  more  dark  about  the  middle  of  the  shadow :  this 
reddish  colour  is  owing  to  the  rays  of  the  sun,  or  to  the 


ECUPSES    OF    THE    MOON.  97 

light  of  the  sun's  atmosphere  refracted  through  the  earth's 
atmosphere,  or  to  the  light  of  the  stars  and  planets  ;  most 
probably  to  the  first  of  these. 

The  sun  or  moon  seen  from  the  earth,  or  the  earth  seen 
from  the  sun  or  moon,  though  spherical,  on  account  of 
their  distance  appear  like  circular  planes  i  these  circular 
planes  are  called  the  disks  of  the  sun,  earth,  or  moon. 
The  apparent  diameter  of  the  disk  of  the  sun  or  moon 
is  by  astronomers  divided  into  12  equal  parts,  which  are 
called  digits  ;  each  digit  into  60  parts,  which  are  called 
minutes  :  as  many  of  these  digits  and  minutes  as  are  co- 
vered by  the  shadow  in  the  middle  of  a  partial  lunar 
eclipse,  so  many  digits  and  minutes  of  the  moon  are  said 
to  be  eclipsed.  In  a  total  eclipse  of  the  moon  without 
continuance,  the  moon  is  eclipsed  1 2  digits ;  in  a  total 
eclipse  with  continuance,  she  is  eclipsed  more  than  12  ; 
thus,  if  her  whole  disk  be  immersed  so  deep  within  the 
shadow,  that  if  her  diameter  contained  15  such  parts  as 
now  it  contains  12  of,  the  whole  15  would  be  eclipsed, 
the  moon  is  then  eclipsed  15  digits.  Sometimes  the  ap- 
parent diameter  of  the  moon  is  observed  near  the  time  of 
the  eclipse,  and  the  greatness  of  the  eclipse  expressed  in 
minutes  of  degrees  and  seconds. 

The  motion  of  the  moon  in  her  orbit  being  eastward, 
the  beginning  of  a  lunar  eclipse  is  when  the  eastern  limb 
or  edge  of  the  moon's  disk  touches  the  western  limb  of 
the  shadow;  the  end  of  a  lunar  eclipse,  when  the  western 
limb  of  the  moon's  disk  leaves  the  eastern  limb  of  the 
shadow  ;  in  a  total  eclipse,  the  time  the  whole  disk  is  in 
the  shadow  is  called  the  stay,  or  time  of  total  immer- 
sion. 

The  beginning  or  end  of  a  lunar  eclipse,  being  instan- 
taneous, serves  to  discover  the  longitude,  but  not  accu- 
rately without  a  telescope ;  for,  by  reason  of  the  penum- 
bra, the  beginning  appears  too  soon,  the  end  too  late  to 
the  naked  eye,  and  not  at  the  same  time  to  all  eyes':  for 
this  reason,  the  longitudes  of  places,  and  the  places  of  the 
moon,  determined  by  eclipses,  before  the  invention  of 
telescopes,  cannot  be  depended  upon.  The  moderns, 
that  they  may  have  a  greater  number  of  opportunities  of 
detei  mining  the  longitude  than  the  beginnings  and  end- 
ings of  eclipses  would  afford,  do  it  by  observing  the  im- 
VOL.  IV.  O 


98  ECLIPSES    OF    THE    SUN. 

mersions  of  the  most  remarkable  spots  of  the  moon  into 
the  shadow,  or  by  their  emerging  out  of  it. 

The  quantity  of  a  lunar  eclipse  depends,  1.  Upon  the 
largeness  of  the  circle  of  the  earth's  shadow,  whose  dia- 
meter may  be  different.  2.  Upon  the  apparent  diameter 
of  the  moon,  which  may  be  different.  3.  Ceteris  paribus, 
upon  the  distance  of  the  moon  from  her  node,  at  the  mo- 
ment of  her  being  at  the  full. 

The  duration  of  a  lunar  eclipse  depends  partly  upon  its 
quantity,  partly  upon  the  velocity  of  the  moon's  motion 
across  the  shadow,  which  is  the  same  as  her  motion  from 
the  sun.  The  moon's  motion  from  the  sun  is  swiftest 
when  she  is  in  her  perigee,  and  the  duration  of  a  central 
eclipse  will  then  be  shortest,  though  the  moon's  diame- 
ter and  the  diameter  of  the  circle  of  the  earth's  shadow 
be  then  greatest ;  because  the  excess  of  the  moon's  way 
through  the  shadow  is  more  than  compensated  by  the 
greater  velocity  of  the  moon's  motion.  The  longest  du- 
ration of  a  central  lunar  eclipse,  u  e.  when  the  earth  is  in 
aphelion,  and  the  moon  in  apogee,  is  about  3  hours,  57 
minutes,  6  seconds.  The  shortest  duration  of  a  central 
lunar  eclipse,  /'.  e.  when  the  earth  is  in  perihelion,  and 
the  moon  in  perigee,  is  3  hours,  37  minutes,  26  seconds. 


OF    ECLIPSES    OF    THE    SUN. 

The  moon,  when  in  conjunction,  if  near  one  of  her 
nodes,  will  be  interposed  between  us  and  the  sun,  and 
will- consequently  hide  the  sun,  or  a  part  of  him,  from 
us,  and  cast  a  shadow  upon  the  earth :  this  is  called  an 
-eclipse  of  the  sun  ;  it  may  be  either  partial  or  total. 

An  eclipse  of  any  lucid  body  is  a  deficiency  or  diminm- 
tion  of  light,  which  would  otherwise  come  from  it  to  our 
eye,  and  is  caused  by  the  interposition  of  some  opake 
bod/. 

The  eclipses  of  the  sun  and  moon,  though  expressed 
by  the  same  word,  are  in  nature  very  different ;  the  sun, 
in  reality,  loses  nothing  of  its  native  lustre  in  the  great- 
est eclipses,  but  is  all  the  while  incessantly  sending  forth 
streams  of  light  every  way  round  him,  as  copiously  as  be* 


ECLIPSES    OF    THE    SUN.  99 

fore.  Some  of  these  streams  are,  however,  intercepted 
in  their  way  towards  our  earth,  by  the  moon  coming  be- 
tween the  earth  and  the  sun :  and  the  moon  having  no 
J^ight  of  her  own,  and  receiving  none  from  the  sun  on  that 
half  of  the  globe  which  is  towards  our  eye,  must  appear 
dark,  and  make  so  much  of  the  sun's  disk  appear  so,  as 
is  hid  from  us  by  her  interposition. 

What  is  called  an  eclipse  of  the  sun,  is  therefore  in  re- 
ality an  eclipse  of  the  earth,  which  is  deprived  of  the  sun's 
light  by  the  moon  coming  between,  and  casting  a  shadow 
upon  it.  The  earth  being  a  globe,  only  that  half  of  it 
which  at  any  time  is  turned  towards  the  sun,  can  be  en- 
lightened by  him  at  that  time  ;  it  is  upon  some  part  of 
this  enlightened  half  of  the  earth,  that  the  moon's  sha- 
dow or  penumbra  falls  in  a  solar  eclipse. 

The  sun  is  always  in  the  plane  of  the  ecliptic ;  but 
the  moon  being  inclined  to  this  plane,  and  only  coin- 
ciding with  it  at  the  nodes,  it  will  not  cover  either  the 
whole  or  a  part  of  the  sun  ;  or,  in  other  words,  the  sun 
will  not  be  eclipsed,  unless  the  moon  at  that  time  be  in 
or  near  one  of  her  nodes. 

The  moon,  however,  cannot  be  directly  between  the 
sun  and  us,  unless  they  be  both  in  the  same  part  of  the 
heavens  ;  that  is,  unless  they  be  in  conjunction.  There- 
fore the  sun  can  never  be  eclipsed  but  at  the  new  moon, 
nor  even  then,  unless  the  moon  at  that  time  be  in  or 
near  one  of  the  nodes. 

The  moon  being  much  smaller  than  the  earth,  and 
having  a  conical  shadow,  because  she  is  less  than  the 
sun,  can  only  cover  a  small  part  of  the  earth  by  her  sha- 
dow ;  though  as  we  have  observed  before,  the  whole 
body  of  the  moon  may  be  involved  in  that  of  the  earth. 
Hence  an  eclipse  of  the  sun  is  visible  but  to  a  few  in- 
habitants of  the  earth  ;  whereas  an  eclipse  of  the  moon 
may  be  seen  by  all  those  that  are  on  that  hemisphere 
which  is  turned  towards  it.  In  other  words,  as  the 
moon  can  never  totally  eclipse  the  earth,  there  will  be 
many  parts  of  the  globe  that  will  suffer  no  eclipse, 
though  the  sun  be  above  their  horizon. 

An  eclipse  of  the  sun  always  begins  in  the  western, 
and  ends  on  the  eastern  side  ;  because  the  moon  mov- 
ing in  her  orbit  from  west  to  east,  necessarily  first  ar- 


100  ECLIPSES    OF    THE    SUN. 

rives  at  and  touches  the  sun's  western  limb,  and  goe\ 
off  at  the  eastern. 

It  is  not  necessary,  in  order  to  constitute  a  central 
eclipse  of  the  sun,  that  the  moon  should  be  exactly  in 
the  line  of  the  nodes,  at  the  time  of  its  conjunction  ; 
for,  it  is  sufficient  to  denominate  an  eclipse  of  the  sun 
central,  that  the  centre  of  the  moon  be  directly  between 
the  centre  of  the  sun  and  the  eye  of  the  spectator  ;  for 
to  him,  the  sun  is  then  centrally  eclipsed.  But,  as  the 
shadow  of  the  moon  can  cover  only  a  small  portion  of 
the  earth,  it  is  obvious  this  may  happen  when  the  moon 
is  not  in  one  of  her  nodes.  Further,  the  sun  may  be 
eclipsed  centrally,  totally,  partially,  and  not  at  all,  at  the 
same  time,  to  different  parts  of  the  earth. 

A  total  eclipse  of  the  sun  is  a  very  curious  spectacle  : 
Clavius  says  that,  in  that  which  he  observed  in  Portu- 
gal, in  .1650,  the  obscurity  was  greater,  or  more  sen- 
sible than  that  of  the  night :  the  largest  stars  made 
their  appearance  for  about  a  minute  or  two,  and  the 
birds  were  so  terrified,  that  they  fell  to  the  ground. 

Thus  in  plate  10,  fig.  3,  let  A  B  C  be  the  sun,  M  N 
the  moon,  fi  1  g  part  of  the  cone  of  the  moon's  shadow  ; 
f  d  the  penumbra  of  the  moon  :  from  this  figure  it  is 
easy  to  perceive, 

1.  That  those  parts  of  the  earth  that  are  within  the 
circle  represented  by  g  h,  are  covered  by  the  shadow  of 
the  moon,  so  that  no  rays  can  come  from  any  part  of 
the  sun  into  that  circle,  on  account  of  the  interposition 
of  the  moon, 

2.  In  those  parts  of  the  earth  where  the  penumbra 
falls,  only  part  of  the  sun  is  visible  ;  thus  between  d  and 
g,  the  parts  of  the  sun  near  C  cannot  be  seen,  the  rays 
coming  from  thence  towards  d  or  g  being  intercepted 
by  the  moon :  whereas  at  the  same  time  the  parts  be- 
tween f  and  h  are  illuminated  by  rays  coming  from  C, 
bat  are  deprived  by  the  moon  of  such  as  come  from  A. 

3.  The  nearer  any  part  of  the  earth,  within  the  pe- 
numbra, is  to  the  shadow  of  the  moon,  as  in  places 
near  g,  1,  or  h,  the  less  is  the  portion  of  the  sun  visi- 
ble to  its  inhabitants  ;  the  nearer  it  is  to  the  outside  of 
the  penumbra,  as  towards  d,  e,  or  f,  the  greater  is  the 
portion  seen. 


ECLIPSES    OF    THE    SUN.  101 

4.  Out  of  the  penumbra,  the  entire  disk  is  visible. 
The  quantity  of  a  solar  eclipse  in  general  is  according 
to  the  size  of  the  moon's  shade  projected  upon  the 
earth  ;  this  shade  is  largest  when  the  earth  is  in  aphe- 
lion, the  moon  in  perigee. 

The  quantity  of  a  solar  eclipse  to  those  within  the 
line,  which  the  centre  of  the  moon's  shade  describes 
upon  the  earth,  depends  upon  the  apparent  diameters 
of  the  sun  and  moon ;  if  they  be  exactly  equal,  the 
eclipse  will  be  barely  total  \  if  the  diameter  of  the  moon 
be  greater  than  that  of  the  sun,  it  will  be  more  than 
total ;  if  the  diameter  of  the  moon's  shade  be  less  than 
the  sun's,  the  eclipse  will  be  annular,  /.  e.  the  sun's  disk 
will  not  be  entirely  covered,  but  there  will  be  a  ring 
of  his  light  visible  round  the  disk  of  the  moon. 

Eclipses  may  be  also  total  or  annular,  in  places  a 
little  distant  from  the  way  of  the  centre  of  the  shade, 
but  not  central.  More  than  total  eclipses  appear  great- 
est in  those  places  which  are  nearest  the  path  of  the 
centre  of  the  shadow.  Partial  eclipses  appear  greatest 
in  those  places  which  are  nearest  the  way  of  the  moon's 
shadow  upon  the  earth.  The  quantity  of  a  solar  eclipse 
in  any  place  is  estimated  by  the  number  of  digits  of  the 
sun's  diameter  covered  by  the  disk  of  the  moon,  in  the 
middle  of  the  eclipse :  in  an  eclipse  barely  total,  the 
sun  is  eclipsed  1 2  digits  :  when  the  eclipse  is  more  than 
total,  he  is  eclipsed  so  much  more  than  12  digits,  as  the 
distance  between  the  limbs  of  the  disks  of  the  sun  and 
moon  amounts  to  in  those  points  where  those  limbs  are 
nearest  to  each  other. 

The  shape  of  the  moon's  shadow  projected  upon  the 
earth  in  the  middle  of  the  eclipse,  depends  upon  the 
moon's  distance  from  her  node.  If  the  moon  be  in  her 
node,  the  centres  of  the  sun,  moon,  and  earth,  are  all  in 
a  straight  line,  which  is  perpendicular  to  the  spherical 
surface  of  the  earth,  and  therefore  the  projection  of  the 
moon's  shadow  upon  the  disk  of  the  earth  will  be  a  circle. 
When  the  moon  has  latitude,  the  axis  of  her  shadowy 
cone  makes  an  oblique  angle  with  the  spherical  surface 
of  the  earth,  and  therefore  the  projection  of  the  shadow 
upon  the  earth's  disk  will  be  an  ellipsis,1  whose  excentri- 
city  will  be  greater,  the  greater  the  moon's  latitude  is. 


102  .ECLIPSES    OF    THE    SUN. 

The  largeness  of  the  moon's  shade  projected  upon  the 
earth,  depends  upon  the  following  circumstances  :  the 
conical  shadow  of  the  moon  is  longer,  and  similar  sec- 
tions at  equal  distances  from  the  moon  are  larger,  the 
greater  the  moon's  distance  is  from  the  sun.  There- 
fore, the  projection  of  the  moon's  shadow  upon  the 
earth  is  largest,  when  the  earth  is  in  aphelion  and  the 
moon  in  perigee  ;  least,  when  the  earth  is  in  perihelion 
and  the  moon  in  apogee,  at  the  same  time.  In  a  solar 
eclipse  that  is  central  and  barely  total,  the  vertex  of 
the  moon's  shadow  does  but  just  reach  the  surface  of 
the  earth  ;  in  an  annular  eclipse,  the  conical  shadow  of 
the  moon  does  not  reach  so  far  as  the  earth. 

The  way  of  the  moon's  shadow  upon  the  earth  is  gene- 
rally from  west  to  east ;  inclining  towards  the  north  pole, 
when  the  moon  is  near  her  ascending  node ;  towards 
the  south  pole,  when  she  is  near  her  descending  node. 
The  way  of  the  shadow  upon  the  earth  may  sometimes, 
but  rarely,  be  from  east  to  west,  and  the  sun  appear  to 
be  eclipsed  first  near  his  eastern  limb.  The  way  of  the 
centre  of  the  moon's  shadow  is  a  straight  line,  only 
when  it  describes  a  diameter  upon  the  earth's  disk; 
otherwise  it  is  an  elliptic  curve,  but  so  near  a  straight 
line,  that  it  may  without  any  sensible  error  be  repre- 
sented by  one. 

The  duration  of  solar  eclipses  depends  on  the  follow- 
ing circumstances.  If  the  moon  be  in  her  node,  the 
centre  of  her  shadow  passes  over  the  centre  of  the  earth's 
enlightened  disk,  and  describes  a  diameter,  u  e.  the 
longest  line  which  can  be  taken  in  a  circle ;  if  the 
moon  have  latitude,  the  centre  of  her  shadow  describes 
a  chord  in  the  circular  disk  of  the  earth,  /.  e.  a  line  less 
than  a  diameter. 

The  whole  time  the  penumbra  of  the  moon  is  passing 
over  the  disk  of  the  earth,  is  called  the  time  of  the 
general  eclipse  ;  because  all  that  time  the  sun  appears 
eclipsed  in  some  place  of  the  earth  or  other.  The  be- 
ginning of  the  general  eclipse  is  when  the  moon's  pe- 
numbra enters  upon  the  disk  of  the  earth  ;  the  end, 
when  the  penumbra  of  the  moon  leaves  it.  The  dura- 
tion of  the  general  eclipse  depends  upon  the  length  of 
the  line  described  upon  the  earth's  disk  by  the  centre 


ECLIPSES    OF    THE    SUN.  103 

of  the  shadow,  the  velocity  of  tihe  moon's  motion  from 
the  sun,  and  the  largeness  and  shape  of  the  projection 
of  the  shadow  and  penumbra. 

The  beginning  of  the  solar  eclipse  in  any  place  upon 
the  earth,  is  when  the  penumbra  which  surrounds  the 
moon's  shadow  first  touches  the  place ;  the  end  of  the 
eclipse  is  when  the  penumbra  leaves  the  place  ;  the  du- 
ration of  the  eclipse  is  while  the  penumbra  passes  over 
the  place.  In  any  place  upon  the  earth  where  the 
eclipse  is  more  than  total,  the  beginning  of  the  total 
darkness  is  when  the  shadow  of  the  moon  first  touch- 
es the  place  ;  the  end,  when  it  leaves  it.  Eclipses  more 
than  total  are  said  to  be  total  with  stay  ;  the  time  of 
stay,  viz.  of  total  darkness,  in  any  place,  is  the  time  of 
the  shadow  passing  over  that  place. 

The  time  the  shadow  is  in  passing  over  any  place, 
(and  the  same  is  true  of  .the  penumbra,  which  is  always 
similar  to  the  shadow)  is  variable,  1st,  from  the  velo- 
city of  the  moon's  motion  from  the  sun  ;  2d,  from  the 
length  of  a  shadow  measured  in  a  line  parallel  to  the 
way  of  the  shadow,  and  drawn  through  the  place  ;  Sd> 
from  the  proximity  of  the  place  to  the  centre  of  the 
earth's  disk. 

The  circumference  of  the  moon's  orbit  is  60  times  as 
great  as  the  circumference  of  the  earth  ;  and  therefore^ 
each  degree  of  the  moon's  orbit  is  equal  to  60  degrees 
of  a  great  circle  on  the  earth's  surface.  And  as  one 
degree  of  such  a  circle  on  the  earth  contains  69^  Eng- 
lish miles,  a  degree  of  the  moon's  orbit  contains  4155 
miles.  The  moon's  motion  in  her  orbit,  considering  it 
as  from  the  sun  to  the  sun  again,  or  from  change  to 
change,  is  through  all  the  360  degree  thereof  in  29- 
days  ;  and  therefore  she  moves  about  half  a  degree, 
or  2077  miles  from  the  sun  in  one  hour  ;  and  with  the 
same  velocity  her  shadow  moves  over  the  earth,  name- 
ly, at  the  rate  of  34^  miles  in  a  minute  ;  which  is  more 
than  four  times  as  swift  as  the  motion  of  a  cannon-ball. 

The  moon  goes  round  the  earth,  not  in  a  circular, 
but  in  an  elliptical  orbit ;  and  the  earth's  centre  is  in 
one  of  the  foci  of  that  orbit.  Hence  the  moon's  dis- 
tance from  the  earth  is  continually  varying  :  at  a  mean 
it  is  240,00Q  miles. 


104  THE    LIMITS    OF    ECLIPSES. 

When  the  moon  changes  at  her  least  distance  from 
the  earth,  her  dark  shadow  may  cover  a  spot  1  TO  miles 
broad  on  the  earth's  surface,  if  the  time  be  about  noon ; 
but  much  more  if  the  time  be  in  the  morning  or  even- 
ing :  and  to  all  who  are  within  that  spot,  the  sun  will 
appear  to  be  totally  eclipsed ;  but  to  no  place  without 
it,  although  he  will  be  partially  eclipsed  to  several  hun- 
dred miles  around.  But,  as  the  moon's  motion  is  then 
the  swiftest  that  it  can  be,  the  dark  shadow  will  be  car- 
ried quite  over  that  spot  in  five  minutes  at  most,  al- 
though the  diurnal  motion  of  the  earth  is  the  same  way 
the  moon's  shadow  goes  ;  and  therefore  the  longest  du- 
ration of  a  total  eclipse  of  the  sun  can  never  be  more 
than  five  minutes,  even  if  it  happen  at  noon.  In  the 
morning  and  evening,  the  earth's  motion  contributes 
very  little  toward  the  duration  of  a  solar  eclipse,  because 
the  dark  shadow  falls  so  obliquely  on  the  earth  ;  and 
indeed,  in  such  an  eclipse,  the  darkness  will  be  over  in 
less  than  five  minutes,  although  the  shadow  then  ^covers 
more  of  the  earth's  surface  than  it  can  do  about  noon. 

When  the  moon  changes  at  her  mean  distance  from 
the  earth,  the  point  of  her  dark  shadow  does  but  just 
reach  the  earth  :  and,  to  the  places  where  it  goes  suc- 
cessively over,  the  sun  will  be  totally  eclipsed  only  for 
an  instant  of  time. 

When  the  moon  changes  at  her  greatest  distance  from 
the  earth,  her  dark  shadow  does  not  reach  the  earth  at 
all :  and  therefore  the  sun  is  not  then  totally  hid  from 
any  part  of  the  earth,  but  appears  like  a  luminous  ring, 
all  round  the  dark  body  of  the  moon,  to  each  part  of  the 
earth  where  the  point  of  her  shadow  is  successively  di- 
rected it  while  she  is  passing  between  the  earth  and  the 
sun. 

Thus  it  is  plain,  that  the  sun  can  never  be  eclipsed 
but  at  the  time  of  new  moon,  nor  the  moon  but  when 
she  is  full. 

OF    THE    LIMITS    OF    SOLAR    AND    LUNAR    ECLIPSES. 

The  earth  goes  round  the  sun  every  year  in  an  orbit 
called  the  ecliptic  ;  and  therefore  the  sun,  as  seen  from 
the  earth,  appears  to  go  round  the  ecliptic  once  a  year. 


THE    PERIOD    OF    ECLIPSES.  105 

If  the  moon's  orbit  lay  quite  even  with  the  ecliptic, 
or,  as  it  is  commonly  expressed,  in  the  plane  of  the 
ecliptic,  the  sun  would  be  eclipsed  at  the  time  of 
every  new  moon,  because  the  moon  would  then  be  di- 
rectly between  the  earth  and  the  sun :  and  the  moon 
would  be  eclipsed  at  every  time  she  was  full,  because  the 
earth  would  then  be  directly  between  her  and  the  sun. 

But  one  half  of  the  moon's  orbit  lies  on  the  north 
side  of  the  plane  of  the  ecliptic,  and  the  other  half  on 
the  south  side  of  it.  Therefore  the  moon's  orbit  in- 
tersects the  plane  of  the  ecliptic  only  in  two  opposite 
points,  which  are  called  the  moon's  nodes.  The  angle 
which  the  moon's  orbit  makes  with  the  plane  of  the 
ecliptic  is  5  degrees,  18  minutes  ;  so  that,  when  the 
moon  is  in  the  northmost  point  of  her  orbit,  she  is  5 
degrees,  1 8  minutes,  north  of  the  ecliptic  ;  and  as  far 
south  of  it,  when  she  is  in  the  southmost  point  of  her 
orbit.  Hence  it  is  plain,  that  the  moon  can  never  be 
in  the  ecliptic  but  when  she  is  in  one  or  other  of  her 
nodes. 

When  the  moon  is  any  more  than  1 8  degrees  from 
either  of  her  nodes,  at  the  time  of  her  change,  she  does 
not  pass  between  the  sun  and  any  part  of  the  earth ; 
but  goes  either  above  or  below  the  sun,  according  as 
she  is  then  north  or  south  of  the  ecliptic  ;  and  there- 
fore she  cannot  then  hide  any  part  of  the  sun  from  any 
part  of  the  earth.  But  when  she  changes  within  18 
degrees  of  either  node,  she  will  hide  the  whole  or  part 
of  the  sun  from  some  part  of  the  earth.  And  if  she 
be  in  either  of  her  nodes  at  the  time  of  change,  the  sun 
will  be  centrally  eclipsed  to  that  point  of  the  earth's 
surface  which  is  then  in  a  straight  line  between  the  sun's 
centre  and  the  earth's.  At  all  other  places,  which  the 
centre  of  the  moon's  shadow  goes  over,  the  sun  will 
likewise  be  centrally  eclipsed. 

When  the  moon  is  any  more  than  12  degrees  from 
either  of  her  nodes  at  the  time  of  full,  she  passes  clear 
of  the  earth's  shadow ;  and  therefore  she  cannot  be 
eclipsed  at  that  time.  But  when  she  is  within  12  de- 
grees of  either  node,  at  the  time  of  her  being  full,  she 

vol.  iv.  p 


106  THE    PERIOD    OF    ECLIPSES. 

is  eclipsed.  And  when  she  is  full  in  either  of  her  nodes, 
she  goes  through  the  middle  of  the  earth's  shadow,  and 
is  totally  eclipsed  with  the  longest  continuance,  which 
may  be  above  an  hour  and  a  half. 


OF    THE    PERIOD    OF    ECLIPSES. 

The  ecliptic  is  divided  into  twelve  equal  parts,  called 
signs  ^  and  each  sign  into  30  equal  parts,  called  degrees. 
If  the  moon's  nodes  had  no  motion  through  the  signs 
of  the  ecliptic,  there  would  be  just  half  a  year  between 
the  times  of  the  sun's  conjunctions  with  the  nodes ; 
and  then,  in  whatever  signs  the  sun  and  moon  were 
eclipsed  in  any  given  year,  they  would  be  eclipsed  every 
year  after.  But  the  eclipses  fall  so  much  sooner  every 
succeeding  year  than  they  did  on  the  year  before,  as  to 
prove,  that  the  nodes  move  backward,  or  contrary  to 
the  motion  of  the  moon,  19|  degrees  every  year,  from 
the  consequent  towards  the  antecedent  signs.  And, 
therefore,  they  go  backwards  through  all  the  signs  and 
degrees  of  the  ecliptic  in  1 8  years  and  225  days. 

If,  in  that  time,  there  were  any  exact  number  of 
courses  of  the  moon  from  change  to  change,  without 
any  fraction,  there  would  be  an  exact  period  or  restitu- 
tion of  eclipses  in  the  same  time.  But,  during  this  re- 
volution of  the  nodes,  there  are  230  courses  of  the  moon, 
and  a  quarter  of  a  course  more  :  so  that  there  can  be  no 
exact  period  of  eclipses  in  any  complete  revolution  of 
the  nodes. 

But  in  1 8  years,  1 1  days,  7  hours,  and  43y  minutes, 
in  which  time  there  are  just  223  courses  of  the  moon, 
from  change  to  change,  there  is  a  conjunction  of  the 
sun  and  moon  with  the  same  node  as  before ;  and,  con- 
sequently, a  period  or  restitution  of  all  the  eclipses  of  the 
sun  and  moon.  And,  therefore,  if  to  the  mean  time  of 
any  eclipse,  either  of  the  sun  or  moon,  you  add  18 
years,  1 1  days,  7  hours,  4>3j  minutes,  you  will  have  the 
mean  time  of  the  return  of  that  eclipse.  Only  note,  that 
when  the  last  day  of  February,  in  leap-year,  comes  but 
four  times  into  this  period,  you  are  to  add  the  abovs 


SUPERNATURAL    DARKNESS,    &C.  107 

number  of  days,  hours,  and  minutes:  but  when  it  comes 
five  times,  as  it  will  sometimes  do,  you  must  add  one 
whole  day  less.  And  thus,  any  one,  who  has  a  set  of  al- 
manacks for  1 9  years,  in  which  all  the  eclipses  are  not- 
ed for  that  time,  may  very  easily  calculate  the  time  of 
any  future  eclipse.  This  is  called  the  Chaldean  Saros, 
or  period  of  eclipses. 

As  the  nodes  go  backwards  at  the  rate  of  1 9T  de- 
grees every  year,  which,  for  the  sake  of  round  numbers, 
we  may  call  19  degrees  ;  these  19  degrees  are  nearly 
equal  to  19  days  of  the  sun's  motion,  and  the  half  of 
of  19  is  9i;  subtract  9l  days  from  182^  days,  which 
make  half  a  year,  and  there  will  remain  173  days  for  the 
time  between  the  sun's  being  in  conjunction  with  either 
of  the  nodes  till  the  time  of  his  being  so  with  the  other. 

Now,  as  the  sun  can  never  be  eclipsed  when  he  is  more 
than  1 8  degrees  from  either  node,  nor  the  moon,  when 
she  is  more  than  12,  as  already  mentioned,  it  is  plain, 
there  must  be  an  eclipse  of  the  sun  at  the  time  of  every 
new  moon  that  falls  within  18  days  before  or  after  the 
time  of  his  being  in  conjunction  with  either  of  the  nodes  ; 
and  that  the  moon  must  be  eclipsed  at  every  time  of  her 
being  full  within  12  days  before  or  after  the  time  of  the 
sun's  being  in  conjunction  with  either  of  the  nodes. 
And,  consequently,  if  we  can  tell  on  what  days  of  the 
year  these  conjunctions  fall,  we  can  easily  tell  at  what 
new  and  full  moons  there  must  be  eclipses  ;  seeing  the 
days  of  new  and  full  moons  are  so  generally  known. 

In  some  years  there  are  six  eclipses,  four  of  which  are 
of  the  sun,  and  two  of  the  moon  :  in  other  years  there 
are  only  two,  and  when  that  happens,  they  are  both  of 
the  sun  :  but  the  most  common  number  is  four  ;  name- 
ly, two  of  the  sun,  and  two  of  the  moon. 


THE    DARKNESS    AT    OUR    SAVIOUR'S    CRUCIFICTION, 
SUPERNATURAL. 

From  the  account  I  have  given  you  of  eclipses  it  plain- 
ly appears  that  the  sun  can  never  be  eclipsed,  in  a  natu- 
ral way,  but  at  the  time  of  new  moon,  nor  the  moon  bufc 


108  SUPERNATURAL    DARKNESS    AT 

when  she  is  full ;  and  that,  when  the  sun  is  totally 
eclipsed,  the  darkness  can  never  continue  above  five  mi- 
nutes at  any  place  on  the  earth. 

But  the  three  evangelists,  St.  Matthew,  St.  Mark, 
and  St.  Luke,  mention  a  darkness  that  continued  three 
hours,  at  the  time  of  our  Saviour's  crucifixion.  If  their 
account  of  that  darkness  had  been  false,  it  would  have 
been  contradicted  by  many  who  were  then  present ;  es- 
pecially as  they  were  great  enemies  both  to  Christ  and 
his  few  disciples,  as  well  as  to  the  doctrine  he  taught. 
But  as  none  of  the  Jews  have  contradicted  the  evangel- 
ists' account  of  this  most  extraordinary  phenomenon,  it 
is  plain  that  their  account  of  it  is  true.  Besides,  the 
evangelists  must  have  known  full  well,  that  it  could  not 
be  their  interest  to  palm  such  a  lie  upon  mankind ; 
which,  when  detected,  must  have  gone  a  great  way  to- 
wards destroying  the  credibility  of  all  the  rest  of  the  ac* 
count  they  gave  of  the  life,  actions,  and  doctrine  of  their 
master  :  and,  instead  of  forwarding  the  belief  of  Christi- 
anity, it  would  have  been  a  blow  at  the  very  root  thereof. 
We  do  not  find  that  they  have  bestowed  any  panegyric 
on  the  life  and  actions  of  Christ,  or  thrown  out  an  in- 
vective against  his  cruel  persecutors ;  but,  in  the  most 
plain,  simple,  and  artless  manner,  have  told  us  what  their 
senses  convinced  them  were  matters  of  fact :  so  that  we 
have  as  good  reason  to  believe,  that  there  was  such  dark- 
ness, as  we  have  to  believe  that  Christ  was  then  upon 
the  earth  :  and  that  he  was,  has  never  been  contradict- 
ed, even  by  the  Jews  themselves. 

But  there  are  other  accounts  of  Christ,  besides  those 
which  the  evangelists  have  left  us.  It  is  expressly  af- 
firmed, by  the  two  Roman  historians,  Tacitus  and  Sue- 
tonis,  that  there  was  a  general  expectation  spread  all  over 
the  eastern  nations,  that  out  of  Judea  should  arise  a  per- 
son who  should  be  governour  of  the  world.  That  there 
lived  in  Judea,  at  the  time  which  the  gospel  relates,  such 
a  person  as  Jesus  of  Nazareth,  is  acknowledged  by  all 
authors,  both  Jewish  and  Pagan,  who  have  written  since 
that  time.  The  star  that  appeared  at  his  birth,  and  the 
journey  of  the  Chaldean  wise  men,  is  mentioned  by  Chal- 
cidius  the  Platonist.     Herod's  causing  the  children  in 


our  saviour's  crucifixion.  109 

Bethlehem  to  be  slain,  and  a  reflexion  upon  him  on  that 
occasion  by  the  emperor  Augustus,  is  related  by  Macro* 
bius.  Many  of  the  miracles  that  Jesus  wrought,  parti- 
cularly his  healing  the  lame,  and  curing  the  blind,  and 
casting  out  devils,  are  owned  by  these  inveterate  and  im- 
placable enemies  of  Christianity,  Celsus  and  Julian,  and 
the  author's  of  the  Jewish  Talmud.  That  the  power  of 
the  heathen  gods  ceased,  after  the  coming  of  Christ,  is 
acknowledged  by  Porphyry,  who  attributed  it  to  their 
being  angry  at  the  setting  up  of  the  christian  religion, 
which  he  calls  impious  and  profane.  The  crucifixion  of 
Christ  under  Pontius  Pilate  is  related  by  Tacitus,  and  the 
earthquake  and  miraculous  darkness  attending  it  were 
recorded  in  the  Roman  public  registers,  commonly  ap- 
pealed to  by  the  first  christian  writers,  as  what  could  not 
be  denied  by  the  adversaries  themselves ;  and  are  in  a 
particular  manner  attested  by  Pblegon,  the  freed  man  of 
Adrian. 

Some  people  have  said,  that  the  above  mentioned 
darkness  might  have  been  occasioned  by  a  natural 
eclipse  oi  the  sun  ;  and,  consequently,  that  there  was 
nothing  miraculous  in  it.  If  this  had  been  the  case,  it 
is  plain,  that  our  Saviour  must  have  been  crucified  at 
the  time  of  new  moon.  But  then,  in  a  natural  way, 
the  darkness  could  not  possibly  have  continued  for 
more  than  five  minutes :  whereas,  to  have  made  it  con- 
tinue for  three  hours,  the  moon's  motion  in  her  orbit 
must  have  been  stopped  for  three  hours,  and  the  earth's 
morion  on  its  axis  must  have  been  stopped  as  long  too. 
And  then,  if  the  power  of  gravitation  had  not  been 
suspended  during  all  that  time,  the  moon  would  have 
fallen  a  great  way  towards  the  earth.  So  that  nothing 
less  than  a  triple  miracle  must  have  been  wrought  to 
have  caused  such  a  long  continued  darkness  by  the  in- 
terposition of  the  moon  between  the  sun  and  any  part 
of  the  earth  :  which  shows,  that  they  who  make  such 
a  supposition  are  entirely  ignorant  of  the  nature  of 
eclipses.  But  there  couid  be  no  natural  or  regular 
eclipse  of  the  sun  on  the  day  of  Christ's  crucifixion, 
as  the  moon  was  full  on  that  day,  and,  consequently,  in 
the  side  of  the  heavens  opposite  to  the  sun.     And, 


110  SUPERNATURAL    DARKNESS,    &C. 

therefore,  the  darkness  at  the  time  of  his  crucifixion 
was  quite  supernatural. 

The  Israelites  reckoned  their  months  by  the  course 
of  the  moon,  and  their  years,  after  they  left  Egypt,  by 
the  revolution  of  the  sun,  computed  from  the  equal 
day  and  night  in  the  spring  to  the  like  time  again.  For 
we  find,  they  were  told  by  the  Almighty,  Exod.  xii.  2, 
that  the  month  Abib,  or  Nisan,  should  be  to  them  the 
first  month  of  the  year.  This  was  the  month  in  which 
they  were  delivered  from  their  Egyptian  bondage,  and 
includes  part  of  March  and  part  of  April  in  our  way 
of  reckoning. 

In  several  places  of  the  Old  Testament,  we  find,  that 
the  Israelites  were  strictly  commanded  to  kill  the  pas- 
chal lamb  on  the  evening,  or,  as  it  is  in  the  Hebrew, 
between  the  evenings,  of  the  fourteenth  day  of  the  first 
month ;  and  Josephus  expressly  says,  "  The  passover 
was  kept  on  the  fourteenth  day  of  the  month  Nisan, 
according  to  the  moon,  when  the  sun  was  in  Aries." 
And  the  sun  always  enters  the  sign  Aries,  when  the 
day  and  night  are  equal  in  the  spring  season. 

They  began  each  month  on  the  first  day  of  the 
moon's  being  visible,  which  could  not  be  in  less  than 
twenty-four  hours  after  the  time  of  her  change  ;  and 
the  moon  is  full  on  the  fifteenth  day  reckoned  from  the 
time  of  change.  Hence,  the  fourteenth  day  of  the 
month,  according  to  the  Israelites'  way  of  reckoning, 
was  the  day  of  full  moon  :  which  makes  it  plain,  that 
the  passover  was  always  kept  on  a  full-moon  day  ;  and 
at  the  time  of  the  full  moon  next  after  the  equal  day 
and  night  in  the  spring  ;  or,  when  the  sun  was  in  Aries. 

All  the  four  evangelists  assure  us,  that  our  Saviour 
was  crucified  at  the  time  of  the  passover  :  and  hence  it 
is  plain,  that  the  crucifixion  was  at  the  time  of  full 
moon,  when  it  is  impossible  that  the  moon  could  hide 
the  sun  from  any  part  of  the  earth.  St.  John  tells  us, 
that  Christ  was  crucified  on  the  day  that  the  passover 
was  to  be  eaten ;  and  we  likewise  find,  that  some  re- 
monstrated against  his  being  crucified  "  on  the  feast 
day,  lest  it  should  cause  an  uproar  among  the  people."* 

*  Ferguson's  Astronomical  Lecture  on  Eclipse?. 


[    in    3 


LECTURE  XXXIX. 


OF    PARALLAX    AND    REFRACTION,    AND    THE 
ABERRATION    OF    LIGHT,    &C. 


ASTRONOMY  is  subject  to  many  difficulties, 
besides  those  which  are  obvious  to  every  eye.  When 
we  look  at  any  star  in  the  heavens,  we  do  not  see  it  in 
its  real  place ;  the  rays  coming  from  it,  when  they  pass 
out  of  the  purer  etherial  medium,  into  our  coarser  and 
more  dense  atmosphere,  are  refracted,  or  bent  in  such 
a  manner,  as  to  show  the  star  higher  than  it  really  is. 
Hence  we  see  all  the  stars  before  they  rise,  and  after  they 
set ;  and  never,  perhaps,  see  any  one  in  its  true  place 
in  the  heavens.  There  is  another  difference  in  the  ap- 
parent situation  of  the  heavenly  bodies,  which  arises 
from  the  station  in  which  an  observer  views  them- 
This  difference  in  situation  is  called  the  parallax  of  an 
object. 

OF    REFRACTION. 

As  one  of  the  principal  objects  of  astronomy  is  to  fix 
the  situation  of  the  several  heavenly  bodies,  it  is  neces- 
sary, as  a  first  step,  to  understand  the  causes  which  oc- 
casion a  false  appearance  of  the  place  of  those  objects, 
and  make  us  suppose  them  in  a  different  situation  from 
that  which  they  really  have.  Among  these  causes  re- 
fraction is  to  be  reckoned.  By  this  term  is  meant  the 
bending  of  the  rays  of  light  as  they  pass  out  of  one  me- 
dium into  another. 

The  earth  is  every  where  surrounded  by  a  hetero- 
geneous fluid,  a  mixture  of  air,  vapour,  and  terrestrial 


112  OF    REFRACTION. 

exhalations,  that  extend  to  the  regions  of  the  sky.  The 
rays  of  light  from  the  sun,  moon,  and  stars,  in  passing 
to  a  spectator  on  the  earth,  come  through  this  medium, 
and  are  so  refracted  in  their  passage  through  it,  that 
their  apparent  altitude  is  greater  than  their  true  altitude. 

Let  A  C,  plate  7,  Jig.  3,  represent  the  surface  of  the 
earth,  T  its  centre,  B  P  a  part  of  the  atmosphere,  H  E  K 
the  sphere  of  the  fixed  stars,  A  F  the  sensible  horizon, 
G  a  planet,  G  D  a  ray  of  light  proceeding  from  the 
planet  to  D,  where  it  enters  our  atmosphere,  and  is  re- 
fracted towards  the  line  D  T,  which  is  perpendicular  to 
the  surface  of  the  atmosphere  ;  and  as  the  upper  air  is 
rarer  than  that  near  the  earth,  the  ray  is  continually  en- 
tering a  denser  medium,  and  is  every  moment  bent  to- 
wards T,  which  causes  it  to  describe  a  curve  as  D  A, 
and  to  enter  a  spectator's  eye  at  A,  as  if  it  came  from 
E,  a  point  above  G.  And  as  an  object  always  appears 
in  that  line  in  which  it  enters  the  eye,  the  planet  will 
appear  at  E,  higher  than  its  true  place,  and  frequently 
above  the  horizon  A  F,  when  its  true  place  is  below 
it  at  G. 

This  refraction  is  greatest  at  the  horizon,  and  de- 
creases very  fast  as  the  altitude  increases,  insomuch  that 
the  refraction  at  the  horizon  differs  from  the  refraction 
at  a  very  few  degrees  above  the  horizon,  by  about  one 
third  part  of  the  whole  quantity.  At  the  horizon,  in 
this  climate,  it  is  found  to  be  about  33  minutes.  In 
climates  nearer  to  the  equator,  where  the  air  is  purer, 
the  refraction  is  less ;  and  in  the  colder  climates,  nearer 
to  the  pole,  it  increases  exceedingly,  and  is  a  happy 
provision  for  lengthening  the  appearance  of  the  light  at 
those  regions  so  remote  from  the  sun.  Gassendus  re- 
lates, that  some  Hollanders,  who  wintered  in  Nova 
Zembla,  in  latitude  75  degrees,  were  agreeably  surpriz- 
ed with  a  sight  of  the  sun  seventeen  days  before  they 
expected  him  in  the  horizon.  This  difference  was 
owing  to  the  refraction  of  the  atmosphere  in  that  lati- 
tude. To  the  same  cause,  together  with  the  peculiar 
obliquity  of  the  moon's  orbit  to  the  ecliptic,  some  of 
these  very  northern  regions  are  indebted  for  an  unin- 
terrupted light  from  the  moon  much  more  than  half 


Of    PARALLAX*  113 

the  month,  and  sometimes  almost  as  long  as  it  is  capa- 
ble of  affording  any  light  to  other  parts  of  the  earth. 

Through  this  refraction  we   are  favoured  with  the 

» sight  of  the  sun,  about  3lT  minutes  before  it  rises  above 

the  horizon  ;  and  also   as  much  every  evening  after  it 

sets  below  it,  which  in  one  year  amounts  to  more  than 

40  hours. 

It  is  to  this  property  of  refraction  that  we  are  also  in- 
debted for  that  enjoyment  of  light  from  the  sun,  when  he 
is  below  the  horizon,  which  produces  the  morning  and 
evening  twilight.  The  sun's  rays,  in  falling  upon  the 
higher  part  of  the  atmosphere,  are  reflected  back  to  our 
eyes,  and  form  a  faint  light,  which  gradually  augments 
till  it  becomes  day.  It  is  owing  to  this,  that  the  sun  illu- 
minates the  whole  hemisphere  at  once ;  deprived  of  the 
atmosphere,  he  would  have  yielded  no  light,  but  when 
our  eyes  were  directed  towards  him ;  and  even  when  he 
was  in  meridian  splendour,  the  heavens  would  have  ap- 
peared dark,  and  as  full  of  stars  as  on  a  fine  winter's  night. 
The  rays  of  light  would  have  come  to  us  in  straight  lines, 
and  the  appearance  and  disappearance  of  the  sun  wrould 
have  been  instantaneous  ;  we  should  have  had  a  sudden 
transition  from  the  brightest  sun- shine  to  the  most  pro- 
found darkness,  and  from  thick  darkness  to  a  blaze  of 
light.  Thus,  by  refraction,  we  are  prepared  gradually  for 
the  light  of  the  sun,  the  duration  of  its  light  is  prolonged, 
and  the  shades  of  darkness  softened. 

To  it  we  must  also  attribute  another  curious  phenome- 
non, mentioned  by  Pliny;  for  he  relates,  that  the  moon 
had  been  eclipsed  once  in  the  west,  at  the  same  time  that 
the  sun  appeared  above  the  horizon  in  the  east.  Masti- 
Hnusy  in  Kepler,  speaks  of  another  instance  of  the  same 
kind,  which  fell  under  his  own  observation. 

OF    PARALLAX. 

The  parallax  of  a  celestial  object  is  the  difference  be- 
tween the  places  that  the  object  is  referred  to  in  the  ce- 
lestial sphere,  when  seen  at  the  same  time  from  two  dif- 
ferent places  within  that  sphere.  Or,  it  may  be  considered 
as  the  angle  under  which  any  two  places  in  the  inferior 

VOL.  I\.  q 


1H  OF    PARALLAX. 

orbits  are  seen,  from  a  superior  planet,  or  from  a  fixed  star. 

The  parallaxes  principally  used  by  astronomy,  are 
those  which  arise  from  considering  the  object  as  viewed 
either  from  the  centre  of  the  earth  and  the  sun,  or  from 
the  surface  and  centre  of  the  earth,  or  from  all  three 
compounded.  t 

The  difference  between  the  place  of  any  planet  as  seen 
from  the  sun,  and  that  of  the  same  planet  as  seen  from 
the  earth,  is  called  the  parallax  of  the  annual  orbit;  in 
other  words,  it  is  the  angle  at  any  planet  subtended  be- 
tween the  sun  and  the  earth. 

The  diurnal  parallax  is  the  change  of  a  celestial  body's 
apparent  place,  arising  from  its  being  viewed  from  two 
different  stations,  one  on  the  surface,  and  the  other  at 
the  centre  of  the  earth. 

The  necessity  of  this  distinction  is  obvious,  for  you 
know,  that  an  object  will  change  its  apparent  situation 
with  respect  to  another,  according  to  the  station  from 
which  it  is  viewed ;  hence  celestial  objects,  viewed  from 
different  parts  of  the  earth's  surface,  will  appear  in  dif- 
ferent situations.  To  facilitate  and  give  certainty  to  cal- 
culation, astronomers  refer  all  celestial  appearances  to  the 
centre  of  the  earth  ;  of  course  they  are  obliged  continu- 
ally to  calculate  parallaxes,  in  order  to  reduce  the  ob- 
served places  of  the  objects  to  that  where  they  would  be 
situate,  if  seen  from  the  centre  of  the  earth. 

Let  a  line,  A  B,  plate  7,  jig*  4,  be  drawn  perpendicu- 
lar to  the  distance  B  C,  between  an  adjacent  object  C,  and 
any  given  station  B :  the  apparent  places  of  the  object, 
when  viewed  from  the  extremities  of  the  line  A  B,  will 
be  different. 

1.  The  perpendicular  line,  A  B,  is  called  the  base* 
*?.  The  extremities,  A,  B,  of  the  base  are  called  stations. 

3.  The  angle  A  C  B,  subtended  by  the  extremities  of  the 
base  at  the  object,  is  called  the  angle  of  the  parallax. 

4.  The  base  is  to  the  lesser  of  the  two  diftances  of  the 
object  from  the  extremities  of  the  base,  as  the  tangent  of 
the  angle  of  parallax  to  radius  ;  and  to  the  greater,  as  the 
sine  of  the  same  angle  to  radius. 

Suppose  lines  to  be  drawn  from  the  two  stations  to  an 
object :  one  of  the  angles  contained  by  these  lines,  as  in 


OF    PARALLAX.  1  1,5 

the  figure,  being  a  right  angle,  the  other  will  be  the  com- 
plement of  the  parallax  to  90  degrees. 

If  the  angles  at  the  stations  terminating  a  given  base  be 
known,  it  is  easy,  by  trigonometry,  to  determine  the  dis- 
tance of  the  object.  N.  B.  We  here  suppose  one  of  the 
angles  at  the  base  to  be  90  degrees. 

When  the  distance  of  an  object  is  greater  than  100,000 
times  the  base,  the  angles  at  the  two  stations  will  not  sen- 
sibly differ  from  two  right  ones  ;  and,  consequently,  the 
lines  drawn  from  the  object  to  the  stations,  are,  physi- 
cally speaking,  parallel. 

Now  the  angle,  whose  tangent  is  to  radius,  as  1  to 
100,000,  is  very  little  more  than  a  second  ;  and  the  most 
accurate  instruments  constructed  for  the  mensuration  of 
angles  can  scarce  be  depended  upon  to  2  seconds. 

Hence,  the  parallax  of  an  object,  whose  distance  is 
above  100,000  times  greater  than  that  between  the  two 
stations  of  observation,  is  insensible. 

We  may  therefore  conclude,  that  if  the  parallax  of  an 
object,  observed  with  an  instrument  sufficiently  exact  to 
measure  an  angle  of  2  seconds,  be  insensible,  the  distance 
of  it  from  either  station  cannot  be  less  than  100,000  times 
the  base,  from  the  extremities  of  which  it  is  observed. 

But  you  are  to  observe,  that  although  the  distance  of 
the  object  cannot  be  less  than  100,000  times  the  base,  it 
may  be  greater  in  any  assignable  ratio. 

Lines  drawn  from  any  given  point  in  a  base,  to  an 
object,  may  in  practice  be  esteemed  parallel,  without 
sensible  error,  if  the  distance  of  the  object  be  more  than 
100,000  times  the  base.        ' 

Having  laid  down  these  few  general  principles,  we  may 
now  proceed  to  explain  the  parallaxes  used  by  astrono- 
mers, which  are  principally  those  which  arise  from  consi- 
dering the  object  as  viewed  from  the  centre  of  the  earth 
and  sun,  from  the  surface  and  the  centre  of  the  earth, 
and  from  these  compounded. 

The  diameter  of  the  earth  is  the  longest  straight  line 
we  can  accurately  obtain,  and  is,  in  general,  the  base  used 
for  determining  the  distances  of  celestial  objects  by  their 
parallaxes. 

The  change  in  the  apparent  place  of  a  planet,  or  fixed 


116  OF    PARALLAX. 

star,  or  any  celestial  body  arising  from  its  being  viewed 
on  the  surface,  or  from  the  centre  of  the  earth,  is  called 
its  diurnal  parallax. 

To  explain  the  parallaxes  with  respect  to  the  earth,  I 
shall  use  the  diagram,  plate  7,  Jig.  2,  where  H  S  W  re- 
presents the  earth ;  T  its  centre ;  ORG  part  of  the 
moon's  orbit ;  P  r  g  a  part  of  the  planet's  orbit ;  Z  a  A 
part  of  the  starry  heavens;  ZS  a  line  which  passes 
through  the  zenith. 

Now  it  is  plain,  from  the  inspection  of  the  diagram, 
that  a  planet,  P,  situate  in  the  zenith-line,  always  an- 
swers to  the  same  point  of  the  heavens,  whether  it  be  re- 
garded from  the  centre  T,  or  from  the  point,  S,  on  the 
surface:  so  that  a  celestial  body  in  the  -zenith  has  no  parallax. 

If  the  planet,  instead  of  being  in  the  zenith,  be  in  the 
horizontal  line  S  A,  perpendicular  to  the  line  Z  S,  its  dis- 
tance, T,  from  the  centre  of  the  earth  is  the  same  as  its 
distance  T  P.  But  the  place  of  the  planet,  seen  from  the 
centre  of  the  earth,  is  at  d,  while  its  place  seen  from  S, 
or  the  surface,  is  at  A  ; — the  difference  between  these  two 
situations  is  their  parallax. 

Let  us  now  compare  these  two  points  or  situations  with 
the  point  Z,  where  the  planet  is  seen,  when  in  the  zenith 
of  the  observer.  The  c\ng\e  Z  Sg,  formed  by  the  vertical 
line  8  Z,  and  the  line  S  A,  in  which  the  planet  appears,  is 
the  apparent  distance  of  the  planet  from  the  zenith  :  but 
if  you  were  at  the  centre  of  the  earth,  the  angle,  Z  Tg, 
would  show  the  true  distance  from  the  zenith. 

The  apparent  distance,  ZS  g,  is  greater  than  the  true 
distance  Z  Tg,  in  the  right  angled  triangle  gT  S.  Geo- 
metry proves,  that  the  angle,  Z  Sg,  is  equal  to  the  two 
angles  S  Tg,  S  g  T.  It  is  therefore  greater  than  the  angle 
S  Tg,  by  the  quantity  Sg T.  Thus  the  apparent  distance 
of  the  planet  from  the  zenith,  is  greater  than  the  true  dis- 
tance ;  and  the  difference  between  these  two  angles  SgT, 
is  the  parallactic  angle,  which  is  in  this  case  called  the  ho- 
rizontal parallax,  the  line  S  T  being  the  base. 

The  parallax  of  a  celestial  body  is  then  the  angle  formed 
at  the  centre  of  the  body  by  two  lines,  one  of  which  pro- 
ceeds to  the  centre  of  the  earth,  the  other  to  its  surface  ; 
or  it  is  the  inclination  of  two  lines  which  proceed  the  one 


OF    PARALLAX,  117 

from  the  centre,  the  other  from  the  surface  of  the  earth, 
to  unite  in  the  centre  of  the  planet ;  or  still,  in  other  words, 
it  is  the  angle  under  which  the  semidiameter  of  the  earth 
would  appear,  if  seen  from  the  centre  of  the  planet. 

The  triangle,  T  S  £,  is  called  the  parallactic  triangle  ; 
it  is  always  situate  vertically,  because  the  line,  S  T,  is  a 
vertical  line ;  thus  the  whole  effect  of  parallax  is  made 
in  a  vertical  circle ;  indeed,  as  the  centre  of  the  earth  is 
under  your  feet,  it  is  in  the  plane  of  all  the  vertical  cir- 
cles. Therefore,  parallax  is  always  reckoned  on  these 
circles,  making  the  object  appear  lower,  but  never  to  the 
right  or  left  of  a  vertical  circle  ;  consequently,  the  paral- 
lax does  not  change  the  azimuth  of  a  planet. 

I  have  hitherto  spoken  only  of  the  parallax  when  the 
planet  is  in  the  horizon,  that  is,  when  Z  S  g  is  a  right 
angle,  and  I  have  called  this  the  horizontal  parallax.  If 
the  planet  be  nearer  the  zenith,  as  at  r,  the  parallactic 
angle  becomes  smaller,  and  is  called  the  parallax  of  alti- 
tude. It  is  evident  by  the  diagram,  that  the  horizontal 
parallax  is  the  greatest  of  all,  and  that  as  the  planet  rises 
above  the  horizon,  it  gradually  diminishes  until  it  come 
to  the  zenith,  where  it  vanishes,  or  becomes  equal  to  no- 
thing. Thus  the  parallax,  A  G  D,  of  the  object  G,  is 
greater  than  the  parallax,  a  R  B,  of  the  same  object  when 
at  R  ;  but  when  it  is  at  O,  in  the  zenith,  there  is  no  pa- 
rallax. 

The  parallax  of  a  planet  is  smaller  in  proportion  as  it 
is  more  distant ;  for  the  nearer  g  is  to  S,  the  greater  is 
the  angle  S  g  T  ;  hence  mathematicians  prove,  that  when 
the  altitudes  are  the  same,  the  parallax  of  altitude  is  in 
the  inverse  ratio  of  the  distance. 

The  horizontal  parallax  of  the  moon,  which  exceeds 
that  of  all  the  other  planets,  scarce  ever  amounts  to  a  de- 
gree. 

The  parallax  of  a  planet  increases  also  with  its  appa- 
rent diameter ;  in  fact,  the  farther  a  planet  is  off,  the  less 
is  its  apparent  diameter,  and  the  diameter  diminishes  like 
the  parallax  in  an  inverse  ratio  of  the  distance  ;  therefore 
the  parallax  is  as  the  diameter.  If  the  parallax  were  les- 
sened one  half,  the  diameter  would  also  be  one  half  less ; 
and  the  same  relation  subsists,  whatever  be  the  distance. 


118  OF    PARALLAX. 

Thus,  the  apparent  diameter  of  the  moon  is  always  A.  of 
its  parallax,  and  the  cube  of  this  fraction,  marks  its 
size  with  respect  to  the  earth. 

When  the  horizontal  parallax  of  a  celestial  object  is 
known,  it  is  easy  to  discover,  by  the  rules  of  trigonome- 
try, the  distance  of  the  object ;  for  in  the  right-angled  tri- 
angle S  T  G,  you  have  the  semidiameter  of  the  earth,  ST, 
known,  the  angle,  S  T  D,  90  degrees,  and  the  parallactic 
angle,  TGS,  given,  from  whence  it  is  easy  to  obtain 
the  rest.  It  is,  indeed,  difficult  to  determine  the  hori- 
zontal parallax  with  accuracy,  on  account  of  the  effects 
of  refraction.  But  the  parallax  of  an  object  at  any  al- 
titude being  observed,  its  horizontal  parallax  may  be 
computed. 

The  diurnal  parallax  of  an  object  according  to  the 
different  situation  of  the  ecliptic  and  equator  in  respect 
to  the  zenith,  will  sometimes  cause  an  apparent  change 
or  parallax'of  the  latitude,  longitude,  declination,  and 
right  ascension  thereof. 

In  finding  the  parallax  of  the  sun,  or,  which  is  the 
same  thing,  the  angle  under  which  the  earth's  semidia- 
meter would  appear  at  that  distance,  the  angle  is  so  very 
small,  that  a  mistake  of  one  second  would  occasion  an 
error  of  about  seven  millions  of  miles  in  the  distance  ; 
from  whence  you  may  judge  of  the  exactness  necessary 
in  finding  the  parallax  of  any  celestial  object. 

The  annual  parallax  is  the  change  in  the  apparent 
place  of  an  object,  which  is  caused  by  its  being  viewed 
from  the  earth  in  different  parts  of  its  orbit. 

The  annual  parallax  of  all  the  planets  is  very  consi- 
derable, that  of  the  fixed  stars  insensible. 

The  sun's  parallax  being  so  small  as  to  be  scarcely 
Sensible  to  the  best  observers,  when  using  the  most  ac- 
curate instruments,  various  indirect  methods  have  been 
proposed  :  of  these,  that  suggested  by  Dr.  Halley  is  al- 
lowed to  be  the  most  perfect.  It  was  to  observe  the 
transit  or  passage  of  Venus  over  the  sun's  disk ;  a  phe- 
nomenon which  happened  in  the  years  1761  and  1769, 
and  by  which  this  difficult  problem  was  resolved  with 
an  accuracy  unlooked  for  by  astronomers  of  ancient 
times. 


t     119     ] 


OF    THE    APPARENT    MOTION    OF    THE    FIXED    STARS, 
OCCASIONED    BY    THE    ABERRATION    OF    LIGHT. 

The  astronomers  of  the  last  century,  in  their  endea- 
vours to  discover  the  parallax  of  the  fixed  stars,  found 
annual  variations  in  the  stars,  following  a  law  contrary 
to  what  would  have  happened,  had  it  arisen  merely  from 
the  earth's  situation  in  its  orbit. 

These  variations  threw  them  into  great  perplexity, 
from  which  they  were  not  relieved  till  Dr.  Bradley,  re- 
applying himself  to  observe  accurately  these  variations, 
at  last  discovered  the  true  cause  thereof;  and  has  given 
rules  for  calculating  the  changes,  and  shown  what  al- 
lowances are  to  be  made  in  consequence  thereof,  in  ob- 
servations of  the  stars. 

He  has  also  proved  clearly,  that  this  aberration  of  the 
fixed  stars,  or  the  motion  which  makes  them  appear  to 
describe  ellipses  of  40  seconds  diameter,  arises  from  the 
motion  of  light  combined  with  the  annual  motion  of 
the  earth. 

This  I  shall  now  endeavour  to  explain,  and  place  in  as 
clear  a  point  of  view  as  possible,  desiring  you  only  to  recol- 
lect the  idea  of  the  decomposition  of  forces  into  parallelo- 
grams, as  explained  in  our  Lectures  on  Mechanics.  Let 
E,  plate  15,  fig.  1,  be  a  star  darting  a  ray  of  light, 
which  I  shall  consider  here  as  a  single  particle,  going 
from  E  to  B.  Let  A  B  be  a  small  portion  of  the  earth's 
orbit,  of  20  seconds,  for  example  ;  and  C  B  the  space 
that  the  ray  of  light  has  passed  through,  while  the  earth 
moved  from  A  to  B ;  thus,  the  particle  was  at  C,  when 
the  earth  was  at  A,  and  arrives  at  B  the  same  time  as  the 
earth.  Hence  C  B  and  A  B  express  the  velocity  of 
light  and  the  earth  respectively  during  20  seconds. 

Draw  C  D  parallel  to  A  B,  and  finish  the  parallelo- 
gram DBA;  now,  according  to  the  known  principle 
of  the  composition  and  decomposition  of  forces,  we  may 
consider  the  velocity,  E  B,  of  the  light,  as  resulcing  from 
the  two  velocities  in  the  directions  CD,  CA  ;  the  velo- 
city, C  D,  being  the  same  in  quantity  and  direction  as 
the  velocity,  A  D,  of  the  earth,  cannot  be  perceived,  and 


120  OF    THE    APPARENT    MOTION 

is  therefore  destroyed  with  respect  to  us  ;  since  the  eye 
cannot  see  by  a  ray  moving  in  the  same  direction  and 
with  the  same  velocity  as  the  eye  itself.  So  that  the 
part,  C  A,  only  of  the  velocity  of  the  light  will  subsist 
to  us,  and  the  ray  coming  to  the  eye  in  the  direction 
C  A,  we  shall  perceive  the  star  in  the  line  A  C,  or,  ac- 
cording to  B  D,  which  is  parallel  thereto  ;  the  angle,  C 
BD,  is  that  termed  the  aberration;  it  is  the  quantity  that 
a  star  appears  out  of  its  true  place,  in  consequence  of 
the  motion  of  light  and  the  earth. 

Perhaps  another  way  of  considering  this  may  render 
it  more  clear  to  your  apprehension.  Suppose  a  tube  to 
be  erected  perpendicular  to  the  horizon  at  a  time  when 
it  rains,  the  drops  to  fall  in  a  perpendicular  direction, 
and  the  tube  to  be  of  such  a  diameter  as  to  admit  but 
one  drop  at  a  time ;  now  it  is  plain,  that  if  a  drop  of 
water  enter  the  orifice  of  the  tube,  it  will  fall  down 
without  touching  the  sides.  But  if  the  tube  be  moved 
along,  still  preserving  its  perpendicular  direction,  any 
drop  that  enters  the  tube  will  strike  against  the  sides, 
and  none  could  pass  freely  through  while  the  tube  is.  in 
motion,  unless  the  tube  have  such  a  direction  as  will 
compensate  the  motion. 

Thus,  let  AB,  plate  15,  fig.  2,  represent  the  hori- 
zon, C  D  the  perpendicular  tube,  and  G  D  the  course 
of  a  drop  of  rain ;  then,  if  C  D  be  moved  towards  A, 
while  the  drop  is  falling  within  the  tube,  it  is  evident, 
that  the  inner  surface  of  the  tube,  which  is  situate  towards 
B,  will  be  carried  against  the  drop,  and  prevent  its  arriv- 
ing at  the  bottom  without  touching.  But  if  the  inclined 
tube  be  moved  with  a  similar  motion  to  that  of  the  drop 
from  E  to  D,  in  the  same  time  that  the  drop  moves 
from  C  to  D,  the  lower  orifice  of  the  tube  and  the  drop 
will  be  found  at  the  same  instant  at  D,  and  the  velocity 
of  the  drop  will  be  expressed  by  C  D,  and  that  of  the 
tube  by  E  D. 

The  same  reasoning  holds  good,  if  instead  of  drops  of 
rain  we  suppose  particles  of  light,  and  a  telescope  instead 
of  a  tube.  For  to  an  observer,  who,  through  the  tube 
C  D,  views  the  vastly  distant  object  C,  if  the  motion  of 
light  be  instantaneous  or  infinitely  swift,  no  finite  mo- 


OF    THE    FIXED    STARS.  121 

tion  of  C  D,  its  position  being  unaltered,  can  prevent 
its  being  visible  ;  because  by  the  supposition  the  light, 
which  enters  at  C,  will  arrive  at  D  before  C  D  can  have 
moved  at  all. 

But  if  light  be  propagated  in  time,  and  the  observer 
be  carried  by  a  motion  similar  as  to  acceleration  to  that 
of  light,  the  tube  must  be  inclined  in  an  angle,  whose 
sine  is  to  the  sine  of  C  E  D,  as  the  velocity  of  the  ob- 
server is  to  the  velocity  of  light. 

By  this  theory,  which  is  established  by  numerous  ob- 
servations of  stars  of  different  magnitudes  and  situa- 
tions, it  appears,  that  the  small  apparent  motion,  which 
the  fixed  stars  have  about  their  real  places,  which  is 
called  their  aberration,  arises  from  the  proportion  which 
the  velocity  of  the  earth's  motion  in  her  orbit  bears  to 
that  of  light. 

This  proportion  is  found  to  be  as  10210  to  1  ;  from 
whence  it  follows,  that  light  moves  or  is  propagated 
from  the  sun  to  the  earth,  in  8  minutes,  12  seconds. 

This  discovery  of  the  aberration  of  light  by  Dr. 
Bradley ',  is  a  direct  proof  of  the  motjion  of  the  earth 
in  its  orbit.  The  motion  of  light,  combined  with  the 
motion  of  the  earth,  produces  an  apparent  difference  in 
the  places  of  the  fixed  stars  ;  and  as  this  motion  is  found 
to  affect  all  the  stars  differently,  according  to  their  situ- 
ations, it  fully  proves  the  truth  of  the  cause  upon  which 
they  were  supposed  to  depend,  and  shows  that  the  Co- 
pernican  system  is  conformable  to  the  nature  and  order 
of  things. 


OF    THE    PRECESSION    OF    THE    EQUINOXES. 

The  stars,  which  compose  the  constellations,  are  found 
to  increase  their  longitude  continually.  The  whole 
starry  firmament  appears  to  have  a  slow  motion,  from 
west  to  east,  about  the  poles  of  the  ecliptic,  so  that  the 
constellations  seem  to  have  deserted  the  places  first  ap- 
propriated to  them  ;  insomuch  that  the  first  star  in  the 
constellation  of  Aries,  which  appeared  in  the  vernal  in- 
VOL.  iv.'  "~~  '  R 


*22  PRECESSION    OF    THE    EQUINOXES. 

tersection  of  the  equator  and  ecliptic  in  the  time  of  Me- 
ton,  the  Athenian,  upwards  of  1900  years  ago,  is  now 
removed  above  SO  degrees  from  that  point ;  so  that 
Aries  is  now  where  Taurus  was,  Taurus  where  Gemini 
was,  &c.  The  discovery  of  this  motion  is  due  to  Hip- 
parchus  of  Rhodes,  one  of  the  most  celebrated  astrono- 
mers of  ancient  times. 

Hence  the  constellations  on  the  zodiac  of  a  celestial 
globe,  do  not  agree  in  figure  and  character,  the  signs  or 
constellations  of  the  zodiac  being  to  the  east  of  those 
signs,  or  arcs  of  the  ecliptic,  which  are  called  by  the 
same  names  ;  for,  in  order  to  avoid  confusion,  astrono- 
mers thought  proper  to  let  the  several  portions  of  the 
ecliptic,  where  those  constellations  were  first  observed  to 
be,  retain  their  old  names,  consequently,  the  vernal  equi- 
nox is  still  considered  as  the  first  point  of  Aries. 

The  spaces  formerly  occupied  by  the  zodiacal  con- 
stellations, retaining  their  ancient  names,  are  called  ana* 
stra,  or  without  their  former  stars;  whereas  the  spaces 
they  now  possess  are  called  stellata. 

This  slow  motion  of  the  stars  forwards,  is  really  caus- 
ed by  a  like  slow  motion  of  the  equinoxial  points  back- 
wards; and  this  is  owing  to  the  revolution  of  the  axis 
of  the  equator  about  the  axis  of  the  ecliptic  ;  the  pole 
of  the  equator  describing  in  the  heavens  a  circle  about 
the  pole  of  the  ecliptic. 

By  this  precession  of  the  equinoxial  points  from  east 
to  west,  they  meet  the  sun  every  year  50  seconds  of 
longitude  before  a  complete  revolution  has  been  made. 
The  time,  in  which  the  sun  appears  to  revolve  from  tro- 
pic to  tropic,  is  called  a  tropical  year ;  this,  with  the  time 
he  has  yet  further  to  go  to  complete  the  revolution, 
namely,  50  seconds,  is  called  the  siderial  year.  Sir  Isaac 
Newton  attributes  this  motion  to  the  spheroidal  figure 
of  the  earth,  deducing  from  this  figure  the  revolution  of 
the  poles  of  the  world  round  those  of  the  ecliptic. 

This  motion  carries  the  stars  about  1  degree,  20  mi- 
nutes, 23  seconds,  in  100  years  ;  so  that  the  total  revo- 
lution of  the  fixed  stars  eastward,  back  to  the  equinoc- 
tial points  again,  will  be  completed  in  25972  years. 


C      123      ] 


LECTURE  XLIL 


OF    SOLAR    AND    SIDERIAL    DAYS  ;    OF    MEAN    TIME  ; 
THE    EQUATION    OF    TIME,  &C. 


IHE  rotation  of  the  earth  about  its  axis  being  uni- 
form, it  necessarily  follows,  that  the  apparent  diurnal 
revolution  of  the  stars  about  the  earth  must  be  also  uni- 
form, that  is,  made  in  equal  times  ;  they  therefore  will 
form  a  very  proper  measure  to  denote  time.  But  then, 
as  they  turn  successively  with  a  constant  motion,  one 
must  be  selected,  by  whose  revolutions  time  may  be 
measured ;  we  must  also  fix  a  term  from  whence  to 
commence  our  reckoning. 

The  sun  being  the  most  conspicuous  object,  was  fixed 
upon  by  the  astronomers  of  early  ages,  as  the  most  pro- 
per measure  for  the  parts  of  time.  But  when  more  ac- 
curate observations  were  made,  the  sun's  motion  was 
found  not  to  be  uniform,  and  consequently  the  time 
measured  thereby  would  be  neither  regular  nor  equal ; 
they  were  therefore  obliged  to  find  out  a  mean  or  regular 
time  for  the  basis  of  their  calculations. 

An  astronomical  or  solar  day  is  divided  into  24  hours, 
reckoning  them  in  numeral  succession,  from  1  to  24. 
The  first  twelve  hours  are  sometimes  distinguished  by 
the  mark  P.  M.  for  after  noon  ;  the  other  twelve  are 
distinguished  by  A.  M.  for  before  noon.  Astronomers 
generally  reckon  through  the  24  hours  from  noon  to 
noon  ;  and  what  is  by  the  common  way  of  reckoning 
called  morning  hours,  is  by  them  reckoned  in  succes- 


124  SOLAR    AND    SIDERIAL    DAYS. 

sion  from  noon  to  midnight.  Thus  5  o'clock  in  the 
morning  of  April  the  10th,  is  by  astronomers  called 
April  9,  17  hours. 

If  the  sun  had  no  other  apparent  motion  but  that  of 
its  diurnal  revolution,  it  would  every  day  describe  the 
same  parallel,  and  be  accompanied  by  the  same  stars. 
But  it  has  also  an  apparent  annual  motion,  by  which  it 
seems  to  be  carried  through  the  zodiac  every  year,  from 
west  to  east,  that  is,  in  a  direction  contrary  to  that  of 
its  diurnal  revolution. 

Hence,  if  on  any  day  the  sun  and  a  star  pass  the  me- 
ridian at  the  same  instant,  on  the  next  day  when  the 
star  returns  to  the  meridian,  the  sun  will  have  departed 
towards  the  east,  as  much  space  as  in  that  interval  it 
has  passed  over  by  its  annual  motion,  and  will  there- 
fore arrive  at  the  meridian  some  moments  after  the  star  ; 
the  day  following  it  will  be  still  later,  so  that  at  the  end 
of  six  months,  it  passes  12  hours  after  the  star,  which 
has  therefore  gained  1 2  hours  on  the  sun  ;  and  at  the 
end  of  the  year  the  star  will  have  passed  366  times  over 
the  meridian,  while  the  sun  has  only  passed  365  times. 

In  this  view  we  have  considered  the  sun's  apparent 
motion ;  the  result  is  the  same  if  you  consider  the  earth's 
real  motion.  If,  indeed,  the  earth  had  no  real  motion, 
and  consequently  the  sun  no  apparent  motion,  the 
length  of  a  natural  day  would  be  about  23  hours  56 
minutes,  for  in  that  time  a  revolution  of  the  earth  is 
completed,  as  appears  by  an  easy  observation  ;  for  any 
fixed  star  that  is  on  the  meridian  at  a  given  hour  of  the 
night,  will,  after  23  hours  56  minutes,  be  on  the  meri- 
dian again  the  night  following.  This  interval  of  time 
is  called  a  siderial  day. 

Thus  you  see  that  there  is  a  distinction  between  a 
solar  day  and  a  siderial  day. 

A  solar  or  astronomical  day  is  the  space  of  time  that 
intervenes  between  the  sun's  departing  from  any  one 
meridian,  and  its  return  to  the  same  again.  The  side- 
rial  day  is  the  space  of  time  that  elapses  between  the 
departure  of  a  star  from  a  given  meridian,  and  its  return 
to  the  same  again. 


SOLAR    AND    SIDERIAL    DAYS.  125 

I  shall  now  endeavour  to  show  you,  why  these  days 
differ  in  length ;  that  is,  why  the  sun  takes  up  more 
time  to  complete  one  revolution  than  a  star. 

This  difference  arises  from  the  sun's  annual  motion. 
The  sun  does  not  continue  always  in  the  same  place 
in  the  heavens  as  the  fixed  stars  do :  but  if  it  be  seen 
at  M,  plate  4,  jig.  2,  one  day,  near  the  fixed  star  R,  it 
will  have  shifted  its  place  the  next  day,  and  will  be 
near  to  some  other  fixed  star  L.  This  motion  of  the 
sun  is  from  west  to  east,  and  one  entire  revolution  is 
completed  in  a  year.  Suppose,  therefore,  that  the  sun, 
when  it  is  at  M,  near  to  the  fixed  star,  R,  appears  in  the 
south  of  any  particular  place,  S  ;  and  then  imagine  the 
earth  to  turn  once  round  upon  its  axis  from  west  to 
east,  or  in  the  direction  S  T  V  W,  so  that  the  place  may 
be  returned  to  the  same  situation.  After  this  rotation 
is  completed,  the  star  R,  will  be  in  the  south  of  the  place 
as  before  ;  but  the  sun  having,  in  the  mean  time,  mov- 
ed eastwards,  and  being  nearer  to  the  star  L,  or  to  the 
east  of  R,  will  not  be  in  the  south  of  the  place  S,  but 
to  the  eastward  of  it :  upon  this  account  the  place,  S, 
must  move  on  a  little  farther,  and  must  come  to  T  be- 
fore it  will  be  even  with  the  sun  again,  or  before  the 
sun  will  appear  exactly  in  the  south. 

This  may  be  illustrated  by  an  instance  :  the  two 
hands  of  a  watch  are  close  together,  or  even  with  one 
another  at  twelve  ;  they  both  turn  round  the  same  way, 
but  the  minute-hand  turns  round  in  a  shorter  time  than 
the  hour  hand  ;  when  the  minute-hand  has  completed 
one  rotation,  and  is  come  round  to  twelve,  the  hour- 
hand  will  be  before  it,  or  will  be  at  one  ;  so  that  the 
minute-hand  must  move  more  than  once  round,  in  order 
to  overtake  the  hour-hand,  and  be  even  with  it  again. 

As  this  subject  is  of  some  importance,  we  shall  en- 
deavour to  render  it  more  clear,  by  placing  it  in  a  dif- 
ferent point  of  view. 

The  diameter  of  the  earth's  orbit  is  but  a  physical 
point,  in  proportion  to  the  distance  of  the  stars ;  for 
which  reason,  and  the  earth's  uniform  motion  on  its 
axis,  any  given  meridian  will  revolve  from  any  star  to 
the  same  star  again,  in  every  absolute  turn  of  the  earth 


126  SOLAR    AND    SIDERIAL    DAYS. 

upon  its  axis,  without  the  least  perceptible  difference 
of  time  being  shown  by  a  clock  which  goes  exactly 
true. 

If  the  earth  had  only  a  diurnal,  without  an  annual 
motion,  any  given  meridian  would  revolve  from  the  sun 
to  the  sun  again,  in  the  same  quantity  of  time  as  from 
any  star  to  the  same  star  again  ;  because  the  sun  would 
never  change  his  place  with  respect  to  the  stars.  But, 
as  the  earth  advances  almost  a  degree  eastward  in  its 
orbit,  in  the  time  that  it  turns  eastward  round  its  axis, 
whatever  star  passes  over  the  meridian  on  any  day  with 
the  sun,  will  pass  over  the  same  meridian  on  the  next 
day,  when  the  sun  is  almost  a-degree  short  of  it,  that  is, 
3  minutes  56  seconds  sooner.  If  the  year  contained 
only  360  days,  the  sun's  apparent  place,  so  far  as  his 
motion  is  equable,  would  change  a  degree  every  day, 
and  then  the  siderial  days  would  be  just  four  minutes 
shorter  than  the  solar. 

Let  ABCDEFGH,  plate  4,  fig.  3,  be  the  earth's 
orbit,  in  which  it  goes  round  the  sun  every  year,  ac- 
cording to  the  order  of  the  letters,  that  is,  from  west 
to  east,  and  turns  round  its  axis  the  same  way,  from 
the  sun  to  the  sun  again,  in  every  24  hours.  Let  S 
be  the  sun  and  R  a  fixed  star,  at  such  an  immense 
distance,  that  the  diameter,  G  C,  of  the  earth's  orbit 
bears  no  sensible  proportion  to  that  distance ;  N  m  n, 
the  earth  is  in  different  points  of  its  orbit.  Let  N  m  be 
any  particular  meridian  of  the  earth,  and  N  a  given 
point  or  place  lying  under  that  meridian. 

When  the  earth  is  at  A,  the  sun  S  hides  the  star  R, 
which  would  always  be  hid  if  the  earth  never  moved 
from  A  ;  and,  consequently,  as  the  earth  turns  round 
its  axis,  the  point  N  would  always  come  round  to  the 
sun  and  the  star  at  the  same  time. 
.  But,  when  the  earth  has  advanced  through  an  eighth 
part  of  its  orbit,  or  from  A  to  B,  its  motion  round  its 
axis  will  bring  the  point  N  an  eight  part  of  a  day,  or 
three  hours,  sooner  to  the  star  than  to  the  sun.  For 
the  star  will  come  to  the  meridian  in  the  same  time  as 
though  the  earth  had  continued  in  its  former  situation 
at  A,  but  the  point  N  must  revolve  from  N  to  n,  before 


SOLAR    AND    SIDERIAL    DAYS.  127 

it  can  have  the  sun  upon  its  meridian.  The  arc,  N  n, 
being  therefore  the  same  part  of  a  whole  circle,  as  the 
arc  A  B,  it  is  plain,  that  any  star  which  comes  to  the 
meridian  at  noon  with  the  sun,  when  the  earth  is  at  A, 
will  come  to  it  at  nine  o'clock  in  the  forenoon,  when 
the  earth  is  at  B. 

When  the  earth  has  passed  from  A  to  C,  one-fourth 
part  of  its  orbit,  the  point,  N,  will  have  the  star  upon  its 
meridian,  or  at  six  in  the  morning,  six  hours  sooner 
than  it  comes  round  to  the  sun  ;  but  the  point,  N,  must 
revolve  six  hours  more,  before  it  has  mid-day  by  the 
sun  :  for  now  the  angle,  A  S  C,  is  a  right  angle,  and 
so  is  N  C  n  ;  that  is,  the  earth  has  advanced  90  degrees 
on  its  axis,  to  carry  the  point  N  from  the  star  to  the 
sun ;  for  the  star  always  comes  to  the  meridian,  when 
N  m  is  parallel  to  R  S  A  ;  because  C  S  is  but  a  point  in 
respect  to  R  S.  When  the  earth  is  at  D,  the  star  comes 
to  the  meridian  at  three  in  the  morning  at  E,  the  earth 
having  gone  half  round  its  orbit ;  N  points  to  the  star 
at  midnight,  it  being  then  directly  opposite  to  the  sun  ; 
and,  therefore,  by  the  earth's  diurnal  motion,  the  star 
comes  to  the  meridian  twelve  hours  before  the  sun ; 
and  then  goes  on,  till  at  A  it  come  to  the  meridian 
with  the  sun  again. 

Thus  it  is  plain,  that  one  absolute  revolution  of  the 
earth  on  its  axis,  which  is  always  completed  when  any 
particular  star  comes  to  be  parallel  to  its  situation  at  any 
time  of  the  day  before,  never  brings  the  same  meridian 
round  from  the  sun  to  the  sun  again ;  but  that  the 
earth  requires  as  much  more  than  one  turn  on  its  axis, 
to  finish  a  natural  day,  as  it  has  gone  forward  in  that 
time,  which  at  a  mean,  is  a  365th  part  of  a  circle,  that 
is  59  minutes  8  seconds  ;  for,  as  365  days  are  to  1  day, 
so  are  360  degrees  to  59  minutes  8  seconds.  Hence, 
in  365  days  the  earth  turns  366  times  round  its  axis, 
and  consequently,  as  one  revolution  of  the  earth  on  its 
axis  completes  a  siderial  day,  there  must  be  one  more 
siderial  day  in  a  year  than  there  are  solar  days. 


L      128     ] 


OF    MEAN    AND    APPARENT    TIME. 

Further  and  more  accurate  observations  showed,  that 
the  solar  days  were  not  equal  to  each  other  ;  after  inves- 
tigating this  subject,  astronomers  were  under  the  neces- 
sity of  distinguishing  two  sorts  of  time,  one  they  called 
apparent  time,  the  other  mean  time. 

Apparent  time  is  that  reckoned  since  the  sun's  centre 
was  last  on  the  meridian  of  the  place,  and  is  that  shown 
by  a  sun-dial,  which  marks  the  hours  every  day  in  such 
a  manner,  that  every  hour  is  a  24th  part  of  the  time  be- 
tween the  noon  of  that  day  and  the  noon  of  the  day  im- 
mediately following. 

Mean  time  is  that  shown  by.  a  clock,  which  goes  uni- 
formly. 

The  time  shown  by  a  sun-dial,  and  the  mean  time, 
or  that  shown  by  a  well-regulated  clock,  agree  only  four 
times  in  the  year  ;  viz.  on  the  1 5th  of  April,  the  1 6th 
of  June,  the  3 1st  of  August,  and  the  24th  of  December. 

The  clock,  if  it  go  equably  and  true  all  the  year 
round,  will  be  before  the  sun  from  the  24th  of  Decem- 
ber to  the  15th  of  April  ;  from  that  time,  to  the  16th 
of  June,  the  sun  will  be  before  the  clock  ;  from  thence, 
to  the  31st  of  August,  the  clock  will  be  again  before 
the  sun  ;  and  from  the  31st  of  August  to  the  24th  of 
December,  the  sun  will  be- faster  than  the  clock.  On 
any  other  day,  if  you  would  set  a  clock  by  a  sun-dial, 
you  must  make  use  of  an  equation-table,  which  shows, 
for  every  day  in  the  year,  how  many  minutes  and  se- 
conds the  sun  is  before  or  behind  the  clock :  the  dif- 
ference between  the  sun  and  the  clock  is  called  the 
equation  of  time.* 

Both  the  solar  and  mean  days  are  divided  into  24 
hours,  or  86400  seconds. 

Three  hundred  and  sixty  degrees  of  the  equator  pass 
under  the  meridian  in  a  mean  day,  and  59  minutes  8 
seconds,  which  is  that  part  of  360  degrees  of  the  sun's 


*  See  my  pamphlet  u  Metnods  of  finding  a  true  Meridian  Line  for 
placing  Sun  Dials,  setting  Clocks,  &c. — E.  Edit. 


THE    EQUATION    OF    TIME.  12$ 

annual  motion  corresponding  to  the  time  of  a  mean 
day. 

In  a  solar  or  true  day,  the  360  degrees  of  the  equator 
pass  under  the  meridian  and  an  arc  thereof  answering 
to  the  ecliptic  arc  described  the  same  day,  called  the 
sun's  motion  in  right  ascension. 

When  the  sun  is  farthest  from  the  earth  or  in  apogee, 
his  motion  in  right  ascension  in  a  day,  is  1  degree,  2 
minutes,  6  seconds  ;  therefore  361  degrees,  2  minutes, 
6  seconds,  pass  the  meridian  in  a  solar  day.  By  work- 
ing  this  proportion,  as  360  degrees,  59  minutes,  8  se- 
conds, is  to  24  hours,  so  is  361  degrees,  2  minutes, 
6  seconds,  we  find  24  hours,  O  minutes,  12  seconds. 
Consequently,  when  the  sun  is  in  apogee,  the  solar  day 
is  12  seconds  longer  than  the  mean  day.  From  hence 
it  follows  : 

1 .  That  in  every  second  of  a  clock  well-regulated  to 
mean  time,  an  arc  of  15  minutes  28  seconds  of  the 
equator  passes  the  meridian  ;  for  this  is  the  quotient  of 
360  degrees,  59  minutes,  8  seconds,  divided  by  86400 
seconds. 

2.  That  a  star's  revolution  answers  to  360  degree  of 
the  equator,  while  the  mean  day  answers  to360  degrees, 
59  minutes,  8  seconds.  This  difference  of  59  minutes 
8  seconds,  being  reduced  to  time,  gives  three  minutes 
56  seconds :  therefore,  the  stars  anticipate  3  minutes 
56  seconds,  every  day  on  mean  time  ;  or,  which  is 
the  same  thing,  a  star's  diurnal  revolution  is  made  in 
23  hours,  56  minutes,  4  seconds. 

To  find  whether  a  clock  be  well  regulated  to  mean 
time,  observe  if  it  show  exactly  23  hours,  56  minutes, 
4  seconds,  from  the  instant  of  any  star's  passage  through 
a  fixed  point,  to  that  of  its  return  to  the  same  point. 
By  what  the  clock  exceeds  this,  it  is  faster,  by  what  it 
wants  thereof,  it  is  slower  than  mean  time. 

OF    THE    EQUATION    OF    TIME. 

I  have  already  observed  to  you,  that  the  equation  of 
time  is  the  difference  between  mean  and  apparent  time, 
or  that  pointed  out  by  a  good  clock,  and  by  a  sun-diai 
respectively. 
Vol.  IV.  S 


130  THE    EQUATION    OF    TIME. 

You  will  soon  perceive  that  there  would  have  been  no 
difference,  and  consequently  no  need  for  any  equation, 
1st,  if  the  earth's  orbit  had  been  a  perfect  circle  with  the 
sun  at  the  centre  ;  2dly,  if  the  earth  had  moved  through 
an  equal  part  or  portion  of  that  circle  every  day;  and  3dly, 
if  the  axis  of  her  diurnal  motion  were  always  perpendi- 
cular to  the  plane  of  her  orbit.  But  neither  of  the  fore- 
going suppositions  is  true ;  for,  1.  the  orbit  of  the  earth 
is  an  ellipse  ;  2.  her  motion  therein  is  not  equable  ;  and 
3.  her  axis  is  inclined  to  the  plane  of  her  orbit :  the  mea- 
sure of  time  therefore,  as  far  as  it  depends  on  these  cir- 
cumstances, must  be  unequal  and  subject  to  an  equation. 

The  equation  of  time  may  then  be  considered  as  aris- 
ing, 1.  from  the  obliquity  of  the  ecliptic  to  the  equator  ; 
2.  from  the  unequal  progression  of  the  earth  through  her 
elliptic  orbit. 

Of  the  first  cause  of 'inequality ',  or  that  arising  from  the 
obliquity  of  the  equator  to  the  ecliptic.  The  motion  of  the 
earth  on  its  axis  is  perfectly  equable,  or  always  at  the  same 
rate;  and,  the  plane  of  the  equator  being  perpendicular 
to  its  axis,  it  is  evident,  that  in  equal  times  equal  portions 
of  the  equator  will  pass  over  the  meridian  ;  and  so  also 
would  equal  portions  of  the  ecliptic,  if  it  were  either  pa- 
rallel to,  or  coincident  with  the  equator. 

But,  as  the  ecliptic  is  oblique  to  the  equator,  the  equa- 
ble motion  of  the  earth  carries  unequal  portions  of  the 
ecliptic  over  the  meridian  in  equal  times,  the  difference 
being  proportionate  to  the  obliquity ;  and,  as  some  parts 
of  the  ecliptic  are  more  oblique  than  others,  those  differ- 
ences are  unequal  among  themselves.  If,  therefore,  we 
should  suppose  two  suns  to  start  from  the  beginning  either 
of  Aries  or  Libra,  and  continue  to  move  through  equal 
arcs  in  equal  times,  one  in  the  equator,  the  other  in  the 
ecliptic,  the  equatorial  sun  would  always  return  to  the 
meridian  in  24  hours  time  as  measured  by  a  good  clock, 
but  the  sun  in  the  ecliptic  would  return  to  the  meridian 
sometimes  sooner,  sometimes  later,  than  the  equatorial 
sun,  and  only  the  same  instant  with  him  on  four  days  in 
the  year. 

To  render  this  plainer,  we  shall  have  recourse  to  a  dia- 
gram, plate  4,  fig.  4.  This  figure  is  to  be  considered  as 


THE    EQUATION    OF    TIME.  lSt 

a  view  of  part  of  the  concave  sphere  of  the  heavens,  where- 
in D  E  represents  a  part  of  the  celestial  equator,  F  G  a 
part  of  the  ecliptic,  A  the  intersection  of  the  two  circles 
at  the  vernal  equinox,  A  B  a  degree  upon  the  equator.  If 
we  imagine  the  plane  of  the  meridian  to  pass  from  the 
situation  M  M,  into  the  situation  N  N,  in  going  through 
the  arc  A  B,  one  degree  of  the  equator,  it  will  also  go 
through  the  arc,  A  C,  more  than  one  degree  of  the  eclip- 
tic. For  in  the  triangle  ABC,  the  angle,  at  B,  is  a  right 
one,  consequently,  the  hypothenuse,  A  C,  is  the  longest 
side. 

At  the  solstices  the  obliquity  of  the  ecliptic  has  a  con- 
trary effect,  and  helps  to  lengthen  the  natural  days :  this 
will  be  easily  comprehended  by  viewing  the  diagram, 
plate  4,  Jig.  5,  where  T  T  is  part  of  the  tropic  of  Capri- 
corn, C  D  part  of  the  ecliptic,  which  may  be  considered 
as  coincident  with  the  tropic  for  some  distance  on  each 
side  of  the  solstitial  point,  as  from  A  to  B  ;  and  therefore 
meridians,  which  are  perpendicular  to  the  tropics,  may 
be  considered  for  that  space  as  perpendicular  also  to  the 
ecliptic.  This  being  supposed,  a  meridian,  in  going  from 
A  towards  B,  will  go  through  as  large  an  arc  in  the  tro- 
pic as  the  ecliptic  :  but  the  tropic  not  being  a  great  cir- 
cle, any  arc,  as  a  b,  taken  in  both  these  circles,  will  mea- 
sure more  minutes  in  the  tropic  than  in  the  ecliptic,  and 
that  in  the  ratio  as  the  ecliptic  exceeds  the  tropic  in  di- 
mensions :  now,  the  circumference  of  the  ecliptic  is  to 
that  of  the  tropic,  nearly  as  60  to  55  ;  and  therefore  the 
arc  ab,  of  55  minutes  in  the  ecliptic,  will  be  60  minutes 
in  the  tropic.  But  every  meridian  passes  in  the  same 
time  through  similar  arcs  in  the  celestial  equator,  and  all 
circles  parallel  to  the  equator,  as  the  tropic's  arc :  con- 
sequently, at  the  solstices  every  arc  of  the  ecliptic  passed 
through  by  any  meridian  in  a  given  time,  will  be  to  the 
arc  of  the  equator  passed  through  in  the  same  time,  as 
55  to  60. 

The  second  cause  of  the  difference  in  the  time  shown 
by  a  well-regulated  clock,  and  a  true  sun-dial,  arises  from 
the  inequality  of  the  sun's  apparent  motion,  which  is  slow- 
est in  summer,  when  the  sun  is  farthest  from  the  earth, 
and  swiftest  in  winter,  when  he  is  nearest  thereto  ;  where- 


132  THE    EQUATION    OF    TIME. 

asthe  earth's  motion  on  its  axis  is  equable  all  theyear  round. 

If  the  sun's  apparent  motion  in  the  ecliptic  were  equa- 
ble, the  whole  difference  between  the  equal  time  as  shown 
by  the  clock,  and  the  unequal  time  as  shown  by  the 
sun,  would  arise  from  the  obliquity  of  the  ecliptic. 
But  this  is  not  the  case,  for  the  sun's  motion  sometimes 
exceeds  a  degree  in  24  hours,  though  it  is  generally 
less.  And  when  his  motion  is  slowest,  any  particular 
meridian  will  return  and  revolve  sooner  to  him  than  when 
his  motion  is  quickest,  for  it  will  overtake  him  in  less  time 
when  he  advances  through  a  less  space,  than  when  he 
moves  through  a  larger  one. 

On  the  1st  of  January,  the  daily  motion  of  the  sun  in 
the  ecliptic  is  nearly  1  degree,  1  minute,  13  seconds; 
but  on  the  1st  of  July,  the  daily  motion  is  57  minutes, 
13  seconds;  the  medium  of  these  is  59  minutes,  13  se- 
conds. The  sun's  place  in  the  ecliptic,  calculated  on  the 
supposition  of  a  daily  motion  of  59  minutes,  13  seconds, 
will  be  behind  his  observed  place  from  the  beginning  of 
January  to  the  beginning  of  July,  and  will  be  before  k 
from  the  beginning  of  July  to  the  beginning  of  January  £ 
the  greatest  difference  is  about  1  degree,  55  minutes,  32 
seconds,  which  is  observed  about  the  beginning  of  April 
and  of  October,  at  which  times  the  observed  daily  motion 
is  59  minutes,  J  3  seconds. 

It  is  necessary  for  an  astronomer  to  know  both  true  and 
mean  time  ;  the  first,  to  ascertain  the  time  of  observation; 
the  second,  because  the  tables  of  the  planets,  &c.  are  cal- 
culated in  conformity  thereto. 

The  relation  between  true  and  mean  time  is  discovered 
by  observing  the  time  marked  by  your  clock,  at  the  in- 
stant when  the  centre  of  the  sun  passes  the  meridian,  and 
adding  what  it  wants  of  12  hours,  or  subtracting  the  ex- 
cess above  it. 

It  is  obtained  for  any  other  hour  besides  12,  by 
taking  the  difference  between  the  times,  per  clock,  of 
the  sun's  passing  the  meridian  on  the  given  day,  and  on 
the  foregoing  or  following  day  ;  and  applying  a  part  of 
this  difference  proportional  to  the  given  time  past  noon. 
*  Example:  March  3,  when  the  sun's  centre  passed  the 
meridian,  the  clock  was  12  hours,  17  minutes,  49  se- 


THE    EQUATION    OF    TIME.  133 

conds ;  the  clock  was  therefore  17  minutes,  49  seconds, 
faster  than  true  or  apparent  time. 

On  the  4th  of  March,  it  was  1 2  hours,  1 7  minutes,  42* 
seconds;  the  difference  is  6  v  seconds,  or  about  one-fourth 
of  a  second  per  hour.  Now  on  the  3d.  the  planet  Mars 
passed  the  meridian  at  14  hours,  27  minutes,  32  seconds; 
the  clock  was  therefore  S\  seconds  more  advanced  than 
at  noon,  which  gives  its  advance  for  that  hour,  1 7  minutes, 
45\  seconds,  and  this,  subtracted  from  14  hours,  27  mi- 
nutes, 32  seconds,  gives  14  hours,  9  minutes,  46*  se- 
conds, for  the  true  or  apparent  time  of  the  transit  of 
Mars. 

From  what  I  have  now  explained  to  you,  it  appears, 
that  there  is  no  body  in  nature,  whose  motion  is  perfectly 
uniform  and  regular ;  that  whenever  we  look  for  com- 
mensurabilities  and  equalities  in  nature,  we  are  always  dis- 
appointed. The  earth  is  spherical,  but  not  perfectly  so ; 
the  summer  is  unequal  when  compared  with  the  winter  ; 
the  ecliptic  disagrees  with  the  equator,  and  never  cuts  it 
twice  in  the  same  equinoctial  point,  the  orbit  of  the  earth 
has  an  eccentricity,  more  than  double  in  proportion  to  the 
spheroidity  of  its  globe  ;  no  number  of  the  revolutions  of 
the  moon  coincide  with  any  number  of  the  revolutions  of 
the  earth  in  its  orbit;  no  two  of  the  planets  measure  one 
another;  and  thus  it  is  wherever  we  turn  our  thoughts, 
so  different  are  the  views  of  the  Creator  from  our  narrow 
conceptions  of  things  ;  where  we  look  for  commensura- 
tion,  we  find  variety  and  infinity.* 

It  is  scarce  possible  to  refrain  here  from  joining  with  an 
elegant  moralist  in  observing,  that  all  the  appearances  of 
nature  uniformly  conspire  to  remind  us  of  the  lapse  of 
time,  and  the  flux  of  life.  The  day  and  night  succeed 
each  other,  the  rotation  of  the  seasons  diversifies  the 
year,  the  sun  attains  the  meridian,  declines  and  sets,  and 
the  moon  every  night  changes  its  form. 

The  day  may  be  considered  as  an  image  of  the  year, 
and  a  year  as  the  representation  of  life.  The  morning  an- 
swers to  the  spring,  and  the  spring  to  childhood  and 
youth ;  the  noon  corresponds  to  the  summer,  and  the 


*  Jones'*  Physiological  Disquisitions. 


134  THE    EQUATION   OF    TIME. 

summer  to  the  strength  of  manhood  ;  the  evening  is  an 
emblem  of  autumn,  and  autumn  of  declining  life.  The 
night,  with  its  silence  and  darkness,  shows  the  winter,  in 
which  all  the  powers  of  vegetation  are  benumbed;  and  the 
winter  points  out  the  time  when  life  shall  cease,  with  its 
hopes  and  pleasures. 

He  that  is  carried  forward,  however  swiftly,  by  a  mo- 
tion equable  and  easy,  perceives  not  the  change  of  place, 
but  by  the  variation  of  objects.  If  the  wheel  of  life,  which 
rolls  thus  silently  along,  passed  on  in  undistinguishable 
uniformity,  we  should  never  mark  its  approaches  to  the 
end  of  the  course.  If  one  hour  were  like  another  ;  if  the 
passage  of  the  sun  did  not  show  its  wasting  ;  if  the  chan- 
ges of  the  seasons  did  not  impress  upon  us  the  flight  of 
the  year  ;  quantities  of  duration,  equal  to  days  and  years, 
would  glide  away  unobserved.  If  the  parts  of  time  were 
not  variously  coloured,  we  should  never  discern  their  de- 
parture or  succession  ;  but  should  live  thoughtless  of  the 
past,  and  careless  of  the  future,  without  will,  and  perhaps 
without  power,  to  compare  the  time  which  is  already  lost, 
with  that  which  may  probably  remain. 


LECTURE  XLIII. 

ON  THE  PLANETARIUM,  TELLURIAN,  AND  LUNARIUM, 


J  O  represent  by  machines  the  motions  and  various 
aspects  of  the  heavenly  bodies,  the  parallelism  of  the 
earth's  axis,  together  with  its  annual  and  diurnal  motions, 
and  by  these  means  to  explain  the  beautiful  variety  of  sea- 
sons, and  other  terrestrial  and  celestial  phenomena,  has 


THE    PLANETARIUM,    &C.  133 

ever  been  considered  as  one  of  the  noblest  efforts  of  me- 
chanical genius.  Among  the  variety  of  machines  contri- 
ved for  these  purposes,  that  before  you,  and  its  parts,  plate 
1 1,  fig  1,  and  plate  12,  fig.  1  and  2,  is  best  adapted  for 
representing  the  celestial  motions. 

It  seems  highly  probable,  that  the  ancients  were  not  un- 
acquainted with  planetary  machines,  but  that  the  same 
powers  of  genius,  which  led  them  to  contemplate  and  rea- 
son upon  the  heavenly  bodies,  induced  them  to  realize 
their  ideas,  and  form  instruments  for  explaining  them ; 
and  we  may  fairly  presume,  that  these  were  carried  to  no 
small  degree  of  perfection,  when  we  consider,  that  of  which 
Archimedes  was  the  maker,  and  Cicero  the  encomiast. 

A  planetarium  may  be  considered  in  some  sort  as  a  dia- 
metrical section  of  our  universe,  in  which  the  upper  and 
lower  hemispheres  are  suppressed. 

The  upper  plate  is  to  answer  for  the  ecliptic  ;  on  this 
are  placed,  in  two  opposite,  but  corresponding  circles,  the 
days,  of  the  month,  and  the  signs  of  the  ecliptic,  with  their 
respective  characters  ;  by  this  plate  you  may  set  the  pla- 
netary balls  so  as  to  be  in  their  respective  places  in  the 
ecliptic,  for  any  day  in  the  year. 

Through  the  centre  of  this  plate,  you  observe  a  strong 
stem,  on  which  is  a  brass  ball  to  represent  the  sun;  round 
the  stem  are  different  sockets  to  carry  the  arms,  by  which 
the  several  planets  are  supported.  The  planets  are  repre- 
sented by  ivory  balls,  having  the  hemisphere  which  is  next 
the  sun  white,  the  other  black,  to  exhibit  their  respective 
phases.  I  can  with  ease  either  take  off,  or  put  on,  any  of 
the  planets,  as  occasion  may  require.  About  the  primary 
planets  are  placed  the  secondary  planets  or  moons,  which, 
are  in  this  instrument  only  moveable  by  the  hand. 

I  turn  the  handle,  and  all  the  planets  are  put  in  motion, 
moving  round  that  ball  which  represents  the  sun.  Now, 
if  you  take  the  earth's  motion  as  a  standard,  they  move 
with  the  same  relative  velocities  and  periodical  times  that 
they  observe  in  the  heavens.  I  scarcely  need  observe,  that 
it  is  impossible  to  give  an  idea  of  the  proportion  and  dis- 
tances of  the  planets  in  the  compass  of  an  instrument  so 
small  as  that  before  you,  or  indeed  of  any  instrument 
whatsoever. 


136  SOLAR    SYSTEM    EXPLAINED. 

The  motions  are  carried  on  by  a  train  of  wheel-work 
concealed  in  the  brass  box,  ABC,  under  the  ecliptic* 


GENERAL    EXPLANATION    OF   THE    SOLAR  SYSTEM,   BY 
THE    PLANETARIUM. 

As  the  centre  of  the  solar  system  is  the  only  place  from 
which  the  motion  of  the  planets  can  be  truly  seen,  let  us 
suppose  ourselves  situate  at  the  centre  of  the  ball  repre- 
senting the  sun.  In  this  situation  the  heavens  would  ap- 
pear perfectly  spherical,  the  stars  being  so  many  lucid 
points  in  the  concave  surface  of  the  sphere. 

Having  attentively  considered  the  stars  for  a  long  time, 
you  will  remark  two  sorts,  the  one  scattered  throughout 
the  heavens  unequally  luminous,  perfectly  at  rest,  and 
therefore  called  fixed  stars ;  the  other  sort,  moving  round 
the  sun  with  unequal  velocities,  called  planets.  By  taking 
one  of  the  fixed  stars  for  a  point  to  set  out  from,  or  for 
this  purpose  in  our  instrument,  using  any  of  the  points 
into  which  the  ecliptic  is  divided,  it  will  be  easy  to  deter- 
mine the  motions  of  the  planets. 

>  Thus,  by  observing  the  earth  as  I  turn  the  winch,  you 
may  perceive,  that  it  continually  approaches  nearer  and 
nearer  to  the  more  eastern  signs ;  in  a  certain  space  of 
time,  it  will  return  to  the  place  from  whence  it  set  out. 

Thus  you  see  how  readily  the  periods  of  the  planets' 
revolutions  may  be  obtained,  by  observing  the  time  that 
elapses  between  their  setting  out  from  any  fixed  point,  and 
returning  to  the  same  again.  The  annual  motion  of  the 
earth  is  the  basis  or  standard,  with  which  the  motions  of 
the  other  planets  are  compared  ;  and  this  is  one' of  the 
reasons,  why  the  months  and  days  of  the  months  are  en- 
graved on  the  ecliptic  circle  of  the  planetarium. 

The  curves,  which  the  planets  describe  in  their  revdlu- 
tions,  are  called  their  orbits. 

If  the  paths  of  the  planets  were  in  one  plane,  as  in  this 
instrument,  they  would  all  be  referred  to  one  circle  in  the 


*  This  complrte  instrument  was  contrived  by  the  late  Mr.  B.  Martin; 
as  it,  in  my  opinion,  deserves  a  fuller  explanation,  I  shall  give  one  in  my 
Appendix  to  this  leciiux* E.  Edit. 


BY    THE    PLANETARIUM.  137 

heavens  ;  but  this  is  not  the  case,  for  their  paths  cross 
each  other  in  different  parts  of  the  heavens. 

When  you  consider  the  motions  of  the  little  system 
before  you,  while  you  are  supposed  to  view  it  from  the 
sun,  all  is  regular ;  but  when  you  view  it  from  the 
earth,  many  of  the  appearances  become  intricate  and 
perplexed.  When  the  works  of  God  are  examined  from 
a  proper  point,  there  is  nothing  but  uniformity,  beauty, 
and  precision,  and  the  heavens  present  you  with  a  plan 
inexpressibly  magnificent,  and  yet  regular  beyond  the 
power  of  invention.  When  properly  examined  and 
looked  into,  you  will  always  find  the  volume  of  the 
universe  perfect  like  its  Author,  containing  mines  of 
truth  for  ever  opening,  fountains  of  good  for  ever  flow- 
ing, being  an  endless  succession  of  brighter  and  still 
brighter  exhibitions  of  the  glorious  Godhead,  always 
answering  the  nature  and  idea  of  infinite  fulness  and 
perfection. 

In  the  centre  of  the  system  is  the  sun,  placed  in  the 
heavens  by  that  Almighty  Power,  who  said,  "  Let  there 
be  light,  and  there  was  light,'*  to  be  the  fountain  of  light 
and  heat  to  all  the  planets  revolving  round  him.  In  this 
machine,  his  situation  is   pointed  out  by  this  brass  ball. 

The  nearest  planet  to  the  sun  is  Mercury;  observe 
the  part  of  the  ecliptic  he  is  at,  and  also  the  place  where 
the  earth  is  situate.  I  now  turn  the  handle,  Mercury  is 
arrived  at  the  place  from  whence  he  set  out,  and  our 
earth  has  gone  over  88  days  of  the  ecliptic  ;  the  velocity 
we  here  give  the  planet  is  inconsiderable,  but  in  his 
course  in  the  heavens  he  is  supposed  to  move  with  a  ve- 
locity equal  to  100,000  miles  in  an  hour. 

Venus  is  the  next  planet  in  the  system  ;  in  the  heavens 
she  is  distinguished  by  the  superiority  of  her  lustre,  ap- 
pearing to  us  the  brightest  and  largest  of  all  the  planets. 
By  observing  her  course  through  the  ecliptic,  and  com- 
paring it  with  the  days  passed  over  by  the  earth  in  the 
same  time,  you  will  find,  in  our  instrument,  Venus  re- 
volving round  the  sun  in  2C25  days  ;  in  the  heavens  she 
moves  at  the  rate  of  80,955  miles  in  an  hour. 

The  third  planet  in  the  solar  system  is  the  Earth;  di- 
minutive as  it  appears  before  you  in  this  instrument,  its 

VOL.  IV.  T 


13H  SOLAR    SYSTEM    EXPLAINED 

real  diameter  is  near  8000  miles ;  it  revolves  round  the 
sun  in  the  space  of  365  days,  into  which  number  the 
brazen  ecliptic  is  divided  ;  this  revolution  constitutes 
our  year,  while  its  revolution  round  its  axis  forms  day 
and  night. 

The  little  ball,  close  and  annexed  to  the  earth,  repre- 
sents the  Moon,  of  which  I  shall  say  nothing  at  present, 
as  there  is  a  part  of  the  instrument  for  explaining  more 
particularly  her  phenomena. 

The  planet  Mars  is  the  next  in  order,  being  the  first 
above  the  earth's  orbit ;  he  revolves  round  the  sun  in 
about  686  days  ;  so  that  our  earth,  as  you  will  observe 
by  the  instrument,  goes  nearly  twice  round,  while  he  is 
performing  his  revolution  ;  he  is  supposed  to  move  at 
the  rate  of  55,783  miles  in  an  hour.  To  this  planet 
our  earth  and  moon  will  appear  like  two  moons,  some- 
times half  or  three  quarters  illuminated,  but  never  full. 

Jupiter ',  the  largest  of  all  the  planets,  is  next  beyond 
Mars;  and  our  earth  must  have  gone  nearly  twelve 
times  round  the  ecliptic  for  one  revolution  of  Jupiter  ; 
yet  so  far  is  its  path  removed  from  the  sun,  that  to  go 
round  it  in  this  space  of  time,  it  moves  at  the  rate  of 
30,193  miles  an  hour.  Though  larger  than  the  earth, 
it  appears  but  small  in  the  heavens,  because,  as  you 
know,  objects  decrease  in  their  apparent  magnitude  in 
proportion  to  their  real  distance.  It  is  attended  by  four 
satellites,  here  represented  by  these  four  balls;  they  are 
invisible  to  the  naked  eye,  but  appear  beautiful  through 
a  telescope. 

Saturn,  the  next  planet,  is  still  higher  in  the  system, 
performing  its  circuit  in  about  thirty  years  of  our  time ; 
so  that  in  this  instrument  its  motion  is  scarcely  sensible, 
while  in  the  heavens  it  goes  at  the  rate  of  22,298  miles 
an  hour,  it  is  accompanied  by  five  satellites,  and  a  large 
luminous  ring,  here  represented  by  this  ivory  circle,  and 
which  is  one  of  the  most  curious  phenomena  of  nature. 

The  Georgium  Sidus,  or  Georgian  planet,  so  called  in 
compliment  to  his  Majesty,  King  George  the  Third,  the 
Royal  patron  and  promoter  of  the  arts  and  sciences,  is 
the  seventh  planet  in  our  system ;  it  is  near  twice  the 
distance  of  Saturn  from  the  sun,  round  which  it  revolves 


BY   THE    PLANETARIUM.  139 

in  about  eighty  years.     Dr.  Herschel  has  discovered  six 
satellites  to  this  planet. 

To  explain,  by  the  planetarium,  why  the  sun,  being  a  fixed 
body,  appears  to  pass  through  all  the  signs  of  the  zodiac 
in  one  year  :  also  showing,  that  this  phenomenon  is  occa- 
sioned by  the  annual  motion  of  the  earth. 

As  the  general  phenomena  of  the  planetary  system 
will  be  best  understood  by  an  induction  of  particulars, 
I  shall  remove  all  the  planets  but  those  whose  motions  I 
am  going  to  explain ;  for  instance,  I  shall  leave  only  the 
earth  and  sun,  and  place  the  earth  over  Libra,  and  it  is 
plain,  that  the  sun  will  then  be  transferred  by  the  eye  of  • 
a  spectator  on  the  earth  to  Aries,  in  which  sign  it  will 
appear  at  the  latter  end  of  March :  move  the  earth  on 
its  orbit  to  Capricorn,  and  the  sun  will  appear  at  Cancer 
in  June,  seeming  to  have  moved  from  v  to  25,  though  it 
has  not  stirred,  the  real  motion  of  the  earth  having  caused 
the  spectator  to  trasfer  the  sun  to  all  the  intermediate 
points  in  the  heavens,  and  thus  given  it  an  apparent  mo- 
tion. Continue  to  move  the  earth  till  it  arrive  at  Aries, 
and  the  sun  will  be  seen  in  Libra  in  the  month  of  Sep- 
tember  :  moving  the  earth  on  to  Cancer,  the  visual  ray 
of  the  spectator  defers  the  sun  to  Capricorn,  as  it  ap- 
pears in  the  month  of  December.  Lastly,  continue 
moving  the  earth,  and  it  will  arrive  at  Aries,  where  we 
set  out.  Thus  I  have  shown,  that  it  is  the  motion  of 
the  earth  which  causes  the  sun  to  appear  in  all  the  dif- 
ferent signs  of  the  zodiac.  Custom,  indeed,  has  taught 
us  to  say,  the  sun  is  in  Aries,  when  it  is  between  us  and 
Aries,  and  so  of  any  other  sign  ;  whereas  it  would  have 
been  more  proper  to  say,  that  the  earth  is  in  Libra. 

To  show  why,  at  different  times  of  the  year,  we  see  the 
heavens  decorated  with  an  entire  different  collection  of 
stars. 

This  phenomenon  is  occasioned  by  the  earth's  pro- 
gressive or  annual  motion  :  while  the  earth  is  traversing 


140       PHENOMENA  OF  THE  PLANETS. 

its  course  under  the  vast  concave  of  fixed  stars,  we  are 
gradually  carried  under  the  different  constellations. 
From  hence  it  is  evident,  that  at  night,  when  the  earth 
is  turned  from  the  sun,  we  shall  in  succession  have  the 
opportunity  of  viewing  from  time  to  time  all  the  stars 
in  the  zodiac,  and  consequent!)  a  different  constellation 
will  present  itself  every  month. 

Thus,  the  Pleiades  in  Taurus  are  not  visible  in  the 
summer  ;  but  in  the  winter  the  earth  is  between  the 
sun  and  them.  These  stars  are  observable  at  night, 
because  they  are  not  intercepted  from  our  sight  by  the 
sun's  rays  ;  and  in  this  manner  they  appear  during  the 
whole  winter,  only  they  seem  to  get  more  westerly 
every  night,  as  the  earth  moves  gradually  by  them  to 
the  east.  To  make  this  more  clear,  place  the  earth  in 
the  planetarium  between  the  sun  and  any  of  the  signs, 
that  side  towards  the  sun  will  be  day,  and  that  towards 
the  sign  night  :  it  follows,  that  at  night  we  are  turned 
towards  the  stars,  which  in  that  sign  (suppose,  as  before, 
the  Pleiades  in  Taurus)  will  then  be  conspicuous  to  us  ; 
but  as  the  spring  approaches,  the  earth  withdraws  itself 
from  between  the  sun  and  the  Pleiades,  till  at  length, 
by  its  progressive  motion,  it  gets  the  sun  between  it  and 
them,  which  then  lie  hid  behind  the  solar  rays  :  after  the 
same  manner,  while  the  earth  performs  its  annual  tract, 
the  sun,  which  always  seems  to  move  the  contrary  way, 
effaces,  by  his  splendor,  the  other  constellations  succes- 
sively ;  but  the  stars  opposite  to  those  hid  by  the  sun, 
are  at  night  presented  to  our  view. 

GENERAL  PHENOMENA  OF  THE  PLANETS. 

I  shall  now  place  the  earth,  Mars,  and  Venus,  on  the 
planetarium,  and  as  each  planet  moves  with  a  different 
degree  of  velocity,  they  are  continually  changing  their 
relative  positions.  Thus,  on  turning  the  handle  of 
the  mechine,  you  find,  1st,  That  the  earth  moves  twice 
as  fast  as  Mars,  making  two  revolutions  while  he  makes 
one  ;  and  Venus,  on  the  other  hand,  moves  much  faster 
than  the  earth.  Secondly,  that  in  each  revolution  of 
the  earth,  these  planets  continually  change  their  relative 


PHENOMENA  OF  THE  PLANETS.        141 

positions,  corresponding  sometimes  with  the  same  point 
of  the  ecliptic,  but  much  oftner  with  different  points. 

To  explain  the  conjunction,  opposition,  elongation,  and  other 
phenomena  of  the  inferior  planets. 

We  may  now  proceed  to  make  some  observations  on 
the  motions  of  Venus,  as  observed  in  the  planetarium. 
If  considered,  as  viewed  from  the  sun,  we  shall  find  that 
Venus  would  appear  at  one  time  nearer  to  the  earth  than 
at  another ;  that  sometimes  she  would  appear  in  the 
same  part  of  the  heavens,  and  at  others  in  opposite 
parts  thereof. 

As  the  planets,  when  seen  from  the  sun,  change  their 
position  with  respect  to  the  earth,  so  do  they  also,  when 
seen  from  the  earth,  change  their  position  with  respect 
to  the  sun,  being  sometimes  nearer  to,  at  others  farther 
from,  and  at  other  times  in  conjunction  with  him. 

But  the  conjunctions  of  Venus  or  Mercury,  seen  from 
the  earth,  not  only  happen  when  they  are  seen  together 
from  the  sun,  but  also  when  they  appear  to  the  solar 
spectator  to  be  in  opposition.  To  illustrate  this,  bring 
the  earth  and  Venus  to  the  first  point  of  Capricorn  ;  then 
by  applying  a  string  from  the  sun  over  Venus  and  the 
earth,  you  will  find  them  to  be  in  conjunction,  or  on 
the  same  point  of  the  ecliptic. 

Whereas,  if  you  turn  the  handle  till  the  sun  is  be- 
tween Venus  and  the  earth,  a  spectator  in  the  sun  will 
see  Venus  and  the  earth  in  opposition  ;  but  an  inhabi- 
tant of  the  earth  will  see  Venus  not  in  opposition  to 
the  sun,  but  in  conjunction  with  him. 

In  the  first  conjunction,  Venus  is  between  the  sun  and 
the  earth ;  this  is  called  the  inferior  conjunction.  In 
the  second,  the  sun  is  situate  between  the  earth  and 
Venus  ;   this  is  called  the  superior  conjunction. 

After  either  of  these  conjunctions,  Venus  will  be 
seen  to  recede  daily  from  the  sun,  but  never  departing 
beyond  certain  bounds,  never  appearing  opposite  to  the 
sun  ;  and  when  she  is  seen  at  the  greatest  distance  from 
him,  a  line  joining  her  centre  with  the  centre  of  the  earth, 
will  be  a  tangent  to  the  orbit  of  Venus. 


142       PHENOMENA  OF  THE  PLANETS. 

To  illustrate  this,  I  take  off  the  sun  from  its  support, 
and  the  ball  of  Venus  from  its  supporting  stem,  and 
place  this  wire,  plate  1  l,y%.  2,  so  that  one  part  P,  may 
be  on  the  stem,  that  supports  the  earth,  and  a  similar 
socket,  Jig.  3,  on  the  pin  which  supports  the  ball  of 
Venus ;  the  wire,  F,  is  to  lie  in  a  notch  at  the  top  of 
the  socket,  which  has  been  put  upon  the  supporting 
stem  of  Venus  :  then  will  the  wire  represent  a  visual 
ray  going  from  an  inhabitant  of  the  earth  to  Venus. 

By  turning  the  handle,  you  will  now  find  that  the 
planet  never  departs  farther  than  certain  limits  from  the 
sun,  which  are  called  its  greatest  elongations,  when  the 
wire  becomes  a  tangent  to  the  orbit,  after  which  it  ap- 
proaches the  sun,  till  it  arrive  at  either  the  inferior  or 
superior  conjunction. 

It  is  also  evident  from  the  instrument,  that  Venus, 
from  her  superior  conjunction,  when  she  is  farthest  from 
the  earth,  to  the  time  of  her  inferior  conjunction,  when 
she  is  nearest,  sets  later  than  the  sun,  is  seen  after  sun- 
set, and  is,  as  it  were,  the  forerunner  of  night  and 
darkness.  But  from  the  inferior  conjunction,  till  she 
come  to  the  superior  one,  she  is  always  seen  westward 
of  the  sun,  and  must  consequently  set  before  him  in  the 
evening,  and  rise  before  him  in  the  morning,  foretelling 
that  light  and  day  are  at  hand. 

Bring  Venus  and  the  earth  to  the  beginning  of 
Aries,  when  they  will  be  in  conjunction  ;  and  turn  the 
handle  for  nearly  225  days,  and  as  Venus  moves  faster 
than  the  earth,  she  will  arrive  at  Aries,  and  have  finish- 
ed her  course,  but  will  not  have  overtaken  the  earth, 
who  has  moved  on  in  the  mean  time  ;  and  Venus  must 
go  on  for  some  time  in  order  to  overtake  her.  There- 
fore, if  Venus  should  be  this  day  in  conjunction  with 
the  sun,  in  the  inferior  part  of  her  orbit,  she  will  not 
come  again  to  the  same  conjunction  till  after  1  year,  7 
months,  and  12  days. 

It  is  plain,  by  inspection  of  the  planetarium,  that 
though  Venus  does  always  keep  nearly  at  the  same 
distance  from  the  sun,  yet  she  is  continually  changing 
her  distance  from  the  earth  •>  her  distance  is  greatest 


PHENOMENA    OF    THE    PLANETS.  143 

when  she  is  in  her  superior,  and  least  when  she  is  in 
her  inferior  conjunction. 

To  explain  the  phases,  the  retrograde,  direct,  and  stationary 
situations  of  the  planets. 

As  Venus  is  an  opake  globe,  and  only  shines  by  the 
light  she  receives  from  the  sun,  that  face  which  is  turn- 
ed towards  the  sun  will  always  be  bright,  while  the  op- 
posite one  will  be  in  darkness ;  consequently,  if  the 
situation  of  the  earth  be  such,  that  the  dark  side  of  Ve- 
nus be  turned  towards  us,  she  will  then  be  invisible, 
except  she  appear  like  a  spot  on  the  disk  of  the  sun.  If 
her  whole  illuminated  face  be  turned  towards  the  earth, 
as  it  is  in  her  superior  conjunction,  she  appears  of  a  cir- 
cular form  ;  and,  according  to  the  different  positions  of 
the  earth  and  Venus,  she  will  have  different  forms,  and 
appear  with  different  phases,  undergoing  the  same  chan- 
ges of  form  as  the  moon.  These  different  phases  are  seen 
very  plain  in  this  instrument,  as  the  side  of  the  planet 
which  is  opposite  to  the  sun  is  blackened ;  so  that  in 
any  position,  a  line  drawn  from  the  earth  to  the  planet, 
will  represent  that  part  of  her  disk  which  is  visible 
to  us. 

The  irregularities  in  the  apparent  motions  of  the  pla- 
nets, is  a  subject  that  this  instrument  will  fully  elucidate; 
and  the  pupil  will  find  that  they  are  only  apparent, 
taking  their  rise  from  the  situation  and  motion  of  the 
observer.  To  illustrate  this,  let  us  suppose  the  fore- 
mentioned  wire,  when  connected  with  Venus  and  the 
earth,  to  be  the  visual  ray  of  an  observer  on  the  earth ; 
it  will  then  point  out  how  the  motions  of  Venus  appear 
in  the  heavens,  and  the  path  she  appears  to  us  to  de- 
scribe among  the  fixed  stars. 

Let  Venus  be  placed  near  her  superior  conjunction, 
and  the  instrument  in  motion,  the  wire  will  mark  out 
the  apparent  motion  of  Venus  in  the  ecliptic.  Thus 
Venus  will  appear  to  move  eastward  in  the  ecliptic,  till 
the  wire  become  a  tangent  to  the  orbit  of  Venus,  in 
which  situation  she  will  appear  to  us  to  be  stationary, 


144  OF    THE    SUPERIOR    PLANETS. 

or  not  to  advance  at  all  among  the  fixed  stars  ;  a  cir- 
cumstance which  is  exceedingly  clear  and  visible  upon 
the  planetarium,.. 

Continue  turning,  till  Venus  be  in  her  superior  con- 
junction, and  you  will  find  by  the  wire  or  visual  ray 
that  she  now  appears  to  move  backward  in  the  ecliptic, 
or  from  east  to  west,  till  she  has  arrived  at  that  part 
where  the  visual  ray  again  becomes  a  tangent  to  her 
orbit;  In  which  position,  Venus  will  again  appear  sta- 
tionary for  some  time  :  after  which  she  will  commence 
anew  her  direct  motion. 

Hence,  when  Venus  is  in  the  superior  part  of  her 
orbit,  she  is  always  seen  to  move  directly,  according 
to  the  order  of  the  signs  ;  but  when  she  is  in  the  infe- 
rior part,  she  appears  to  move  in  a  contrary  direction. 

What  has  been  said  concerning  the  motions  of  Venus 
is  applicable  to  those  of  Mercury  ;  but  the  conjunctions 
of  Mercury  with  the  sun,  as  well  as  the  times  of  his  be- 
ing direct,  stationary,  or  retrograde,  are  more  frequent 
than  those  of  Venus. 


OF    THE    SUPERIOR    PLANETS,    AS    SEEN    FROM    THE 
EARTH. 

If  you  extend  your  observations  on  the  instrument 
to  Mars,  you  will  find  by  the  visual  ray  that  Mars, 
when  in  conjunction  and  when  in  opposition,  will  ap- 
pear in  the  same  point  of  the  ecliptic,  whether  it  be  seen 
from  the  sun  or  the  earth  ;  and  in  this  situation  only  is 
its  real  and  apparent  place  the  same,  because  then  only 
the  ray  proceeds  as  if  it  came  from  the  centre  of  the 
universe. 

You  will  find,  that  the  direct  motion  of  a  superior 
planet  is  swifter  the  nearer  it  is  to  the  conjunction,  and 
slower  when  it  is  nearer  to  quadrature  with  the  sun  ; 
but  that  the  retrograde  motion  of  a  superior  planet  is 
swifter  the  nearer  it  is  to  opposition,  and  slower  the 
nearer  it  is  to  quadrature ;  but  at  the  time  of  change 
from  direct  to  retrograde,  its  motion  becomes  insensible. 


[      145     ] 


TO  PROVE  BY  THE  PLANETARIUM  THE  TRUTH  OF 
THE  COPERNICAN,  AND  ABSURDITY  OF  THE  PTOLE- 
MAIC   SYSTEM. 

Of  all  the  prejudices  which  philosophy  contradicts, 
there  is  none  so  general  as  that  the  earth  keeps  its  place 
unmoved.  This  opinion  seems  to  be  universal,  till  it 
be  corrected  by  instruction,  or  by  philosophical  specu- 
lation. Those  who  have  any  tincture  of  education, 
are  not  now  in  danger  of  being  held  by  it  ;  but  yet 
they  find  at  first  a  reluctance  to  believe  that  there  are 
antipodes,  that  the  earth  is  spherical,  and  turns  round 
its  axis  every  day,  and  round  the  sun  every  year.  They 
can  recollect  the  time  when  reason  struggled  with  pre- 
judice upon  these  points,  and  prevailed  at  length,  but 
not  without  some  efforts.* 

The  planetarium  gives  ocular  demonstration  of  the 
motion  of  the  earth  about  the  sun,  by  showing  that  it 
is  thus  only  that  the  celestial  phenomena  can  be  explain- 
ed, and  making  the  absurdity  of  the  Ptolemaic  system 
evident  to  the  senses  of  young  people.  For  this  pur- 
pose, I  take  off  the  brass  ball  which  represents  the  sun, 
and  put  on  a  small  ivory  ball,  jig.  b,  in  its  place  to  re- 
present the  earth,  and  place  a  small  brass  baling,  a, 
for  the  sun,  on  that  arm  which  carries  the  earth. 

The  instrument  in  this  state  will  give  an  idea  of  the 
Ptolemaic  system,  with  the  earth  immoveable  in  the 
centre,  and  the  heavenly  bodies  revolving  about  it  in 
the  following  order :  Mercury,  Venus,  the  Sun,  Mars, 
Jupiter,  and  Saturn.  Now,  in  this  disposition  of  the 
planets,  several  circumstances  are  to  be  observed,  that 
are  contrary  to  the  real  appearances  of  the  celestial  mo- 
tions, and  which  therefore  prove  the  falsity  of  this  sys- 
tem. 

It  will  appear  from  the  instrument,  that  on  this  hy- 
pothesis Mercury  and  Venus  could  never  be  seen  to  go 
behind  the  sun,  from  the  earth,  because  the  orbits  of 


Reid's  Essays  on  the  Intellectual  Powers  of  Man. 
VOL.  IV.  U 


146 


TO    RECTIFY    THE    PLANETARIUM, 


both  of  them  are  contained  between  the  sun  and  the 
earth  ;  but  these  planets  are  seen  to  go  as  often  behind 
the  sun  as  before  it ;  we  may,  therefore,  from  hence 
conclude  that  this  system  is  erroneous. 

It  is  also  apparent  from  the  planetarium,  that  on  this 
scheme  these  planets  might  be  seen  in  conjunction  with, 
or  in  opposition  to  the  sun,  or  at  any  distance  from  it. 
But  this  is  contrary  to  experience ;  for  they  are  never 
seen  in  opposition  to  the  sun,  or  on  the  meridian  of  Lon- 
don, for  instance,  at  midnight ;  nor  do  they  ever  re- 
cede from  the  sun  beyond  certain  limits. 

Again,  on  the  Ptolemaic  system  all  the  planets  would 
be  at  an  equal  distance  from  the  earth,  in  all  parts  of 
their  orbit,  and  would  therefore  necessarily  appear  al- 
ways of  the  same  magnitude,  and  moving  with  equal 
and  uniform  velocities  in  one  direction  ;  circumstances 
which  are  known  to  be  repugnant  to  observation  and 
experience. 

To  rectify  the  planetarium^  or  place  the  planets  in  their  true 
situations  ^  as  seen  from  the  sun. 

The  situation  of  the  planets  in  the  heavens  are  accu- 
rately calculated  by  astronomers,  and  published  in  al- 
manacks appropriated  to  the  purpose,  as  the  Nautical 
Almanack,  White's  Ephemeris,  &c.  An  ephemeris  is 
a  diary  or  daily  register  of  the  motions  and  places  of 
the  heavenly  bodies,  showing  the  situation  of  each  pla» 
net  at  12  o'clock  each  day.  These  situations  it  exhibits 
both  as  seen  from  the  sun,  and  from  the  earth  ;  but,  as 
the  former,  or  the  heliocentric,  is  the  only  one  of  any 
use  for  this  purpose,  I  shall  here  explain  so  much  of 
that  part  of  Mr.  White's  Ephemeris,  for  the  year  1790, 
as  will  enable  you  to  rectify  the  planetarium. 


SI 

Day 

Length 

Helioc. 

Helioc. 

Helioc.      Helioc. 

Helioc. 

Helioc. 

increa 

of  Day. 

long. 

long. 

long.         long. 

long. 

long. 

h 

% 

*    1    e 

9 

s 

1 

7       4  14    48|27X  35 

2«JU4 

5J7|^16 

11  til  U 

8  ^  35 

18  18 

7 

7  24 

15       8  27       47 

2      42 

29      57 

17        2 

18           7 

26rJ53 

13 

7    44 

15    2827       59 

3        0 

2=£=3921       52 

7        37 

3  SI    4 

19 

8      0 

15    44  28       11 

3      37 

5       20128      36 

7  V$    7 

4^15 

25 

8    10 

16       0|28       23 

4        5 

8         3)  4/22 

16        3fy0  ^    0 

TO    USE    IT    AS    A    TELLURIAN.  147 

In  the  foregoing  table  for  May,  1790,  you  have  the 
heliocentric  places  calculated  to  every  six  days  of  the 
month,  which  is  sufficiently  accurate  for  general  pur- 
poses. Thus  on  the  19th,  you  have  Saturn  in  28°  IT 
of  Pisces,  Jupiter  in  3°  37'  of  Virgo,  Mars  in  5°  20'  of 
Libra,  the  Earth  28°  36'  of  Scorpio,  Venus  7°  7'  of  Ca- 
pricorn, and  Mercury  4°  1 3'  of  Virgo  ;  to  which  places 
on  the  ecliptic  of  the  planetarium,  the  several  planets 
are  to  be  set,  and  they  will  then  exhibit  their  real  situa- 
tions, both  with  respect  to  the  sun  and  the  earth  for  that 
day. 

To  use  the  instrument  as  a  tellurian^  plate  12,  jig.  1. 

The  sun,  the  earth,  and  the  moon,  are  bodies,  which, 
from  our  connection  with  them,  are  so  interesting  to  us, 
that  it  is  necessary  to  enter  into  a  minute  detail  of  their 
respective  phenomena.  To  render  this  instrument  a 
tellurian,  all  the  planets  are  first  to  be  taken  off,  the 
piece  of  wheel-work,  A  B,  is  to  be  placed  on  in  their 
stead,  in  such  a  manner  that  the  wheel,  c,  may  fall  into 
the  teeth  that  are  cut  upon  the  edge  of  the  ecliptic. 
The  milled  nut,  D,  is  then  to  be  screwed  on,  to  keep 
the  wheel-work  firmly  in  its  place.  It  is  best  to  place 
this  wheel-work  in  such  a  manner,  that  the  index,  E, 
may  point  to  the  21st  of  June,  and  then  to  move  the 
globe,  so  that  the  north  pole  may  be  turned  towards 
the  sun. 

The  instrument  will  then  show,  in  an  accurate  and 
clear  manner,  all  the  phenomena  arising  from  the  an- 
nual and  diurnal  motions  of  the  earth  :  as  the  globe  is 
of  three  inches  diameter,  all  the  continents,  seas,  king- 
doms, &c.  may  be  distinctly  seen  ;  the  equator,  the 
ecliptic,  tropics,  and  other  circles,  are  very  visible,  so 
that  the  problems  relative  to  peculiar  places  may  be  sa- 
tisfactorily solved.  The  axis  of  the  earth  is  inclined  to 
the  ecliptic  in  an  angle  of  66j  degrees,  and  preserves 
its  parallelism  during  the  whole  of  its  revolution.  About 
the  globe  there  is  a  circle  to  represent  the  terminator, 
or  boundary  between  light  and  darkness,  dividing  the 
enlightened  from  the  dark  hemisphere.     At  N  O  is  an 


148  TO    EXPLAIN    THE    CHANGES    OF 

hour-circle,  to  determine  the  time  of  the  sun's  rising  or 
setting. 

The  brass  index,  G,  represents  a  central  solar  ray  ; 
it  serves  to  show  when  it  is  noon,  or  when  the  sun  is 
upon  the  meridian  at  any  given  place  :  it  also  shows 
what  sign  and  degree  of  the  ecliptic  on  the  globe  the 
sun  describes  on  any  day,  and  the  parallel  it  describes. 

The  plane  of  the  terminator,  HI,  passes  through  the 
centre  of  the  earth,  and  is  perpendicular  to  the  central 
solar  ray.  The  index,  E,  points  out  the  sun's  place  in 
the  ecliptic  of  the  instrument  for  any  given  day  in  the 
year. 

To  explain  the  changes  of  seasons  by  the  tellurian. 

The  first  thing  to  be  done,  is  to  rectify  the  tellurian  ; 
or,  in  other  words,  to  put  the  globe  into  a  position  si- 
milar to  that  of  the  earth  for  any  given  day.  Thus,  to 
rectify  the  tellurian,  for  the*21st  of  June,  turn  the  han- 
dle till  the  annual  index  come  to  the  given  day ;  then 
move  the  globe  by  the  arm  K  L,  so  that  the  north  pole 
may  be  turned  towards  the  sun  ;  and  adjust  the  termi- 
nator, so  that  it  may  just  touch  the  edge  of  the  arctic 
circle.  The  globe  is  then  in  the  situation  of  the  earth 
for  the  longest  day  in  our  northern  hemisphere,  the  an- 
nual index  pointing  to  the  first  point  of  Cancer  and  the 
21st  of  June  ;  brings  the  meridian  of  London  to  co- 
incide with  the  central  solar  ray  G,  and  move  the  hour- 
circle,  N  O,  till  the  index  L,  points  to  XII ;  we  then 
have  the  situation  of  London  with  respect  to  the  longest 
day. 

Now,  on  gently  turning  the  handle  of  the  machine, 
the  point  representing  London  will,  by  the  rotation  of 
the  earth,  be  carried  away  towards  the  east,  while  the 
sun  seems  to  move  westward ;  and  when  London  has 
arrived  at  the  eastern  part  of  the  terminator,  the  index 
will  point  on  the  hour-circle  to  the  time  of  sun-setting  for 
that  day  ;  continue  to  turn  on,  and  London  will  move 
in  the  shaded  part  of  the  earth,  on  the  other  side  of  the 
terminator  ;  when  the  index  is  again  at  XII,  it  is  mid- 
night at  London  :  by  moving  on,  London  will  emerge 


SEASONS    BY    THE    TELLURIAN.  149 

from  the  western  side  of  the  terminator,  and  the  index 
will  point  out  the  time  of  sun-rising,  the  sun  at  that 
instant  appearing  to  rise  above  the  horizon  in  the  east 
to  an  inhabitant  of  London. 

It  will  be  evident  by  the  instrument,  while  in  this  po- 
sition, that  the  central  solar  ray,  during  the  whole  revo- 
lution of  the  earth  on  its  axis,  only  points  to  the  tropic 
of  Cancer,  and  that  the  sun  is  vertical  to  no  other  parts 
of  the  earth,  but  those  which  are  under  this  tropic. 

By  observing  how  the  terminator  cuts  the  several 
parallels  of  the  globe,  we  shall  find  that  all  those  be- 
tween the  northern  and  southern  polar  circles,  except 
the  equator,  are  divided  unequally  into  diurnal  and  noc- 
turnal arcs,  the  former  being  greatest  on  the  north  side 
of  the  equator,  and  the  latter  on  the  south  side  of  it. 

In  this  position  the  northern  polar  circle  is  wholly  on 
that  side  the  terminator  which  is  nearest  the  sun,  and 
therefore  altogether  in  the  enlightened  hemisphere,  and 
the  inhabitants  thereof  enjoy  a  continual  day.  In  the 
same  manner,  the  inhabitants  of  the  southern  polar  cir- 
cle continue  in  the  dark  at  this  time,  notwithstanding 
the  diurnal  revolution  of  the  earth  ;  it  is  the  annual  mo- 
tion only  which  can  relieve  them  from  this  situation  of 
perpetual  darkness,  and  bring  to  them  the  blessings  of 
day  and  the  enjoyments  of  summer.  While  in  this 
state,  the  inhabitants  in  north  latitude  are  nearest  to  the 
central  solar  ray,  and  consequently  to  the  sun's  perpen- 
dicular beams  ;  and  of  course,  a  greater  number  of 
his  rays  will  fall  upon  any  given  place,  than  at  any  other 
time :  the  sun's  rays  do  now  also  pass  through  a  less 
quantity  of  the  atmosphere,  which,  together  with  the 
length  of  the  day  and  the  shortness  of  the  night,  are 
the  reasons  of  the  increase  of  the  heat  in  summer  to- 
gether with  all  its  other  delightful  effects ;  the  season 
when  the  Lord  pours  forth  his  blessings  upon  every 
living  creature  in  the  greatest  abundance. 

While  the  earth  continues  to  turn  round  on  its  axis 
once  a  day,  it  is  continually  advancing  from  west  to 
east,  according  to  the  order  of  the  signs,  as  is  seen  by 
the  progress  of  the  annual  index,  E,  which  points  suc- 
cessively to  all  the  signs  and  degrees  of  the  ecliptic  ; 


150  TO    EXPLAIN    THE    CHANGES    OF 

the  sun  in  the  mean  time  seems  to  describe  the  ecliptic 
also,  going  from  west  to  east,  at  the  distance  of  six  signs 
from  the  earth  ;  that  is,  when  the  earth  really  sets  out 
from  the  first  point  of  Capricorn,  the  sun  seems  to  set 
out  from  the  first  point  of  Cancer,  as  is  plain  from 
the  index. 

But  as,  during  the  annual  revolution  of  the  earth,  the 
axis  always  remains  parallel  to  itself,  the  situation  of 
this  axis  with  respect  to  the  sun  must  be  continually 
changing. 

As  the  earth  moves  on  in  the  ecliptic,  the  northern 
polar  circle  gets  gradually  under  the  terminator  ;  so  tnat 
when  the  earth  is  arrived  at  the  first  point  of  Aries,  and 
the  annual  index  is  at  the  first  point  of  Libra,  on  the 
22d  of  September,  this  circle  is  divided  into  two  equal 
parts  by  the  terminator,  as  is  also  every  other  parallel 
circle,  and  consequently  the  diurnal  and  nocturnal  arcs 
are  equal :  this  is  called  the  time  of  equinox  ;  the  days 
and  nights  are  then  equal  all  over  the  earth,  being  each 
of  them  twelve  hours  long,  as  will  be  seen  by  the  horary 
index,  L.  The  central  solar  ray,  G,  having  successively 
pointed  to  the  parallels  that  may  be  supposed  to  be  be- 
tween the  equator  and  the  tropic  of  Cancer,  is  at  this 
period  perpendicular  to  the  inhabitants  that  live  at  the 
equator. 

By  continuing  to  turn  the  handle,  the  earth  advances 
in  the  ecliptic,  and  the  terminator  shows  how  the  days 
are  continually  decreasing,  and  the  diurnal  arcs  shorten- 
ing :  till  by  degrees  the  whole  space  contained  by  the 
northern  polar  circle  is  on  that  side  of  the  terminator 
which  is  opposite  to  the  sun  ;  this  happens  when  the 
earth  has  got  to  the  first  point  of  Cancer,  and  rhe  annu- 
al index  is  at  the  first  point  of  Capricorn,  on  the  21st 
of  December.  In  this  state  of  the  globe,  the  northern 
polar  circle,  and  all  the  countries  within  that  space,  have 
no  day  at  all  ;  while  the  inhabitants  that  live  within  the 
southern  polar  circle,  being  on  that  side  of  the  termina- 
tor which  is  next  the  sun,  enjoy  perpetual  day.  By 
this,  and  the  former  situation  of  the  earth,  you  will  ob- 
serve that  there  are  nations  to  whom  a  great  portion  of 
the  year  is  darkness,  who  are  condemned  to  pass  weeks 


SEASONS    BY    THE    TELLURIAN.  151 

and  months  without  the  benign  influence  of  the  solar 
rays.  The  central  solar  ray  is  now  perpendicular  to  the 
tropic  of  Capricorn  ;  the  length  of  the  days  is  inversely 
what  it  was  when  the  sun  entered  Cancer,  the  days  be- 
ing now  at  their  shortest,  and  the  nights  longest  in  the 
northern  hemisphere  :  the  length  of  each  is  pointed  out 
by  the  horary  index. 

The  earth  being  again  carried  on  till  it  enter  Libra, 
and  the  sun  Aries,  we  shall  again  have  all  the  pheno- 
mena of  the  equinoctial  seasons.  The  terminator  will 
divide  all  the  parallels  into  two  equal  parts ;  the  poles  will 
again  be  in  the  plane  of  the  terminator  ;  and  conse- 
quently as  the  globe  revolves,  every  place  from  pole  to 
pole  will  describe  an  equal  arc  in  the  enlightened  and  ob- 
scure hemispheres,  entering  into  and  going  out  of  each 
exactly  at  six  o'clock,  as  shown  by  the  hour-index. 

As  the  earth  advances,  more  of  the  northern  polar 
circle  comes  into  the  illuminated  hemisphere,  and  conse- 
quently the  days  increase  with  us,  while  those  on  the 
other  side  of  the  equator  decrease,  till  the  earth  arrive 
at  the  first  point  of  Capricorn,  the  place  from  which  we 
first  began  to  make  our  observations. 

To  explain  the  phenomena  that  take  place  in  what  is  called 
a  parallel^  direct ,  and  right  sphere. 

,  Take  off  the  globe  and  its  terminator,  and  put  on  in 
its  place  the  globe  which  accompanies  the  instrument, 
and  which  is  furnished  with  a  meridian,  horizon,  and 
quadrant  of  altitude  ;  the  edge  of  the  horizon  is  gradu- 
ated from  the  east  and  west,  to  the  north  and  south  points, 
and  within  these  divisions  are  the  points  of  the  compass 
to  the  under  side  of  this  horizon  ;  but  at  1 8  degrees 
from  it  another  circle  is  affixed,  to  represent  the  twilight- 
circle  :  the  meridian  is  graduated  like  the  meridian  of  a 
globe  ;  the  quadrant  of  altitude  is  divided  into  degrees, 
beginning  at  the  zenith,  and  finishing  at  the  horizon. 

This  globe,  if  the  horizon  be  differently  set  with  res- 
pect to  the  solar  ray,  will  exhibit  the  various  phenomena 
arising  from  the  situation  of  the  horizon  with  respect  to 
the  sun,  either  in  a  right,  a  parallel,  or  an  oblique  sphere ; 


152  PHENOMENA    IN    A    PARALLEL, 

or  having  set  the  horizon  to  any  place,  you  will  see 
by  the  central  solar  ray  how  long  the  sun  is  above  or 
below  the  horizon  of  that  place,  and  at  what  point  of 
the  compass  he  rises,  his  meridian  altitude,  and  many 
other  curious  particulars,  of  which  we  shall  give  a  few 
examples. 

Set  the  horizon  to  coincide  with  the  equator,  and 
place  the  earth  in  the  first  point  of  Libra  ;  then  will  the 
globe  be  in  the  position  of  a  parallel  sphere,  and  of  the 
inhabitants  of  the  poles  at  that  season  of  the  year,  which 
inhabitants  are  represented  by  a  pin  at  the  upper  part  of 
the  quadrant  of  altitude  ;  the  handle  being  turned  round 
gently,  the  earth  will  revolve  upon  its  axis,  and  the  solar 
ray  will  coincide  with  the  horizon,  without  deviating  in 
the  least  to  the  north  or  south  ;  showing,  that  on  the 
21st  of  March  the  sun  does  not  appear  to  rise  or  set  to 
the  terrestial  poles,  but  passes  round  through  all  the  points 
of  the  compass,  the  plane  of  the  horizon  bisecting  the 
sun's  disk. 

Now  place  the  horizon  so,  that  it  may  coincide  with 
the  poles,  and  the  pin  representing  an  inhabitant  to  be 
over  the  equator,  the  globe  in  this  position  is  said  to  be 
in  that  of  a  right  sphere  ;  the  equator,  and  ail  the  pa- 
rallels of  latitude,  are  at  right  angles,  or  perpendicular 
to  the  horizon  ;  by  turning  the  handle  till  the  earth  has 
completed  a  year,  or  one  revolution  about  the  sun,  we 
shall  perceive  all  the  solar  phenomena  as  they  happen  to 
an  inhabitant  of  the  equator,  which  are,  1.  That  the 
sun  rises  at  six,  and  sets  at  six,  throughout  the  year,  so 
that  the  days  and  nights  there  are  perpetually  equal.  2. 
That  on  the  21st  of  March,  and  22d  of  September,  the 
sun  is  in  the  zenith,  or  exactly  over  the  heads  of  the  in- 
habitants. 3.  That  one  half  of  the  year  between  March 
and  September,  the  sun  is  every  day  full  north,  and  the 
other  half,  between  September  and  March,  is  full  south 
of  the  equator,  his  meridian  altitude  being  never  less 
than  66-  degrees. 

If  the  pin,  representing  an  inhabitant,  be  now  removed 
out  of  the  equator,  and  set  upon  any  place  between  it  and 
the  poles,  the  horison  will  not  then  pass  through  either 
of  the  poles,  nor  coincide  with  the  equator,  but  cut  it 


DIRECT,    AND    RIGHT    SPHERE.  153 

obliquely,  one  half  being  above,  the  other  half  below  the 
horizon  ;  the  globe  in  this  state  is  said  to  be  in  that  of  an 
oblique  sphere,  of  which  there  are  as  many  varieties  as 
there  are  places  between  the  equator  and  both  poles.  But 
one  example  will  be  sufficient ;  for  whatever  appearance 
happens  to  one  place,  the  same,  as  to  kind,  happens  to 
every  other  place,  differing  only  in  degree,  as  the  lati- 
tudes differ.  Bring  the  pin,  therefore,  over  London, 
then  will  the  horizon  represent  the  horizon  of  London, 
and  in  one  revolution  of  the  earth  round  the  sun,  we 
shall  have  all  the  solar  appearances  through  the  four 
seasons  clearly  exhibited,  as  they  really  are  in  nature ; 
that  is,  the  earth  standing  at  the  first  degree  of  Libra, 
and  the  sun  then  entering  into  Aries,  the  meridian  turn- 
ed to  the  solar  ray,  and  the  hour-index  set  to  XII,  you 
will  then  have  the  globe  standing  in  the  same  position 
towards  the  sun,  as  our  earth  does  at  noon  on  the  21st 
of  March.  If  the  handle  be  turned  round,  when  the  so- 
lar ray  comes  to  the  western  edge  of  the  horizon,  the 
hour-index  will  point  to  VI,  which  shows  the  time  of 
sun-setting ;  London  then  passes  into,  and  continues  in 
darkness,  till  the  hour-index  having  passed  over  XII 
hours,  come  again  to  VI,  at  which  time  the  solar  ray 
gains  the  eastern  edge  of  the  horizon,  thereby  defining 
the  time  of  sun-rising ;  six  hours  afterwards  the  meri- 
dian again  comes  to  the  solar  ray,  and  the  hour-index 
points  to  XII ;  thereby  evidently  demonstrating  the 
equality  of  the  day  and  night,  when  the  sun  is  in  the 
equinoctial.  You  may  then  also  observe,  that  the  sun 
rises  due  east,  and  sets  due  west. 

Continuing  to  move  the  handle,  you  will  find,  that  the 
solar  ray  declines  from  the  equator  towards  the  north,  and 
every  day  at  noon  rises  higher  upon  the  graduations  of  the 
meridian  than  it  did  before,  continually  approaching  to 
London,  the  days  at  the  same  time  growing  longer  and 
longer,  and  the  sun  rising  and  setting  more  and  more  to- 
wards the  north,  till  the  21st  of  June,  when  the  earth 
gets  into  the  first  degree  of  Capricorn,  and  the  sun  ap- 
pears in  the  tropic  of  Cancer,  rising  about  40  minutes 
past  3  in  the  morning,  and  setting  about  20  minutes  past 
8  in  the  evening,  and  after  continuing  about  seven  hours 

VOL.  iv.  x 


154  PHENOMENA    IN    A    PARALLEL    SPHERE. 

in  the  nether  hemisphere,  appears  rising  in  the  north- 
east, as  before.  From  the  21st  of  June  till  the  22d  of 
September,  the  sun  recedes  to  the  south,  and  the  days 
gradually  decrease  till  the  autumnal  equinox,  when  they 
again  become  equal. 

During  the  three  succeeding  months,  the  sun  continues- 
to  decline  towards  the  south  pole,  till  the  21st  of  Decem- 
ber, when  the  sun  enters  the  tropic  of  Capricorn,  rising 
on  the  south-east  point  of  the  compass  about  20  minutes 
past  8  in  the  morning,  and  setting  about  40  minutes  past 
3  in  the  evening,  at  the  south-west  point  upon  the  hori- 
zon ;  after  which,  the  sun  continues  in  the  dark  hemi- 
sphere for  17  hours,  and  then  appears  again  in  the  south- 
east, as  before.  From  this  chill  solstice  the  sun  returns 
towards  the  north,  and  the  days  continually  increase  in 
length  till  the  vernal  equinox,  when  all  things  are  re- 
stored in  the  same  order  as  at  the  beginning. 

Thus  all  the  varieties  of  the  seasons,  the  time  of  sun 
rising  and  setting,  and  at  what  point  of  the  compass ;  as 
also  the  meridian  altitude  and  declination  every  day  of 
the  year,  the  duration  of  twilight,  and  to  what  place  the 
sun  is  at  any  time  vertical,  are  fully  exemplified  by  this 
globe  and  its  apparatus. 

Before  we  quit  the  phenomena  particularly  arising  from 
the  motion  and  position  of  the  earth,  let  the  globe,  with 
the  meridian  and  horizon,  be  removed,  and  the  ivory 
ball,  which  fits  upon  a  pin,  be  placed  thereon,  to  repre- 
sent the  earth. 

As  the  axis  of  this  globe  stands  perpendicular  to  the 
plane  of  the  ecliptic,  you  will  find,  that  the  solar  ray  con- 
tinually points  to  the  equator  of  this  little  ball,  and  will 
never  deviate  to  the  north  or  south  ;  though,  by  turning 
the  handle,  the  ball  is  made  to  complete  a  revolution 
round  the  sun.  This  shows,  that  the  earth  in  this  position 
would  have  had  the  days  and  nights  equal  in  every  part 
of  the  globe,  all  the  year  long ;  there  would  have  been 
no  difference  in  the  climates  of  the  earth,  no  distinction  of 
seasons ;  an  eternal  summer,  or  never-ceasing  winter, 
would  have  been  our  portion ;  an  unvaried  sameness, 
that  would  have  limited  inquiry,  and  satiated  curiosity; 
and  that  the  variety  of  the  seasons  is  owing  to  its  axis 
being  inclined  to  the  plane  of  its  orbit. 


[      155     ] 


of  the  lunarium,   plate  12,  Jig.  % 

Having  thus  illustrated  the  phenomena,  which  arise 
particularly  from  the  inclination  of  the  earth's  axis  to  the 
plane  of  the  ecliptic,  from  its  rotation  round  its  axis,  and 
revolution  round  the  sun ;  we  now  proceed  to  explain, 
by  this  instrument,  the  phenomena  of  the  moon.  But, 
in  order  to  this,  it  will  be  necessary  to  speak  first  of  the 
instrument,  which  is  put  in  motion  like  the  preceding 
one,  by  the  teeth  on  the  fixed  wheel ;  it  is  also  to  be 
placed  upon  the  same  socket  as  the  tellurian,  and  con- 
fined down  by  the  same  milled  nut,  D,   fig.  1 . 

The  sloping  ring,  P  O,  represents  the  plane  of  the 
moon's  orbit,  or  path  round  the  earth;  so  that  the  moon, 
in  her  revolution  round  the  earth,  does  not  move  paral- 
lel to  the  plane  of  the  ecliptic,  but  on  this  inclined  plane; 
the  two  points  of  this  plane,  that  are  connected  by  the 
brass  wires,  are  the  nodes,  one  of  which  is  marked  &, 
for  the  ascending  node,  the  other  t3 ,  for  the  descending 
node.  The  moon  is  therefore  sometimes  on  the  north, 
and  sometimes  on  the  south  side  of  the  ecliptic,  which 
deviations  from  the  ecliptic  are  called  her  north  or  south 
latitude ;  her  greatest  deviation,  which  is  when  she  is  at 
her  highest  and  lowest  points,  called  her  limits,  is  5  de- 
grees, 18  minutes ;  this,  with  all  the  other  intermediate 
degrees  of  latitude,  are  engraved  on  this  ring,  beginning 
at  the  nodes,  and  numbered  both  ways  from  them.  At 
each  of  the  nodes,  and  at  about  18  degrees  distance  from 
them,  we  find  this  mark  o,  and  at  about  22  degrees 
this  3 ,  to  indicate,  that  when  the  full  moon  is  got  as  far 
from  the  nodes  as  the  mark  y ,  there  can  be  no  eclipse 
of  the  moon  ;  nor  any  eclipse  of  the  sun,  when  the  new 
moon  has  passed  the  mark  g  ;  these  points  are  generally 
termed  the  limits  of  eclipses.  The  nodes  of  the  moon 
do  not  remain  fixed  at  the  same  point  of  the  ecliptic,  but 
have  a  motion  contrary  to  the  order  of  the  signs. 

T  V  is  a  small  circle  parallel  to  the  ecliptic ;  it  is  di- 
vided into  12  signs,  and  each  sign  into  30  degrees;  this 
circle  is  moveable  in  its  socket,  and  is  to  be  set  by  the 
hand,  so  that  the  same  sign  may  be, opposite  to  the  sun 


156  PHENOMENA    OF    THE    MOON. 

that  is  marked  by  the  annual  index.  These  signs  always 
keep  parallel  to  themselves,  as  they  go  round  the  sun,  but 
the  inclined  plane,  with  its  nodes,  go  backwards,  so  that 
each  node  recedes  through  all  the  above  signs  in  about 
19  years.  R  S  is  a  circle,  on  which  are  divided  the  days 
of  the  moon's  age  ;  X  Y  is  an  ellipsis,  to  represent  the 
moon's  elliptical  orbit,  the  direct  motion  of  the  apogee, 
or  the  line  of  the  apsides,  with  the  situation  of  the  ellip- 
tical orbit  of  the  moon,  and  place  of  the  apogee  in  the 
ecliptic  at  all  times. 

To  rectify  the  lunarium. 

Set  the  annual  index,  E,  on  the  large  ecliptic  to  the 
first  of  Capricorn  ;  then  turn  the  plate,  with  the  moon's 
signs  upon  it,  until  the  beginning  of  Capricorn  point  di- 
rectly to  the  sun  ;  turn  the  handle  till  the  annual  index 
come  to  the  first  of  January  ;  then  find  the  place  of  the 
north  node  in  an  ephemeris,  to  which  place,  among  the 
moon's  signs,  set  the  north  node  of  her  inclined  orbit,  by 
turning  it  till  it  be  in  its  proper  place  in  the  circle  of  signs; 
set  the  moon  to  the  day  of  her  age. 

GENERAL  PHENOMENA  OF  THE  MOON. 

Having  rectified  the  lunarium  for  use,  on  putting  it 
into  motion,  it  will  be  evident, 

1.  That  the  moon,  by  the  mechanism  of  the  instru- 
ment, always  moves  in  an  orbit  inclined  to  that  of  the 
ecliptic,  and  consequently  in  an  orbit  analogous  to  that 
in  which  the  moon  moves  in  the  heavens. 

2.  That  she  moves  from  west  to  east. 

3.  That  the  white,  or  illuminated  face  of  the  moon,  is 
always  turned  towards  the  sun. 

4.  That  the  nodes  have  a  revolution  contrary  to  the 
order  of  the  signs,  that  is  from  Aries  to  Pisces ;  that  this 
revolution  is  performed  in  about  nineteen  years,  as  in 
nature. 

5.  That  the  moon's  rotation  upon  her  axis  is  effected 
and  completed  in  about  277  days  ;  whereas  it  is  29j  days 
from  one  conjunction  with  the  sun  to  the  next. 


PHASES    OP    THE    MOON.  157 

6.  That  every  part  of  the  moon  is  turned  to  the  sun, 
in  the  space  of  her  monthly  or  periodic  revolution. 

To  be  more  particular.  On  turning  the  handle,  you 
will  observe  another  motion  of  the  earth,  which  has  not 
yet  been  spoken  of,  namely,  its  monthly  motion  about  the 
common  centre  of  gravity  between  the  earth  and  moon, 
which  centre  of  gravity  is  represented  by  the  pin  Z. 
From  hence  we  learn,  that  it  is  not  the  centre  of  the 
earth  which  describes  what  is  called  the  annual  orbit,  but 
the  centre  of  gravity  between  the  earth  and  moon,  and 
that  the  earth  has  an  irregular,  vertical,  or  spiral  motion 
about  this  centre,  so  that  it  is  every  month  at  one  time 
nearer  to,  at  another  time  farther  from  the  sun.  It  is  evi- 
dent, from  this  instrument,  that  the  moon  does  not  regard 
the  centre  of  the  earth,  but  the  centre  of  gravity,  as  the 
centre  of  her  proper  motion  ;  that  the  centre  of  the  earth 
is  farthest  from  the  sun  at  new  moon,  and  nearest  at  full 
moon  ;  that  in  the  quadratures  the  monthly  parallax  of 
the  earth  is  so  sensible,  as  to  require  a  particular  equation 
in  astronomical  tables.  These  particulars  were  first  ap- 
plied to  the  orrery  by  the  late  ingenious  Mr.  Benjamin 
Martin. 

To  explain  the  phases  of  the  moon. 

The  moon  assumes  different  phases  to  us,  1.  On  ac- 
count of  her  globular  figure.  2.  On  account  of  the  mo- 
tion in  her  orbit,  between  the  earth  and  the  sun :  for 
whenever  the  moon  is  between  the  earth  and  the  sun,  we 
call  it  new  moon,  the  enlightened  part  being  then  turned 
from  us  ;  but  when  the  earth  is  between  the  sun  and  the 
moon,  we  call  it  full  moon,  the  whole  of  the  enlightened 
part  being  then  turned  towards  us. 

The  phases  of  the  moon  are  clearly  exhibited  in  this 
instrument  ;  for  we  here  see  that  the  half  which  is  oppo- 
site to  the  sun  is  always  dark,  while  that  which  is  next  to 
the  sun  is  white,  to  represent  the  illuminated  part.  Thus, 
when  it  is  new  moon,  you  will  see  the  whole  white  part 
next  the  sun,  and  the  dark  part  turned  towards  the  earth, 
showing  thereby  its  disappearance,  or  the  time  of  its  con- 
junction and  change  :  on  turning  the  handle,  a  small  por- 
tion of  the  white  part  will  begin  to  be  seen  from  the  earth, 


158  TO    EXPLAIN    THE    PHASES 

which  portion  will  gradually  increase,  and  towards  the 
end  of  the  7  th  day,  you  will  perceive,  that  half  of  the 
light,  and  half  of  the  dark  side,  is  turned  towards  the 
earth,  thus  exhibiting  the  appearance  of  the  moon  at  the 
first  quarter.  From  hence  the  light  side  will  continually 
show  itself  more  and  more  in  a  gibbous  form,  till  at  the 
end  of  fourteen  days  the  whole  white  side  will  be  turned 
towards  the  earth,  and  the  dark  side  from  it,  the  earth 
now  standing  in  a  line  between  the  sun  and  moon  ;  and 
thus  the  instrument  explains  the  opposition,  or  full  moon. 
On  turning  the  handle  again,  some  of  the  shaded  part 
will  begin  to  turn  towards  the  earth,  and  the  white  side 
to  turn  away  from  it,  decreasing  in  a  gibbous  form  till 
the  last  quarter,  when  the  moon  will  appear  again  as  a 
crescent,  which  she  preserves  till  she  has  attained  ano- 
ther conjunction. 

In  this  lunarium  the  moon  has  always  the  same  face 
or  side  to  the  earth,  as  is  evident  from  the  spots  deline- 
ated on  the  surface  of  the  ivory  ball  revolving  about  its 
axis  in  the  course  of  one  revolution  round  the  earth  ;  in 
consequence  of  which,  the  light  and  dark  parts  of  the 
moon  appear  permanent  to  us,  and  the  phases  are  shown 
as  they  appear  in  the  heavens. 

As  the  earth  turns  round  its  axis  once  in  24  hours,  it 
must  in  that  time  exhibit  every  part  of  its  surface  to  the 
inhabitants  of  the  moon,  and  therefore  its  luminous  and 
opake  parts  will  be  seen  by  them  in  constant  rotation.  As 
the  half  of  the  earth  which  is  opposed  to  the  sun  is  al- 
ways dark,  the  earth  will  exhibit  the  same  phases  to  the 
lunarians  that  they  do  to  us,  only  in  a  contrary  order, 
that  is,  when  the  moon  is  new  to  us,  we  shall  be  full  to 
them,  and  vice  versa.  But  as  one  hemisphere  only  of 
the  moon  is  ever  turned  towards  us,  it  is  only  those  that 
are  in  this  hemisphere  who  can  see  us ;  our  earth  will 
appear  to  them  always  in  one  place,  or  fixed  in  the  same 
part  of  the  heavens  ;  the  lunarians  in  the  opposite  hemi- 
sphere never  see  our  earth,  nor  do  we  ever  view  that  part 
of  the  moon  which  they  inhabit.  The  moon's  apparent 
diurnal  motion  in  the  heavens  is  produced  by  the  daily 
revolution  of  our  earth. 


OF    THE    MOON,    &C.  159 

If  we  consider  the  moon  with  respect  to  the  sun,  the 
instrument  shows  plainly,  that  one  half  of  her  globe  is 
always  enlightened  by  the  sun  ;  that  every  part  of  the  lu- 
nar ball  is  turned  to  the  sun,  in  the  space  of  her  monthly 
or  periodical  revolution  ;  and  that,  therefore,  the  length 
of  the  day  and  night  in  the  moon  is  always  the  same,  and 
equal  to  14-  of  our  days.  When  the  sun  sets  to  the  lu- 
narians in  that  hemisphere  next  the  earth,  the  terrestrial 
moon  rises  to  them,  and  they  can  therefore  never  have 
any  dark  night ;  while  those  in  the  other  hemisphere  can 
have  no  light  by  night,  but  what  the  stars  afford. 


OF  THE  PERIODICAL  AND  SYNODICAL  MONTH. 

The  difference  between  the  periodical  month,  in  which 
the  moon  exactly  describes  the  ecliptic,  and  the  synodi- 
cal,  or  time  between  any  two  new  moons,  is  here  ren- 
dered very  evident.  To  show  this  difference,  observe  at 
any  new  moon  her  place  in  the  ecliptic,  then  turn  the 
handle,  and  when  the  moon  has  got  to  the  same  pojnt  in 
the  ecliptic,  you  will  see  that  the  dial  shows  27y  days, 
and  the  moon  has  finished  her  periodic  revolution.  But 
the  earth  at  the  same  time  having  advanced  in  its  annual 
path  about  27  degrees  of  the  ecliptic,  the  moon  will  not 
have  got  round  into  a  direct  line  with  the  sun,  but  will  re- 
quire 28  days  and  4  hours  more,  to  bring  it  into  con- 
junction with  the  sun  again. 


OF  ECLIPSES  OF  THE  SUN  AND  MOON. 

There  is  nothing  in  astronomy  more  worthy  of  our 
contemplation,  nor  any  thing  more  sublime  in  natural 
knowledge,  than  rightly  to  comprehend  those  sudden 
obscurations  of  the  heavenly  bodies,  that  are  termed 
eclipses,  and  the  accuracy  with  which  they  are  now  fore- 
told. "  One  of  the  chief  advantages  derived  by  the  pre- 
sent generation  from  the  improvement  and  diffusion  of 
philosophy,  is  delivery  from  unnecessary  terror,  and  ex- 
emption from  false  alarms.  The  unusual  appearances, 
whether  regular  or  accidental,  which  once  spread  con- 


160  ECLIPSES    OF    THE    SUN    AND    MOON. 

sternation  over  ages  of  ignorance,  are  now  the  recreations 
of  inquisitive  security.  The  sun  is  no  more  lamented  when 
it  is  eclipsed,  than  when  it  sets ;  and  meteors  play  their 
coruscations  without  prognostic  or  prediction." 

We  have  already  observed,  that  the  sun  is  the  only 
real  luminary  in  the  solar  system,  and  that  none  of  the 
other  planets  emit  any  light  but  what  they  have  received 
from  the  sun  ;  that  the  hemisphere,  which  is  turned  to- 
wards the  sun,  is  illuminated  by  his  rays,  while  the  other 
side  is  involved  in  darkness,  and  projects  a  shadow,  which 
arises  from  the  luminous  body. 

When  the  shadow  of  the  earth  falls  upon  the  moon,  it 
causes  an  eclipse  of  the  moon  ;  when  the  shadow  of  the 
moon  falls  upon  the  earth,  it  causes  an  eclipse  of  the  sun. 

An  eclipse  of  the  moon,  therefore,  never  happens  but 
when  the  earth's  opake  body  interposes  between  the  sun 
and  the  moon,  that  is,  at  the  full  moon ;  and  an  eclipse 
of  the  sun  never  happens  but  when  the  moon  comes  in  a 
line  between  the  earth  and  the  sun,  that  is,  at  the  new 
moon. 

From  what  we  have  already  seen  by  the  instrument,  it 
appears,  that  the  moon  is  once  every  month  in  conjunc- 
tion, and  once  in  opposition ;  from  hence  it  would  ap- 
pear, that  there  ought  to  be  two  eclipses,  one  of  the  sun, 
the  other  of  the  moon,  every  month  ;  but  this  is  not  the 
case,  and  that  for  two  reasons;  first,  because  the  orbit  of 
the  moon  is  inclined  in  an  angle  of  about  five  degrees  to 
the  plane  of  the  ecliptic  ;  and  secondly,  because  the  nodes 
of  this  orbit  have  a  progressive  motion,  which  causes  them 
to  change  their  place  every  lunation.  Hence  it  often 
happens,  that  at  the  time  of  opposition  or  conjunction 
the  moon  has  so  much  latitude,  or  what  is  the  same 
thing,  is  so  much  below  or  above  the  plane  of  the  eclip- 
tic, that  the  light  of  the  sun  will  in  the  first  case  reach 
the  moon  without  any  obstacle,  and  in  the  other  the  earth; 
but  as  the  nodes  are  not  fixed,  but  run  successively  through 
all  the  signs  of  the  ecliptic,  the  moon  is  often,  both  at  the 
times  of  conjunction  and  opposition,  in  or  very  near  the 
plane  of  the  ecliptic  ;  in  these  cases  an  eclipse  happens, 
either  of  the  sun  or  moon,  according  to  her  situation. 
The  whole  of  this  is  rendered  clear  by  the  lunarium. 


OF    A    NEW    TERRESTRIAL    GLOBE.  161 

where  the  wire  projecting  from  the  earth,  shows  when 
the  moon  is  above,  below,  or  even  with  the  earth,  at 
the  times  of  conjunction  and  opposition,  and  thus  when 
there  will  be,  or  not,  any  eclipse. 

The  distance  of  the  moon  from  the  earth  varies  sen- 
sibly with  respect  to  the  sun  ;  it  does  not  move  in  a  cir- 
cular, but  in  an  elliptic  orbit  round  us,  the  earth  being 
at  one  of  the  foci  of  this  curve.     The  longest  axis  of  the 
lunar  orbit  is  not  always  directed  to  the  same  point  of 
the  heavens,  but  has  a  movement  of  its  own,  which  is 
not  to  be  confounded  with  that  of  the  nodes  ;  for  the  mo- 
tion of  the  last  is  contrary  to  the  order  of  the  signs,  but 
that  of  the  line  of  apsides  is  in  the  same  direction,  and 
turns  to  the  same  point  of  the  heavens  in  about  nine 
years.     This  motion  is  illustrated  in  the  lunarium  by 
means  of  the  brass  ellipsis  X  Y,  fig.  2,  which  is  carried 
round  the  earth  in  less  than  nine  years ;  thus  showing 
the  situation  of  the  elliptical  orbit  of  the  moon,  and  the 
place  of  the  apogee  in  the  ecliptic. 


OF  A  NEW  TERRESTRIAL  GLOBE,*   plate    13,  fig%   2, 

And  of  a  new  apparatus  adapted  thereto,  for  solving,  in  an 
easy  and  natural  manner,  the  several  phenomena  of  the 
sun,  moon,  and  earth. 

Though  globes  have  ever  been  considered  as  the  best 
instruments  for  conveying  general  ideas  of  astronomy 
and  geography,  yet  they  have  always  been  mounted  in 
a  way  that  must  perplex  and  confuse  the  learner,  and 
furnish  him  with  ideas  that  are  altogether  false,  and  con- 
trary to  the  nature  of  things. 

That  you  may  clearly  perceive  the  great  advantages 
of  a  globe  mounted  like  that  before  you,   I  shall  first 


*  The  terrestrial  globe  was  first  improved  by  my  father,  and  placed  in  a 
fixed  position,  &cc.  The  floating  meridian -and  horizon  were  added  by  Mr. 
Newman. 

VOL.  IV.  Y 


162  OF    A    NEW    TERRESTRIAL    GLOBE. 

point  out  a  few  of  the  imperfections  of  globes  mounted 
in  the  common  way.* 

Now,  in  the  first  place,  what  is  rectifying  a  globe  thus 
mounted,  but  a  continual  absurdity  ?  For  to  rectify  the 
globe  to  any  particular  latitude,  the  axis  of  the  earth  is 
continually  shifted  from  one  false  position  to  another, 
the  mind  of  the  pupil  is  confused,  and  he  with  difficulty 
conceives,  that  the  axis  of  the  earth  never  varies  its  po- 
sition, but  always  preserves  the  same  inclination  to  the 
plane  of  its  orbit. 

The  broad  paper  circle  of  the  common  globes  is  de- 
signed to  represent  the  ecliptic  and  the  horizon  ;  but  on 
examination  you  will  find,  it  represents  neither  the  one 
nor  the  other.  Now  the  ecliptic  is  the  apparent  path  of 
the  sun,  with  which  the  earth's  axis  always  makes  an 
angle  of  661  degrees ;  but  by  shifting  the  axis  of  the 
globe,  to  rectify  for  the  latitude,  this  circle  can  never 
be  in  its  position  as  ecliptic,  except  when  the  axis  is  at  66± 
degrees,  and  consequently,  cannot  be  used  as  the  ecliptic. 
Now,  let  us  consider  it  as  the  horizon.  Every  place  is 
in  the  zenith  of  its  horizon,  and  the  place  and  horizon 
always  move  together ;  but  in  the  common  globes,  the 
broad  paper  circle  is  the  horizon  in  one  situation  only, 
that  is,  when  the  place  is  in  the  zenith  ;  after  having 
rectified  the  globe  to  the  latitude,  the  moment  you  move 
the  globe,  the  broad  paper  circle  is  no  longer  the  hori- 
zon. Thus  it  is  plain,  that  this  circle  cannot  with  pro- 
priety be  considered  either  as  a  horizon  or  an  ecliptic.  As 
if  it  were  to  multiply  confusion,  a  circle  is  laid  down  on 
the  terrestrial  globe  to  represent  the  ecliptic,  and  used  as 
such  in  solving  problems  upon  the  common  globes, 
though  it  involves  the  pupil  in  numerous  absurdities : 
thus,  having  marked  the  sun's  place  in  the  ecliptic,  and 
rectified  the  globe  to  the  latitude,  then  turn  the  globe, 
and  the  sun  and  earth  have  a  diurnal  motion  together  ; 
of  course,  if  you  have  day  when  you  begin,  you  will 


*  As  I  do  not  agree  entirely  with  our  Author,  in  what  he  considers  the 
imperfections  oi  gl  bes  moun  t d  in  the  common  manlier;  in  my  Appen- 
dix to  this  lecture  I  have  added  a  few  explanatory  observations  thereon* 
E.  Edit. 


OF    A    NEW    TERRESTRIAL    GLOBE.  163 

have  the  same  daring  the  whole  twenty-four  hours.  Ma- 
ny other  errors  might  easily  be  pointed  out,  but  these 
are  sufficient  to  show  you  that  no  one  can  be  properly 
taught  by  globes  so  mounted. 

The  globe  before  you,  plate  1  S^Jig.  2,  does  not  hang 
in  a  frame  like  the  common  globes,  but  is  mounted  on 
a  pedestal,  and  is  supported  by,  and  moveable  on,  an 
axis,  which  is  enclined  66\  degrees  to  the  ecliptic,  and  of 
course  is  always  parallel  to  the  axis  of  the  earth,  sup- 
posing the  path  of  the  globe  to  be  parallel  to  the  eclip- 
tic*. On  the  pedestal,  but  under  the  globe  is  a  gradu- 
ated circle  C  D,  marked  with  the  signs  and  degrees  of  the 
ecliptic  ;  adjoining  thereto  is  a  circle  of  months  and 
days,  answering  to  every  degree  of  the  ecliptic  ;  within 
this  circle  is  the  sun's  declination  for  every  day  of  the 
month. 

There  is  a  moveable  arm  A  B,  which,  being  set  to  the 
day  of  the  month,  immediately  points  out  the  sun's  place 
in  the  ecliptic,  and  his  declination.  On  this  moveable 
arm,  but  nearer  the  index,  you  observe  a  pillar  E ;  on 
the  top  of  the  pillar  is  fixed  a  small  ball,  through  which 
a  steel  wire  passes,  to  represent  a  ray  proceeding  from 
the  centre  of  the  sun. 

Round  the  globe  is  a  brass  circle  F,  to  represent  the 
horizon  of  any  place  ;  and  at  right  angles  to  this  horizon, 
is  fixed  a  semicircle  G,  to  answer  for  a  general  meridian. 
The  middle  point  of  the  semicircle  answers  for  the  situ- 
ation of  any  inhabitant  on  the  earth,  for  which  reason  a 
steel  pin  is  fixed  over  the  middle  point  of  this  semicircle. 

One  supposition  only  is  necessary  for  performing  every 
problem  with  this  globe  ;  namely,  that  a  spherical  lumi- 
nous body  will  enlighten  one  half  a  spherical  opake  bo- 
dy, and,  consequently,  that  a  circle  at  right  angles  to  the 
central  solar  ray,  and  dividing  the  globe  in  halves,  will 
be  a  terminator,  showing  the  boundaries  of  light  and 
darkness  for  any  given  day. 


*  Globes  of  9  or  12  inches  in  diameter,  when  mounted  in  the  i  bove  man- 
ner, are  ot  the  most  convenient  dimensions.  If  much  accuracy  be  want- 
ed, and  no  limited  expense  given,  an  18  inch  globe,  so  mounted,  makes  a 
very  useful  and  illustrative  instrument. — E  E«it. 


164  OF    A    NEW    TERRESTRIAL    GLOBE. 

For  this  purpose,  at  the  end  of  the  moveable  arm 
opposite  to  the  sun,  there  is  a  pillar  H,  from  the  top  of 
which  projects  a  piece  to  carry  a  circle  I,  that  surrounds 
the  globe,  and  always  divides  it  into  equal  portions,  sepa- 
rating the  enlightened  from  the  dark  parts.  Eighteen 
degrees  behind  this  circle,  but  parallel  thereto,  is  another 
circle,  to  represent  the  limits  of  twilight. 

There  are  two  brass  plates,  K  and  L,  under  the  globe, 
which  are  turned  by  the  diurnal  revolution  thereof ;  each 
of  them  is  divided  into  twice  twelve  hours  and  parts  of 
an  hour,  to  answer  for  the  hours  of  day  ;  on  the  out- 
side are  laid  down  the  degrees  of  longitude  for  every 
hour  ;  so  that  these  circles  give  you  at  sight  the  hour  of 
the  day  or  night,  at  any  two  places  on  the  globe,  and 
the  difference  of  longitude  corresponding  thereto. 

There  is  one  thing  more  relative  to  the  globe,  which 
renders  it  a  planetary  globe  ;  for,  by  setting  this  pillar, 
plate  13,y%.  4,  to  the  place  of  any  planet  in  the  eclip- 
tic, and  the  ball  to  the  latitude  of  the  planet,  it  will 
solve  all  problems  relative  to  that  planet,  or  the  moon ; 
as  it  does  for  the  sun,  by  means  of  a  central  solar  ray. 

To  rectify  this  globe,  set  the  division  under  the  repre- 
sentative inhabitant  over  the  given  place,  set  the  solar 
index,  L,  to  the  day  of  the  month  ;  then  turn  the  globe 
round  on  the  axis  till  the  meridian  coincide  with  the 
central  solar  ray,  and  the  hour-index  under  the  globe  to 
XII ;  and  this  globe  is  then  in  the  position  of  our  globe, 
with  respect  to  the  sun  and  that  place,  &c. 

Place  the  inhabitant  on  the  western  side  of  the  termi- 
nator, and  he  will  see  the  sun  or  the  central  solar  ray 
rising  in  the  horizon,  and  this  ray  will  mark  thereon  the 
sun's  amplitude,  and  the  hour-circle  gives  you  the  hour 
and  minute  of  the  sun's  rising  on  that  day  and  place  ; 
turn  the  globe  gradually  till  the  meridian  coincide  with 
the  central  solar  ray,  and  the  point  will  mark  out  the 
sun's  meridian  altitude  for  that  day.  As  the  globe  goes 
on,  the  altitude  decreases,  and  when  the  inhabitant  is 
arrived  at  the  other  side  of  the  terminator,  the  solar  ray- 
is  in  the  horizon,  points  out  the  sun's  amplitude,  and 
you  have  the  time  of  his  setting  on  the  hour-circle.  If 
you  proceed  to  turn  the  globe  till  the  inhabitant  be  under 


OF    THE    CELESTIAL    CLOBE.  165 

that  circle  which  is  behind  the  terminator,  the  hour-circle 
will  give  the  time  that  twilight  finishes.  The  sun's  alti- 
tude for  any  given  time  of  the  day,  is  obtained  by  stopping 
the  globe  when  the  index  points  out  that  hour,  and  the 
quadrant  of  altitude  over  the  inhabitant,  and  then  bring- 
ing it  to  the  central  ray,  which  will  point  out  thereon  the 
altitude  for  that  hour. 

In  this  manner  you  may  solve  the  same  questions  for 
any  other  place,  or  any  other  day,  always  observing,  1. 
To  fix  the  inhabitant  over  the  given  place.  2.  To  set  the 
sun's  annual  index  to  the  given  day  of  the  month.  3.  To 
bring  the  meridian  to  the  central  solar  ray,  and  the  hour- 
index  under  the  globe  to  XII.  By  placing  the  small 
pillar  to  the  moon's  place  in  the  ecliptic,  and  the  ball  to 
her  latitude,  the  same  problems  may  be  solved  at  the  same 
time  for  the  moon  ;  and  so  respectively  of  any  other 
planet. 

By  this  globe,  a  person  entirely  unacquainted  with  as- 
tronomy may,  in  a  few  hours,  acquire  a  competent  and 
natural  notion  of  the  principal  phenomena,  and  be  en- 
abled to  solve  the  greatest  part  of  the  most  interesting 
problems  concerning  the  sun,  moon,  and  planets. 


OF    THE    CELESTIAL    GLOBE,  Jig.  3,  plate   13. 

As  the  terrestrial  globe  js  mounted  to  correspond  ex- 
actly with  the  globe  of  our  earth,  and  every  problem 
answered  as  the  phenomena  are  really  occasioned  by  the 
annual  and  diurnal  motion  of  the  earth  ;  so  the  celestial 
globe,  to  be  comfortable  to  nature,  should  be  as  nearly 
as  possible  an  exact  imitation  of  the  heavens,  and  their 
situation  with  respect  to  the  earth  ;  which  is  far  from 
being  the  case  with  the  common  globes. 

To  make  the  celestial  globe  thus  comfortable  to  na- 
ture, it  should  have  no  motion  ;  the  appearance  of 
motion  in  the  firmament  arises  from  the  diurnal  motion 
of  the  earth  ;  it  is  plain,  therefore,  that  whatever  gives 
a  true  representation  of  the  heavens  will  have  no  motion. 
The  celestial  globe  before  you,  is  therefore  fixed  on  an 
axis,  making,  like  that  of  the  terrestrial  globe^  an  angle 


\66  OF    THE    CELESTIAL    GLOBE. 

of  66*  degrees  with  the  plane  of  the  ecliptic ;  and  the 
ecliptic  on  this  globe  exactly  coincides  with  the  sun's 
apparent  path  round  the  earth.  All  problems  concern- 
ing the  sun,  moon,  and  planets,  are  performed  by  the 
terrestrial  globe.  This  globe  needs  only  be  used  for  the 
stars,  and  one  or  two  problems  will  give  you  a  suffi- 
cient idea  of  the  manner  of  solving  all  that  relates  to 
the  stars. 

Tojind  the  latitude  and  longitude  of  a  given  star.  Find 
the  star  on  the  globe,  and  then  place  the  index  and  clip, 
A,  on  the  ecliptic-plate,  slide  the  siderial  index  till  it  be 
exactly  over  the  star  ;  then  the  latitude  is  shown  on  the 
arc,  and  the  longitude,  by  the  index,  on  the  ecliptic. 

Tojind  the  rising,  setting,  amplitude,  and  meridian  alti- 
tude of  the  same  star.  Take  the  clip  from  the  celestial 
globe,  and  put  it  to  the  same  degree  of  longitude  on  the 
terrestrial  ecliptic  plate  ;  turn  the  globe  on  its  axis,  and 
the  time  of  its  rising  and  setting  is  immediately  pointed 
out  by  the  hour-index  ;  its  amplitude  is  shown  on  the 
horizon  ;  its  meridian  altitude,  by  the  meridian  ;  and 
its  azimuth  and  altitude  for  any  hour,  by  applying  the 
quadrant  of  altitude  under  the  siderial  index  for  that 
hour. 

I  shall  conclude  my  lectures  on  these  instruments 
with  some  observations  that  naturally  arise  from  consi- 
dering the  art  and  ingenuity  with  which  they  are  con- 
structed. For,  when  we  see  materials  working  with 
an  art  and  contrivance  that  is  not  in  their  nature,  we 
are  at  once  convinced  that  a  superior  intelligence  has 
been  concerned  in  their  arrangement,  &c. 

Thus  also  in  nature,  whatever  bears  the  marks  of 
a  wisdom  not  belonging  to  the  known  causes  produc- 
ing it,  may  be  properly  stiled  providential :  for,  when 
agents  void  of  wisdom  act  wisely,  it  is  plain  there 
must  be  some  hand  to  conduct  them,  though  we  may 
not  be  able  to  perceive  by  what  springs  or  channels  of 
communication  it  operates.  There  wants,  therefore,  no 
long  train  of  reasoning  to  lead  us  into  the  knowledge 
of  a  Providence.  Penetration  and  closeness  of  thought 
have  no  further  use  in  this  case,  than  to  discover  the  fal- 
lacy of  those  sophisms,  wherein  infidel  writers  endea- 


CONCLUSIVE    OBSERVATIONS.  167 

vour  to  overcloud  the  most  apparent  truths.  The  plain 
man  needs  no  assistance  here  from  the  philosopher,  but 
may  say  to  him  as  Diogenes  did  to  Alexander •,  "  Only 
please  to  stand  out  of  my  sun-shine." 

Intelligence*  is  manifested  two  ways,  either  by  means 
supplied  to  answer  the  endwe  conceive  to  have  been  had 
in  view,  though  we  do  not  discern  the  method  by  which 
they  were  prepared  ;  or  else  by  the  contrivance  appa- 
rent in  productions,  though  we  do  not  see  what  end  they 
answer  :  the  former  more  particularly  gives  us  the  dis- 
play of  providence  ;  the  latter,  of  the  wisdom  where- 
with it  is  administered. 

If  you  saw  a  house  stored  with  furniture,  utensils, 
and  victuals  ;  the  gardens  planted  with  herbs  and  fruit 
trees  ;  the  ground  stocked  with  cows,  horses,  deer,  and 
poultry,  all  in  a  manner  fitted  for  the  entertainment 
and  convenience  of  a  family  ;  you  would  certainly  con- 
clude, there  was  some  master  who  had  taken  care  to  pro- 
vide for  the  uses  whereto  they  were  respectively  proper. 
Or,  if  an  ignorant  person  went  into  a  room  where, 
among  scales,  weights,  compasses,  rules,  and  other  things 
of  common  use,  he  should  find  quadrants,  theodolites, 
armillary  spheres,  planetariums,  tellurians,  &c.  of  whose 
use,  as  well  as  of  the  figures  upon  them,  he  was  entire- 
ly ignorant ;  yet  he  would  know,  without  being  told, 
that  they  were  the  work  of  some  artificer  proceeding 
with  skill  and  contrivance,  and  who  made  them  for 
purposes  worthy  the  care  with  which  they  were  finished. 

In  this  manner  we  constantly  reason  upon  common 
occasions,  and  there  wants  only  a  proper  attention  to 
lead  us  into  the  like  train  of  thinking  upon  the  pheno- 
mena of  visible  nature.  For  there  you  may  perceive 
ample  provision  made  in  vast  variety  for  the  numerous 
family  of  Adam  ;  corn,  fruit,  herbs,  cattle,  and  fowl, 
for  our  sustenance  ;  wool,  flax,  and  cotton,  for  our 
cloathing ;  drugs  and  simples  for  our  relief ;  air  for 
our  breathing  ;  timber,  stone,  lime,  and  brick-earth  for 
our  habitation  ;  wood  and  coal  for  our  firing  ;  beasts  of 


Tucker's  Light  of  Nature,  vol.  iii.  part  1,  page  192. 


168  CONCLUSIVE    OBSERVATIONS. 

burden  for  our  assistance  ;  winds  to  purify  our  atmos- 
phere, to  re  fresh  our  heats,  and  waft  us  from  shore  to 
shore  ;  variety  of  climes  and  soils  to  bear  us  a  produce 
of  every  kind  ;  dews  and  rains  to  make  them  yield  us 
their  increase.  The  sea,  that  original  source  of  water, 
so  necessary  to  us  for  many  uses,  serves  likewise  to  as- 
sociate distant  nations  by  opening  the  communication 
of  commerce.  The  sun  diffuses  his  warmth  and  light 
to  cherish  us.  The  distant  stars  guide  us  over  the 
boundless  ocean  and  inhospitable  desert,  extend  the 
fields  of  science  to  an  immensity  of  space,  and  turn  the 
rugged  brow  of  night  into  a  cheerful  scene  of  contem- 
plation. 

Even  within  the  narrow  compass  of  our  own  bodies, 
we  carry  about  no  inconsiderable  stores,  without  which 
we  could  not  receive  benefit  from  those  arround  us. 
We  have  engines  of  digestion  and  secretion,  springs  and 
channels  of  circulation,  limbs  for  instruments  of  action, 
bones  for  our  support  and  protection,  organs  of  speech 
for  our  mutual  intercourse.  What  a  multitude  of  ves- 
sels, glands,  and  ducts,  to  concoct  and  distribute  our 
aliment !  What  artificial  structure  and  excellent  dispo- 
sition of  muscles  and  joints,  to  serve  for  instruments  of 
action !  What  amazing  nicety  in  the  organs  of  sense ! 
The  eye,  with  her  humours  and  coats  mathematically 
arranged,  and  duly  proportioned  one  among  the  other ; 
the  ear  in  winding  and  modulating  the  vibrations  of  air 
into  sounds  ;  the  nerves  in  imperceptible  threads  running 
every  where  through  the  fleshy  parts,  yet  returning 
their  notices  without  impediment  from  the  farthest  ex- 
tremities of  our  limbs  !  And  all  this  complicated  machine, 
containing  an  infinitude  of  multiform  works,  is  bound 
up  in  a  small  compass,  yet  with  such  stupendous  skill, 
that  they  do  not  interfere  with  each  others  operations, 
nor  fall  into  discord  upon  our  motions ! 

We  have  appetite  to  stimulate,  senses  to  inform,  the 
faculties  of  comparing,  distinguishing,  judging,  to  en- 
lighten, and  reason  to  direct  us.  In  the  capacity  of  our 
senses  and  affections,  we  have  sources  of  pleasure,  en- 
joymentj,  and  innocent  mirth. 


CONCLUSIVE    OBSERVATIONS.  169 

In  the  multitude  of  the  objects  of  creation,  we  find 
a  provision  made  and  suited  to  our  various  organs, 
tastes,  and  faculties,  a  fund  for  bodily  support,  sub- 
jects for  intellectual  inquiries  and  mental  gratification. 
"  Which  shall  we  admire  most,  the  multitude  of  our 
organs,  their  finished  form  and  faultless  order,  or  the 
power  which  the  mind  exercises  over  them  ?  Ten  thou- 
sand veins  are  put  into  her  hands,  and  yet  she  manages 
and  conducts  them  all  without  the  least  perplexity  or 
irregularity  ;  with  a  promptitude,  a  consistency^  that 
nothing  else  can  equal ;  touching  every  spring  of  the 
human  machine  with  the  most  masterly  skill,  though 
she  knows  nothing  of  the  nature  of  her  instrument,  or 
the  process  of  her  operation. 

If  you  turn  your  eyes  upon  the  vegitable  tribes,  you 
perceive  them,  in  countless  multitudes  of  trees,  shrubs, 
weeds,  mosses,  &c.  each  growing,  spreading,  and  flou- 
rishing, by  laws  adapted  to  its  own  kind  ;  and  all  work- 
ed with  such  exactness  and  nicety  of  art,  as  the  greatest 
human  ingenuity  could  not  imitate ;  their  sap-vessels 
curiously  woven  within  the  stem,  and  dispersed  among 
the  roots  and  branches  ;  their  leaves  wrought  finer  than 
needle-work.  The  finest  works  of  the  loom  and  the 
needle,  when  examined  with  a  microscope,  appear  so 
rude  and  coarse,  that  a  savage  might  be  ashai;;ed  to 
wear  them  :  but,  when  the  work  of  God  is  brought  to 
the  same  test,  we  see  how  fibres,  too  minute  for  the 
naked  eye,  are  composed  of  others  still  more  minute; 
and  these  again  of  others  ;  till  the  primordial  threads, 
or  first  principles  of  the  texture,  are  utterly  undiscern- 
able ;  while  the  whole  substance  presents  a  celestial  ra- 
diance in  its  colouring,  as  if  it  were  intended  for  the 
cloathing  of  an  angel. 

Yet  are  these  wonders  of  the  vegetable  world  sur- 
passed by  those  of  the  animal,  whose  frame  contains  a 
more  complicated  machinery,  capable  of  more  admira- 
ble play  :  for,  besides  the  engines  of  growth  and  nutri- 
ment analogous  in  both,  the  animal  is  furnished  with 
organs  of  sensation,  and  instruments  of  activity.  What 
a  richness  of  invention  is  displayed  in  the  variety  of 
VOL.  iv.  z 


110  CONCLUSIVE    OBSERVATIONS. 

their  forms,  and  the  diversity  of  their  cloathing.  Nor 
can  we  help  remarking  those  surprising  instincts  that 
Severally  guide  them  to  their  harbours,  their  fo<.  ds, 
their  ways  of  breeding  and  preservation,  instruct  them 
to  build  their  nests,  to  make  their  comb,  to  spin  their 
webs,  and  provide  for  the  future  without  knowledge  of 
their  wants. 

Nor  must  we  omit  the  uses  and  qualities  assigned  to 
animals,  that  we  can  turn  commodiously  to  our  advan- 
tage :  we  have  not  to  seek  our  wool  from  the  fierce 
lion,  nor  want  the  untameable  tyger  to  plow  our 
grounds  ;  but  the  ox,  the  horse,  and  the  sheep,  have 
docility  and  manageableness  given  them  for  their  cha- 
racteristics. Creatures  saleable  in  the  market  are  more 
prolific  than  those  of  the  savage  kind.  Poultry  and  rab- 
bits keep  within  their  accustomed  purlieus  ;  but  nobo- 
dy knows  where  to  find  the  coarse-grained  heron,  or 
the  worthless  cuckoo.* 

"  O  Lord,  how  manifold  are  thy  works ;  in  wisdom 
hast  thou  made  them  all ;  the  earth  is  full  of  thy  riches! 
All  creatures  wait  upon  thee,  that  thou  mayest  give  them 
their  meat  in  due  season.  When  thou  givest  it  them 
they  gather  it,  and  when  thou  openest  thine  hand  they 
are  filled  with  good."  How  great  and  beautiful  is  this 
idea !  The  hand  of  man  scatters  food  to  the  few  crea- 
tures that  are  about  him  ;  but  when  the  hand  of  God 
is  opened,  a  world  is  fed  and  satisfied.! 


Rev.  W,  Jones's  Sermons,  vol,  ii.  p.  63.        f  Ibid.  p.  104. 


[     171     ] 


APPENDIX  TO  LECTURE  XLIII. 


BY  THE  E.  EDITOR. 


CONTAINING  A  PARTICULAR  DESCRIPTION  OF  THE 
BEST  SIMPLE  PORTABLE  ORRERY;  COMPARATIVE 
OBSERVATIONS  ON  THE  DIFFERENT  MODES  OF 
MOUNTING  GLOBfcS  ;  AND  A  DESCRIPTION  OF  THE 
PORTABLE    EQUATORIAL    INSTRUMENT.       Plate  14, 

Jig-   2. 


J\S  our  author  has  not  given  a  full  reference  to 
the  several  parts  of  the  orrery,  as  noticed  in  page  135, 
and  its  being  of  the  kind  the  most  portable  and  complete 
hitherto  made,  I  have  thought  it  better  to  add  here  a 
further  description  of  the  machine,  and  some  observa- 
tions on  globes  and  the  equatorial. 

The  words,  orrery  and  planetarium,  are  frequently 
used  indifferently  to  signify  the  same  instrument.  Among 
instrument-makers,  the  term,  orrery,  is  generally  given 
to  a  large  and  complete  machine,  showing  all  the  mo. 
tions  of  the  planetary  bodies  in  the  most  perfect  man- 
ner possible  by  wheel-work.  The  term,  planetarium, 
is  given  to  an  instrument  showing  chiefly  the  periodical 
revolutions  of  the  primary  planets  ;  the  word,  tellurian, 
to  an  instrument  showing  completely  the  annual  and 
diurnal  motions  of  the  earth  only  ;  and  lunarium,  to  an 
instrument  showing  the  motions  and  various  appearances 
of  the  moon  only. 

These  instruments  have  been  made  of  various  de- 
grees of  magnitude  and  perfection,  and  the  completest 
sort  are  those  that  exhibit  by  wheel-work  the  periodical 
revolutions  of  the  satellites,  as  well  as  of  the  primary 


172  martin's  orrery  described. 

planets  and  their  diurnal  motions.  The  quantity  of 
wheel-work,  as  well  as  other  machinery,  to  produce 
such  motions,  creates  an  expense  unavoidably  great. 
Rowlefs  grand  orrery,  as  made  of  late  years  by  Wright, 
appears  to  be  the  most  expensive  one  ever  made  in  this 
country  ;  the  expense  of  which,  I  am  informed,  is  not 
less  than  5001.  It  will  be  unnecessary  to  enter  here  into 
a  detail  of  the  very  considerable  varieties  of  inferior  orre- 
ries since  made  by  Ferguson,  Martin,  and  myself;  we  now 
make  them  from  100  guineas  downwards  to  one  guinea, 
and  therefore  suitable  to  the  different  purposes  and 
purses  of  all  students  and  amateurs  of  astronomy. 

The  orrery  now  selected  by  our  Author,  see  plate  1 1 , 
Jig.  ],  and  plate  12,  Jig.  1  and  2,  was  contrived  by  the 
late  learned  and  ingenious  instrument-maker  Mr.  B. 
Martin,  about  the  year  1770,  and  in  a  small  pamphlet 
published  by  him  in  1771,  was  announced  as  an  orrery 
of  a  new  construction,  representing  in  the  various  parts 
of  its  machinery  all  the  motions  and  phenomena  of  the 
planetary  system. 

For  a  portable  instrument,  it  is  in  my  opinion  the 
most  complete  and  elegant  ever  made ;  excepting  the 
ivory  balls  representing  the  planets,  it  is  all  made  of 
brass.  The  brass  box,  ABC,  plate  11,  fig.  1,  is  about 
1 1  inches  in  diameter,  and  instead  of  being  supported 
upon  three  short  feet,  as  represented  in  the  plate,  we 
make  the  box  to  be  supported  on  a  brass  pillar  and  claw 
feet,  like  the  stand  of  the  tellurian,  plate  IS,  fig.  3,  as 
originally  done  by  the  inventor :  in  this  manner,  it  is 
found  to  be  more  commodious  and  conspicuous  when 
in  use.  To  represent  all  the  various  motions  of  the  pla- 
nets by  wheel-work  in  one  machine,  occasions  it  to  be 
of  great  bulk  and  weight ;  Mr.  Martin,  by  constructing 
his  machine  in  different  parts,  certainly  diminished  that 
unity  and  magnificence  of  one  great  and  elegant  ma- 
chine, but  at  the  same  time  decreased  very  considerably 
the  expense,  and  rendered  the  apparatus,  notwithstand- 
ing, very  simple,  complete,  and  instructive. 

The  component  parts  of  this  orrery  are  as  follows  : 

1 .  A  planetarium,  plate  1 1 ,  fig.  1 ,  exhibiting  the  or- 
der, motion,  and  aspects,  of  all  the  primary  planets  of 


martin's  orrery  described.  173 

our  system.  The  new  Georgian  planet  is  not  represent- 
ed in  the  figure,  but  is  usually  applied  by  us  to  the  instru- 
ment. The  planets  are  easily  put  on  or  taken  off  from 
their  respective  sockets  occasionally.  Their  heliocentric 
places  in  the  ecliptic  below,  for  the  day,  are  to  be  first  set 
by  the  astistance  of  White's  Ephemeris  or  the  Nautical 
Ephemeris  for  the  year  ;  then,  by  turning  the  winch  or 
handle  they  will  all  move  from  west  to  east,  with  the 
same  respective  motions  and  periodical  times  as  they  have 
in  the  heavens  ;  thus  representing  in  a  just  manner  the 
Pythagorean,  and  Copernican  or  Solar  System. 

For  the  representation  of  the  Ptolemaic,  or  vulgar  sys- 
tem, our  Author  has  already  given  directions  at  page 
145,  et  seq. 

The  apparatus,  plate  1 1,  Jig*  2  and  3,  is  used  to  ex- 
emplify the  apparent  retrograde  and  direct  motions,  and 
sometimes  stationary  positions,  of  the  planets.  You  take 
off  the  ivory  earth  ®,  and  place  the  socket,  P,  upon  the 
wire  in  its  stead  ;  the  socket,^.  3,  is  to  be  applied  upon 
either  the  arms  of  Mercury  $ ,  or  Venus  9 ,  instead  of 
the  ivory  balls.  The  retrograde  arm  or  wire,  Jig.  2, 
about  the  part  F,  is  then  to  be  placed  on  the  nut-piece, 
as  shown  at  Jig.  3  ;  this  wire  represents  a  ray  of  light 
coming  from  the  planet  to  the  object ;  and  the  small 
ball,  the  planet  as  it  appears  among  the  stars  in  the  hea- 
vens. When  the  winch  is  turned,  the  direct  motion, 
stationary  position,  and  apparent  retrograde  motion  of 
the  inferior  planets,  as  seen  from  the  earth,  will  be 
clearly  shown  agreeable  to  what  our  Author  has  before 
related,  see  page  142,  and  plate  \\^  Jig.  2. 

2.  The  tellurian,  plate  12 ,  jig.  1,  is  the  second  part 
of  this  orrery.  When  the  planetary  arms  are  all  taken 
off  from  their  central  sockets,  this  part  is  to  be  applied  ; 
it  is  made  fast  on  the  socket  by  the  brass  screw-nut,  D. 
The  globe  is  three  inches  in  diameter,  by  which  is  accu- 
rately and  evidently  shown  all  the  phenomena  arising 
from  the  annual  and  diurnal  motions  of  the  earth.  For 
this  purpose,  the  axis  of  the  earth  keeps  a  perfect  paral- 
lelism and  constant  inclination  to  the  plane  of  the  eclip- 
tic :  the  circle  of  illumination  or  terminator,  H,  divides 
the  globe  into  its  enlightened  and  dark  hemispheres,  and 


174  martin's  orrery  described. 

and  by  the  dial-plate,  N  O,  under  the  globe,  it  will  appear 
at  what  hour  the  sun  rises  and  sets  to  every  country  and 
on  every  day  of  the  year.  There  is  an  index,  G,  to  show 
when  it  is  noon,  or  when  the  sun  is  upon  the  meridian 
of  any  particular  place  ;  by  the  same  index  is  also  shown 
what  sign  or  degree  of  the  ecliptic  the  sun  is  in  for  every 
day,  the  parallel  of  declination  it  describes,  and  the  length 
of  the  diurnal  and  nocturnal  parts  of  that  parallel  in  the 
light  and  dark  hemispheres.  At  times,  it  is  convenient 
to  have  a  slower  motion  of  the  earth,  for  which  there  is 
in  the  box  a  provision  made,  and  you  have  only  to  shift 
the  winch  from  one  hole  in  the  side  of  the  box  to  the 
other. 

When  the  tellurian  is  used,  it  is  to  be  put  upon  the 
socket  of  the  earth,  and  placed  exactly  over  the  degree 
of  the  ecliptic  which  the  earth  possesses  that  day,  or  so 
that  the  index,  E,  at  the  end  of  the  arm,  may  point  to 
the  sun's  place  in  the  ecliptic  ;  and  then  screwing  it  fast 
upon  the  socket  by  the  nut  D,  and  turning  the  winch, 
the  proper  motions  will  commence. 

To  explain  more  completely  the  various  phenomena  of 
the  earth,  another  three- inch  globe  is  made,  furnished 
with  a  brass  divider,  meridian,  horizon,  and  quadrant  of 
altitude,  similar  to  those  shown  at  plate  13,  Jig.  2,  all 
graduated  ;  and  also  a  circle  to  represent  the  twilights. 
By  this,  all  the  chief  problems  on  the  terrestrial  globe 
may  be  performed  and  illustrated  in  a  very  natural  man- 
ner, and  has  been  described  by  our  Author  at  page  151. 

3.  The  lunarium\  plate  1 2,  fig.  2,  is  placed  upon  the 
same  socket,  -fig.  1 ,  instead  of  the  tellurian.  Its  machi- 
nery is  also  put  into  motion  by  the  teeth  of  the  large  fixed 
ecliptic  plate  P  Q^  The  motions  shown  by  this  part  an- 
swer all  the  phenomena  of  a  satellite  or  moon  revolving 
about  its  primary,  while  that  moves  about  the  sun.  These 
motions  and  appearances  are  as  follows : 

1 .  The  menstrual  motion  of  the  earth  and  moon  about 
the  common  centre  of  gravity,  at  z,fig.  2,  between  them. 

2.  The  circular  motion  of  this  centre  of  gravity  about 
the  sun,  which  describes  the  true  annual  orbit,  and  in 
which  the  earth  has  a  very  irregular,  or  rather  vermicu- 


martin's  orrery  described.  175 

lar  motion,  from  one  side  to  the  other,  being  in  each 
month  nearer  to  and  farther  from  the  sun. 

3.  The  monthly  motions  of  the  moon,  viz.  the  peri- 
odical month,  in  which  the  moon  describes  exactly  the 
ecliptic ;  and  the  synodical  month,  which  shows  the  space 
or  time  between  two  moons  or  conjunctions. 

4.  The  annual  parallel  motion  of  the  ecliptic  showing 
the  place  of  the  moon ;  by  the  ring,  T  Q,  the  place  of 
the  nodes  ;  by  the  oval  ring,  X  V,  the  apogee  and  peri- 
gee,  as  also  the  geocentric  motions,  places,  and  aspects 
of  the  sun  and  all  the  planets. 

5.  The  retrograde  motion  of  the  nodes,  with  the  in- 
clination of  the  lunar  orbit,  and  the  degree  of  her  latitude 
from  the  ecliptic  in  every  part. 

6.  The  direct  motion  of  the  apogee,  or  the  line  of  the 
apsides,  with  the  situation  of  the  ecliptic  orbit  of  the  moon, 
and  the  place  of  the  apogee  in  the  ecliptic  at  any  time. 

7.  The  mechanism  of  the  wheels,  &c.  being  such  as  to 
show  the  phases  of  the  moon,  and  always  to  show  the 
same  face  to  the  earth,  every  way  similar  to  the  face  of 
the  moon  in  the  heavens  at  the  same  time. 

By  such  a  variety  of  motions  in  the  lunarium,  it  will 
be  easy  to  see  the  rationale  of  most  of  the  lunar  irregu- 
larities and  variable  phenomena  ;  also  every  thing  relative 
to  the  nature  and  doctrine  of  eclipses,  both  solar  and 
lunar,  total,  annular,  and  partial. 

A  small  brass  lamp,  to  be  lighted  with  oil  and  cotton, 
or  a  wax-taper,  usually  accompanies  the  instrument,  which 
is  fitted  to  the  stem  where  the  sun  is  applied.  In  a  dark- 
ened room,  with  the  lamp  lighted,  the  several  phenomena, 
exhibited  by  the  parts  of  the  machine,  are  more  illustra- 
tive and  striking. 

A  machine,  or  secondary  planetarium,  is  sometimes 
made,  either  to  correspond  with  that  of  plate  1 1,  fig.  1, 
or  to  be  occasionally  connected  with  it,  and  is  called  the 
Jovian  system.  This  part  shows  the  motion  of  Jupiter's 
four  moons  or  satellites,  as  nearly  corresponding  to  their 
motions  in  the  heavens  as  the  latest  observations  upon 
them  will  admit  of.  The  distances  of  the  satellites  have 
here  the  same  proportions  as  in  the  heavens,  which  are 
expressed  in  semidiameters  and  decimal  parts  of  Jupiter's 


176  OBSERVATIONS    ON    GLOBES. 

globe.  In  a  similar  way,  machines  are  made  to  repre- 
sent the  motions  of  the  satellites  of  Saturn,  and  ot  the 
Georgian  planet. 

To  illustrate  the  nature  and  manner  of  the  precessions 
of  the  equinoxes,  and  the  phenomena  of  the  transirs  of 
Mercury  and  Venus  over  the  face  of  the  sun,  apparatuses 
might  easily  be  adapted  ;  but,  like  the  Jovian  and  Satur- 
nian  machines,  they  are  only  made  from  particular  orders, 
as  not  being  the  most  material  parts  of  the  orrery. 

COMPARATIVE  OBSERVATIONS  ON  GLOBES,    MOUNTED 
IN    THE    COMMON   MANNER,  AND  THOSE   MOUNTED 

in  the  improved  method,  see platel3,  fg. 2  and  3. 

As  I  do  not  agree  in  opinion  with  our  late  Author  in 
his  assertion,  that  the  mounting  of  globes  in  the  common 
way,  renders  them  unfit  for  the  intended  purposes  of  in- 
struction, and  as  conveying  unnatural  and  confused  ideas 
to  the  learner,  see  page  162, 1  will  only  trouble  the  reader 
with  a  few  remarks,  and  leave  him  to  judge  which  he  con- 
ceives to  be  the  most  perspicuous  and  useful  for  his  pur- 
pose ;  both  methods,  in  many  cases,  have  their  peculiar 
advantages. 

Globes  are  not  designed  as  instruments  of  accurate  cal- 
culation, or  universal  illustration  ;  they  are  made  for  the 
assistance,  in  a  familiar  way,  of  beginners,  in  the  perform- 
ance of  a  variety  of  useful  and  instructive  problems  in  the 
sciences  of  astronomy,  geography,  navigation,  spherical 
trigonometry,  dialling,  chronology,  &c.  &c.  The  more 
simple  the  manner  of  mounting  globes  for  these  purposes, 
the  more  easy  and  familiar  will  the  operation  be  found  by 
the  learner.  On  this  account,  1  always  preferred  the  me- 
thod of  a  well-divided  horary  circle  at  the  north  pole,  to 
the  semicircular  wires  and  sliding  points,  adopted  origi- 
nally by  our  late  Author's  father.  In  the  new  18  inch 
globes,  noticed  in  my  note  in  page  .501,  vol.  iii.  I  have 
constructed  the  hour  circles  with-such  clear  and  fine  divi- 
sions, as  to  show  the  time  to  five  minutes  of  a  degree, 
which  is  quite  sufficient  for  ail  the  material  problems,  and 
is  as  accurate  as  the  fitting  up  of  a  globe  to  be  depended 
upon,  can  be  made. 


OBSERVATIONS    ON    THE    GLOBES.  177 

No  terrestrial  globes  convey  just  representations  of  the 
position  of  our  earth  and  the  heavens,  for  any  particular 
day,  till  they  be  rectified  completely,  and  duly  set  by  the 
compass.  In  respect  to  the  shifting  of  the  inclination  of 
the  axis  or  the  north  pole,  considered  by  our  Author  as 
an  absurdity,  it  must  be  observed,  that  when  properly  set, 
the  horizon  is  a  true  representation  of  the  place  it  is  rec- 
tified for.  The  relative  position  of  the  horizon  to  the 
axis  is  the  same  in  both  methods  of  mounting. 

In  the  globe  mounted,  as  shown  in  plate  13,  Jig.  2, 
the  moveable  horizon  is  shifted,  as  it  would  naturally 
shift,  should  an  inhabitant  move  from  one  latitude  to  an- 
other. In  the  common  globe  you  elevate  the  pole  to  the 
same  effect;  and,  though  in  London,  should  you  elevate 
the  pole  for  the  latitude  of  Jamaica,  the  axis  at  London 
would  not  point  to  the  north  pole,  or  polar  star;  yet, 
supposing  you  vvere  at  Jamaica,  it  would  then  point  justly. 
The  axis  of  the  earth,  with  respect  to  the  horizon,  may 
therefore  be  said,  relatively  to  the  horizon,  to  be  conti- 
nually shifting,  as  an  inhabitant  changes  his  latitude. 
This  can  be  easily  understood  by  the  learner,  and  cannot 
be  the  means  of  any  confused  ideas. 

The  horizon  on  the  broad  paper-circle  upon  the  stand 
of  the  globes,  containing  the  ecliptic,  divided  into  signs 
and  degrees,  and  a  contiguous  calendar,  is  not  designed 
to  represent  more  than  the  horizon  of  a  place.  The  gra- 
duated circle  of  the  ecliptic  is  useful  for  finding  the  place 
of  the  sun  for  any  day  in  a  ready  manner,  without  the 
trouble  of  referring  to  an  ephemeris  ;  the  sun's  place 
being  found,  the  ecliptic  circle  upon  the  globe  is  imme- 
diately used  to  have  the  requisite  mark  set  on  it.  The 
sun's  place  being  thus  fixed  on  the  globe,  the  perform- 
ance of  a  problem,  by  turning  the  sun  and  globe  round 
together,  is  certainly  not  strictly  agreeable  to  nature, 
but  in  part  so,  and  is  quite  sufficient  for  the  result  of  the 
problem. 

In  the  inspecting  of  particular  countries  on  the  terres- 
trial globe,  it  is  proper  to  have  it  quite  clear,  and  with- 
out being  obliged  to  push  away  any  contiguous  appenda- 
ges. For  the  performance  of  the  problems  on  the  pla- 
nets, and  tracing  the  paths  of  comets,  &c.  the  celestial 

VOL.  IV.  A  2 


178  OBSERVATIONS    ON    THE    GLOBES. 

globe,  mounted  in  the  common  way,  will  be  found  more 
convenient.  Upon  the  whole,  therefore,  I  judge  the  fol- 
lowing particulars  to  be  the  chief  advantages  in  the  im- 
proved mounted  globes,  and  those  mounted  in  the  com- 
mon manner  respectively. 

Advantages  peculiar  to  the  new-mounted  globes  y  as  repre- 
sented in  plate  13,  fig.  2  and  3. 

1.  The  axis  always  retains  a  natural  and  permanent 
position,  directed  to  the  north  pole  in  the  heavens,  when 
duly  set  by  the  compass. 

2.  The  sun  upon  the  stem  E,  jig.  2,  moon,  or  planets, 
being  in  a  fixed  position,  and  the  globe  turning  about  its 
axis,  is  the  natural  and  just  representation  of  the  cause 
of  all  the  various  phenomena  respecting  their  rising,  cul- 
minating, setting,  lengths  of  days  and  nights,  &c.  result- 
ing from  the  diurnal  motion  of  the  earth,  and  the  respec- 
tive situations  of  the  planets  in  their  orbits. 

3.  The  moveable  horizon,  meridian,  &c.  applied  to 
the  globe,  are  to  have  their  positions  naturally  changed, 
according  to  the  motion  or  situation  of  an  inhabitant  of 
the  globe. 

4.  The  globe  being  placed  in  a  darkened  room,  and  a 
candle  or  lamp  being  placed  at  a  proper  distance,  in  a 
line  with  the  centre  of  the  sun  ;  the  globe  will  be  divided, 
as  in  nature,  into  the  enlightened  and  darkened  hemi- 
sphere, and  show  in  what  degree  the  various  countries 
enjoy  the  presence  of  the  sun  or  day,  and  the  lengths, 
at  that  time  of  the  year,  of  the  day  and  night. 

5.  By  the  wheel- work,  contained  underneath  the  plate 
C  D,  the  relative  position  of  the  axis  to  the  terminator  of 
light  and  darkness  is  shown,  consequently,  the  state  of  the 
light  and  darkness  of  the  globe  at  any  season  of  the  year. 
This  change  of  the  position  of  the  axis  is  not  agreeable  to 
nature,  but  sufficient  to  explain  the  phenomena.  In  this 
instance  it  makes  a  complete  tellurian  upon  a  large  scale; 
but,  in  respect  to  the  permanent  parallelism  of  the  axis, 
it  will  be  best  explained  by  the  tellurian  of  the  improved 
orrery  before  described :  see  plate  1 2,  jig.  1 . 


OBSERVATIONS    ON    THE    GLOBES.  179 

Advantages  peculiar  to  the  globes,  mounted  in  the  common 

manner, 

1.  Their  having  no  brass  circles,  or  other  appendages, 
contiguous  to  their  surfaces,  renders  them  easy  and  accu- 
rate for  inspection,  and  in  the  performance  of  a  variety 
of  problems. 

2.  The  external  appendages  are  less  in  quantity  than 
in  the  globe  shown  at  plate  1 3,  jig.  2,  consequently  more 
easy  and  ready  to  the  learner. 

3.  In  many  cases,  where  the  use  of  a  quadrant  of  alti- 
tude is  necessary,  they  are  done  in  a  ready  manner,  which 
could  not  be  the  case  with  the  new-mounted  one. 

4  Ail  great  circles  of  the  sphere,  being  imaginary, 
and  referred  to  in  the  heavens,  the  position  of  right,  pa- 
rallel, and  oblique,  to  the  inhabitants  of  the  globe,  can 
only  be  represented  by  the  common  mounted  globe, 
where  there  is  a  contrivance  of  the  horary  circle  being 
under  the  meridian,  or  to  shift  away  occasionally  from 
the  pole. 

5.  Globes  of  large  dimensions,  such  as  1  8  inches  in 
diameter  and  upwards,  are  much  less  expensive  and  port- 
able, mounted  in  the  common  way,  than  in  the  other 
way. 

These  observations  are  quite  sufficient  to  give  the  reader 
a  general  idea  of  the  merits,  and  unavoidable  defects  of 
both  mountings.  In  the  performance  of  a  variety  of  pro- 
blems upon  the  common  globe,  he  will  find  many  others 
that  are  not  necessary  to  be  noticed  here ;  and  to  con* 
vince  him,  that  a  rational  knowledge  of  the  celestial  phe- 
nomena can  only  be  obtained  by  joining  observations  of 
the  heavenly  bodies  with  the  portions  of  his  studies  on 
globes,  orreries,  and  other  astronomical  instruments. 

Besides  the  methods  of  mounting  globes  just  described, 
other  mountings  are  applied  for  the  following  purposes, 

1.  A  globe  to  show  the  phenomena  of  the  transits  of 
Mercury  and  Venus  over  the  sun. 

2.  A  globe  to  show  the  phenomena  of  solar  and  lunar 
eclipses  on  all  places  of  the  terrestrial  globe,  called  an 
eclipsareon. 


180  DESCRIPTION    OF    THE    EQUATORIAL. 

3.  A  globe  to  show  the  nature  and  manner  of  the  pre- 
cession of  the  equinoxes,  and  thereby  the  difference  be- 
tween the  sidereal  and  tropical  years,  as  also  the  apparent 
and  direct  motion  of  the  fixed  stars. 

4.  A  celestial  globe,  with  a  telescope  to  fix  on  the  north 
pole  of  the  globe  occasionally,  with  a  divided  arc,  &c.  &c. 
An  apparatus  of  my  invention  for  observing  any  celestial 
body,  and  thereby,  in  an  instantaneous  manner,  obtain- 
ing all  the  particulars  of  any  phenomena  presenting  them- 
selves in  the  heavens. 

5.  A  lunar  globe,  or  the  selenegraphia,  invented  by 
Mr.  John  Russel,  R.  A.  forming  an  apparatus  for  exhi- 
biting the  phenomena  of  the  moon,  and  the  useful  pur- 
poses to  which  it  may  be  applied. 

A    DESCRIPTION    OF    THE    EQUATORIAL,    OR    UNIVER- 
SAL   SUN-DIAL. 

In  the  former  edition  of  this  Work,  our  Author  omit- 
ted the  description  of  the  equatorial,  as  represented  in 
plate  14,  fig,  2.  A  description,  with  a  variety  of  pro- 
blems to  be  performed  with  it,  he  published  in  his  Astro- 
nomlcal  and  Geographical  Essays ,  8vo.  4th  edit.  1794, 
and  to  which  I  must  refer  the  reader,  as  the  limits  of  this 
work  will  admit  but  of  a  concise  description  and  use  of 
the  instrument. 

An  equatorial  instrument  is  the  most  general  and  com- 
prehensive of  all  instruments  made  for  the  purposes  of 
practical  astronomy.  It  is  the  best  of  any  to  exercise  the 
beginner  in  the  science.  For  when  he  has  obtained  a  per- 
fect knowledge  of  the  management  of  this,  there  will  be 
no  other  construction  of  an  astronomical  machine,  but 
what  he  will  comprehend  the  use  of  in  a  ready  manner. 

An  equatorial  is  considered  by  astronomers  as  an  in- 
strument useful  in  taking  the  following  particulars  of  the 
heavenly  bodies:  their  altitudes  and  azimuths ;  their  right 
ascensions  and  declinations ;  to  determine  the  latitudes 
and  longitudes  of  places ;  to  find,  by  observations  on  the 
sun  and  stars,  the  hour  of  the  day  and  night ;  to  mea- 
sure angles  universally,  or  the  distances  of  objects  on 
land,  and  to  take  angles  in  general  for  the  resolution  of 


DESCRIPTION    OF    THE    EQUATORIAL.  181 

many  problems  in  practical  astronomy,  trigonometry, 
&c.  &c. 

Plate  14,  jig.  2,  represents  one  of  the  smallest  dimen- 
sions usually  made.  Its  equatorial  circle,  M  N,  is  about 
four  inches  in  diameter,  and  the  rest  in  proportion.  The 
instrument  consists  of  the  following  parts :  a  horizontal 
circle  E  F,  divided  into  four  quadrants  of  90°  each,  with 
a  fixed  nonius  or  vernier  scale  at  N,  and  the  circle  itself 
may  be  turned  by  the  hand  on  its  centre  or  axis.  A  strong 
pillar  is  fixed  to  the  centre  of  the  horizontal  circle,  sup- 
porting the  centre  of  a  vertical  semicircle  A  B,  divided 
into  two  quadrants  of  90°  each.  This  is  called  the  semi- 
circle of  altitude,  as  it  is  used  to  take  angles  of  altitude  and 
depression.  There  is  a  vernier  scale  at  K. 

At  right  angles  to  the  plane  of  this  semicircle,  the  equa- 
torial circle,  M  N,  is  fixed,  representing  the  equator  of  the 
globe,  and  divided  into  24  hours,  or  twice  12  hours,  each 
hour  being  subdivided  into  five  minutes.  Upon  this  cir- 
cle moves  another  with  a  chamfered  edge,  carrying  a  no- 
nius, by  which  the  divisions  of  the  equatorial  are  further 
subdivided,  and  are  read  off  to  single  minutes.  At  right 
angles  to  this  circle  is  fixed  the  semicircle  of  declination, 
D,  divided  into  two  quadrants  of  90°  each. 

The  brass  bar,  that  carries  the  sight  O  P,  is  fixed  to  an 
index  moveable  on  the  semicircle,  and  carrying  a  nonius 
at  Q^  The  sight  O,  to  which  the  eye  is  to  be  applied,  has 
two  small  holes,  and  a  dark  glass  for  screening  the  eye 
from  the  sun ;  and  the  sight,  P,  has  two  small  pieces 
screwed  on,  the  lower  with  a  small  hole  to  admit  the  rays 
from  the  sun,  and  the  upper  two  cross  wires  for  observa- 
tions by  their  intersection.  There  are  two  spirit-levels, 
L,  L,  fixed  on  the  horizontal  circle,  at  right  angles  to 
each  other,  which,  with  the  three  adjusting-screws,  I,  G, 
H,  are  useful  for  duly  levelling  the  instrument. 

A  small  telescope  is  sometimes  applied  in  place  of  the 
two  sight-pieces  P,  O. 

I  shall  here  insert  our  Author's  directions  for  adjusting 
the  instrument,  and  one  -problem,  as  an  example  of  its 
use;  for  further  information,  the  reader  may  consult  his 
Astronomical  Essays. 


182  ADJUSTMENT   OF    THE    EQUATORIAL. 


Probl   m  I.    To  adjust  the  equatorial  for  observation. 

Set  the  instrument  on  a  firm  support.  First,  to  adjust 
the  levels,  and  the  horizontal  or  azimuth  circle.  Turn 
the  horizontal  circle  till  the  centre-line,  or  o,  of  the  divi- 
sions coincide  with  the  middle  stroke  of  the  nonius,  or 
near  it.  In  this  situation,  one  of  the  levels  will  be  found 
to  lie  either  in  a  right  line  joining  the  two  foot-screws, 
which  are  nearest  the  nonius,  or  else  parallel  to  such  a 
right  line.  By  means  of  the  two  last-mentioned  screws, 
cause  the  bubble  in  the  level  to  become  stationary  in  the 
middle  of  the  glass ;  then  turn  the  horizontal  circle  half 
round,  by  bringing  the  other  o  to  the  nonius ;  and  if  the 
bubble  remain  in  the  middle,  as  before,  the  level  is  well 
adjusted ;  if  it  do  not,  correct  the  position  of  the  level, 
by  turning  one  or  both  of  the  screws  which  pass  through 
its  ends,  by  means  of  a  turn-screw,  till  the  bubble  have 
moved  half  the  distance  it  ought  to  come  to  reach  the 
middle  ;  and  cause  it  to  move  the  other  half,  by  turning 
the  foot-screws  already  mentioned.  Return  the  horizon- 
tal circle  to  its  first  position,  and  if  the  adjustments  have 
been  well  made,  the  bubble  will  remain  in  the  middle ; 
if  otherwise,  the  process  of  altering  the  level  and  the 
foot-screws,  with  the  reversing,  must  be  repeated  till  it 
bear  this  proof  of  its  accuracv.  Then  turn  the  horizon- 
tal circle  till  90°  stand  opposite  to  the  nonius;  and  by  the 
foot-screw  immediately  opposite  the  other  90°,  without 
touching  the  others,  cause  the  bubble  of  the  same  level 
to  stand  in  the  middle  of  the  glass.  Lastly,  by  its  own 
proper  screws  set  the  other  level,  not  yet  attended  to,  so 
that  its  bubble  may  occupy  the  middle  of  its  glass. 

Secondly,  to  adjust  the  line  of  sight.  Set  the  nonius  on 
the  declination-semicircle  at  o,  the  nonius  on  the  horary 
circle  at  VI,  and  the  nonius  on  the  semicircle  of  altitude 
at  90°.  Look  through  the  sights  towards  some  part  of  the 
horizon,  where  there  is  a  diversity  of  remote  objects. 
Level  the  horizontal  circle,  and  then  observe  what  object 
appears  on  the  centre  of  the  cross-wires.  Reverse  the 
semicircle  of  altitude,  so  that  the  other  90°  may  apply  to 
the  nonius  ;  taking  care,  at  the  same  time,  that  the  other 


ADJUSTMENT    OF    THE    EQUATORIAL.  183 

three  nonii  continue  at  the  same  parts  of  their  respective 
graduations  as  before.  If  the  remote  object  continue  to  be 
seen  on  the  centre  of  the  cross-wires,  the  line  of  sight  is 
truly  adjusted  ;  but  if  not,  unscrew  the  two  screws  which 
carry  the  frame  of  the  cross- wires,  and  move  the  frame 
till  the  intersection  appear  to  lie  on  a  new  object,  half-way 
between  the  object  first  observed,  and  that  to  which  the 
wires  are  applied  in  the  last  position.  Return  the  semi- 
circle of  altitude  to  its  original  position  :  if  the  intersex 
tion  of  the  wires  be  then  found  to  be  on  the  object  to 
which  they  were  last  directed,  the  line  of  sight  is  truly 
adjusted  ;  but  if  not,  the  frame  must  be  again  altered  as 
before:  and  the  same  general  operation  must  be  repeat- 
ed, till  the  cross- wires  in  both  positions  apply  to  the  same 
object. 

Besides  this  adjustment  of  the  centre  of  intersection,  it 
is  necessary  that  one  of  the  wires  should  be  in  the  plane 
of  the  declination-semicircle,  and  the  other  at  right  an- 
gles to  that  plane.  As  the  wires  are  fixed  at  right  angles 
to  each  other,  the  adjustment  of  one  of  them  will  be 
sufficient.  For  this  purpose,  observe  any  small  object 
on  one  of  the  wires ;  if  it  be  the  vertical  wire,  move  the 
index  of  the  semicircle  of  declination  ;  or  if  the  other, 
move  the  last-mentioned  semicircle  on  the  axis  of  the 
equatorial  circle.  In  either  case,  the  object  will  coincide 
with  the  wire  during  its  motion,  if  the  position  be  right ; 
if  not,  alter  that  position,  taking  care  not  to  displace  the 
centre  from  its  adjustment. 

To  adjust  the  piece  which  carries  the  hole  for  forming 
the  solar  spot,  direct  the  sights  to  the  sun,  so  that  the 
centre  of  the  luminous  circle,  formed  by  the  aperture 
which  carries  the  cross- wires,  may  fall  precisely  on  che 
upper  sight-hole.  Then  move  the  frame,  with  the  small 
perforation,  till  the  solar  spot  fall  exactly  on  the  lower 
$ighNhole. 

Thirdly,  to  find  the  correction  to  be  applied  to  obser- 
vations by  the  semicircle  of  altitude.  Set  the  nonius  on  the 
declination-semicircle  to  o,  and  the  nonius  on  the  horary 
circle  to  XII ;  direct  the  sights  to  any  fixed  and  distant 
object,  by  moving  the  horizontal  circle  and  semicircle  of 
altitude,  and  nothing  else  \  note  the  degree  and  minute 


184  TO    OBSERVE    BY    THE    EQUATORIAL"! 

of  altitude  or  depression  ;  reverse  the  declination-semi- 
circle, by  directing  the  nonius  on  the  horary  circle  to  the 
opposite  XII ;  direct  the  sights  again  to  the  same  object 
by  means  of  the  horizontal  circle  and  semicircle  of  alti- 
tude, as  before.  If  its  altitude,  or  depression  be  the  same 
as  was  observed  in  the  other  position,  no  correction  will 
be  required  ;'  but  if  otherwise,  half  the  difference  of  the 
two  angles  is  the  correction  to  be  added  to  all  observa- 
tions or  rectifications  made  with  that  quadrant,  or  half  of 
the  semicircle,  which  shows  the  least  angle;  or  to  be  sub- 
tracted from  all  observations  or  rectifications  made  with 
the  other  quadrant,  or  half. 

When  the  levels  and  cross- wires  are  once  truly  set, 
they  will  preserve  their  adjustment  a  long  time  if  not  de- 
ranged by  violence  ;  and  the  correction  to  be  applied  to 
the  semicircle  of  altitude  is  a  constant  quantity. 

For  the  observations  on  the  sun  and  the  rest  of  the  hea- 
venly bodies,  a  suitable  fixed  position  in  a  room  should 
be  determined  for  the  instrument. 


Problem  II.    To  measure  angles,  cither  of  azimuth,  alti- 
tude, or  depression. 

Set  the  middle  mark  of  the  nonius  on  the  declination 
at  o,  and  fix  it  by  means  of  the  milled  screw  behind.  Set 
the  horary  circle  at  XII,  on  the  equator,  and  the  instru- 
ment, previously  adjusted,  is  ready  for  observation.  Then, 
if  the  sights  be  directed  successively  to  any  two  objects, 
the  degrees  and  minutes  contained  between  the  two  posi- 
tions of  the  nonius,  on  the  limb  of  the  horizontal  circle, 
will  show  the  horizontal  angle  of  the  quadrant.  And 
likewise,  if  the  sights  be  directed  to  any  object,  by  moving 
the  horizontal  circle  and  semicircle  of  altitude,  the  degree 
and  minute  marked  by  the  nonius  on  the  last-mentioned 
semicircle  will  be  the  angle  of  altitude,  if  on  the  quadrant 
or  part  nearest  the  eye ;  or  of  depression,  if  on  the  re- 
moter quadrant. 

Remark.  It  is  proper  in  this  place  to  describe  the  na- 
ture and  use  of  the  admirable  contrivance,  commonly 
called  a  vernier  or  nonius.    It  depends  on  the  simple  cirr 


TO    OBSERVE    BY    THE    EQUATORIAL.  185 

cumstance,  that  if  any  line  be  divided  into  equal  parts, 
the  length  of  each  part  will  be  greater,  the  fewer  the  di- 
visions ;  and,  contrariwise,  it  will  be  less  in  proportion 
as  those  divisions  are  more  numerous.  Thus  it  may  be 
observed,  that  the  distance  between  the  two  extreme 
strokes  on  the  nonius,  in  the  equatorial  before  us,  is  ex- 
actly equal  to  eleven  degrees  on  the  limb,  but  that  it  is 
divided  into  twelve  equal  parts.  Each  of  these  last  parts 
will  therefore  be  shorter  than  the  degree  in  the  propor- 
tion of  11  to  Vl\  that  is  to  say,  it  will  be  one-twelfth 
part,  or  five  minutes  shorter.  Consequently,  if  the  mid- 
dle stroke  be  set  precisely  opposite  to  any  degree,  the  rela- 
tive positions  of  the  nonius  and  the  limb  must  be  altered 
live  minutes  of  a  degree,  before  either  of  the  two  adja- 
cent strokes  next  the  middle,  on  the  nonius,  can  be 
brought  to  coincide  with  the  nearest  stroke  of  a  degree  ; 
and  so,  likewise,  the  second  strokes  on  the  nonius  will 
require  a  change  often  minutes;  the  third  of  fifteen,  and 
so  on  to  thirty,  when  the  middle  line  of  the  nonius  will 
be  seen  to  be  equidistant  from  two  of  the  strokes  on 
the  limb;  after  which,  the  lines  on  the  opposite  side  of 
the  nonius  will  coincide  in  succession  with  ihe  strokes  on 
the  limb. 

It  is  clear  from  this,  that  whenever  the  middle  stroke 
of  the  nonius  does  not  stand  precisely  opposite  to  any  de- 
gree, the  odd  minutes,  or  distance  between  it  and  the  de- 
gree immediately  preceding,  may  be  known  by  the  num- 
ber of  the  stroke  on  the  nonius,  which  coincides  with  any 
of  the  strokes  on  the  limb.  It  must  be  observed,  how- 
ever, that  as  the  degrees  in  the  several  quadrants  are  rec- 
koned in  opposite  directions,  so  likewise  the  nonius  has 
two  sets  of  numbers  ;  for  the  use. of  which  it  need  only 
be  remembered,  that  they  always  begin  .from  the  middle, 
and  go  to  30  minutes,  and  thence  from  the  opposite  SO 
minutes  in  the  same  direction  to  the  middle ;  and  that, 
they  must  always  be  reckoned  in  the  opposite  direction 
to  the  degrees  on  the  limb. 

In  this  instrument  they  must  be  read  in  the  opposite 
direction;  but  when  the  nonius-plate  has  its  divisions 
fewer  than  the  number  of  parts  on  the  limb  to  which  it 
\0L.  IV.  b2 


186        OBSERVATIONS    ON    THE    EQUATORIAL. 

is  equal,  they  coincide  successively  in  the  same  direction 
as  that  of  the  motion  of  the  index. 

The   angles  by   this   small  equatorial  are  usually 
shown  to  5'  of  a  degree,  and  the  time  to  1  minute. 

Equatorials,  when  made  for  accuracy,  are  of  much 
larger  dimensions,  and  the  various  circles  moved  by  teeth 
and  pinion.  The  equatorial  circle  is  commonly  made 
from  6  to  18  inches  diameter,  and  the  angles  shown  to 
1  minute  of  a  degree,  and  the  time  to  10  seconds. 

It  is  an  instrument  not  now  so  much  in  repute  as  for* 
merly.  The  several  circles  are  very  difficult  to  be  made 
without  some  small  eccentricity.*  The  better  kind  are 
therefore  now  made  upon  a  simpler  construction ;  or,  in- 
stead  of  them,  a  vertical  and  horizontal  circle,  called  a 
circular  instrument ,  which,  having  but  two  principal  cen- 
tres or  axes,  is  found  more  accurate  and  permanent.  The 
altitude  and  azimuth  of  the  objects  are  only  by  this  ob- 
tained ;  but  these  are  the  chief  data  in  practical  astrono- 
my and  topography,  and  from  them  all  the  requisite  de- 
ductions and  purposes  can  readily  be  obtained. 

The  usual  diameters  of  the  principal  vertical  circle  of 
the  circular  instrument  are  from  1 2  inches  to  2  feet,  but 
there  are  no  fixed  dimensions ;  the  construction  of  the 
observatory,  or  place  of  observation,  is  the  chief  guide  to 
the  astronomer  in  the  choice  of  his  instruments. 

To  these  instruments,  as  well  as  to  the  equatorial  of 
the  better  kind,  achromatic  telescopes  are  applied,  that 
will  enable  the  observer  to  see  the  planets  and  stars  in  the 
day  time. 


*  It  is  now  the  practice  in  all  the  best  and  large  angular  instruments  to 
apply  two  opposite  verniers  or  nonii,  by  which  any  small  eccentricity  is  easily 
observed  and  allowed  for  after  the  observation.-' 


[     187     ] 


LECTURE  XLIV. 


OF    THE    FIXED    STARS. 


JN  O  part  of  astronomy  gives  such  enlarged  ideas 
of  the  structure  and  magnificence  of  the  heavens,  as  the 
consideration  of  the  number,  magnitude,  and  distance  of 
the  fixed  stars. 

We  admire,  indeed  with  propriety,  the  vast  bulk  of  our 
own  globe ;  but  when  we  consider  how  much  it  is  sur- 
passed by  most  of  the  heavenly  bodies,  what  a  point  it  de- 
generates into,  and  how  little  more  even  the  vast  orbit  in 
which  it  revolves  would  appear,  when  seen  from  some  of 
the  fixed  stars,  we  begin  to  conceive  more  just  ideas  of 
the  extent  of  the  universe,  and  the  boundaries  of  crea- 
tion. 

The  most  conspicuous  and  brightest  of  the  fixed  stars 
of  our  horizon  is  Sirius.  The  earth,  in  moving  round 
the  sun,  is  190,OCO,000  miles  nearer  to  this  star  in  one 
part  of  its  orbit,  than  in  the  opposite ;  yet  the  magnitude 
of  the  star  does  not  appear  to  be  in  the  least  altered,  or 
its  distance  affected  by  it ;  so  that  the  distance  of  the 
fixed  stars  is  great  beyond  all  computation.  The  un- 
bounded space  appears  filled  at  proper  distances  with 
these  stars,  each  of  which  is  probably  a  sun,  with  attend- 
ant  planets  rolling  round  it.  In  this  view,  what,  and  how 
amazing,  is  the  structure  of  the  universe  ! 

Though  the  fixed  stars  are  the  only  marks  by  which 
astronomers  are  enabled  to  judge  of  the  course  of  the 
moveable  ones,  and  we  have  asserted  that  their  relative 
positions  do  not  vary ;  yet  this  assertion  must  be  confined 
within  some  limits,  for  many  of  them  are  found  to  under- 


188  OF    THE    FIXED    STARS. 

go  particular  changes,  and  perhaps  the  whole  are  liable 
to  some  peculiar  motion,  which  connects  them  with  the 
universal  system  of  created  nature.  Dr.  Herschel  even 
goes  so  far  as  to  suppose,  that  there  is  not,  in  strictness 
of  speaking,  one  fixed  star  in  the  heavens  ;  but  that  there 
is  a  general  motion  of  all  the  starry  systems,  and  conse- 
quently of  the  solar  one  among  the  rest. 

There  are  some  stars,  whose  situation  and  place  were 
heretofore  known,  and  marked  with  precision,  that  are 
no  longer  to  be  seen  ;  new  ones  have  also  been  disco- 
vered, that  were  unknown  to  the  ancients,  while  num- 
bers seem  gradually  to  vanish.  There  are  others,  which 
are  found  to  have  a  periodical  increase  and  decrease  of 
magnitude  ;  and  it  is  probable,  that  the  instances  of  these 
changes  would  have  been  more  numerous,  if  the  ancients 
had  possessed  the  same  accurate  means  of  examining  the 
heavens,  as  are  used  at  present. 

New  stars  offer  to  the  mind  a  phenomenon  more  sur- 
prising, and  less  explicable,  than  almost  any  other  in  the 
science  of  astronomy ;  I  shall  select  a  few  instances  of 
the  more  remarkable  ones  for  your  instruction  :  a  consi- 
deration of  the  changes  that  take  place,  at  so  immense  a 
distance  as  the  stars  are  known  to  be  from  you,  may  ele- 
vate your  mind  to  consider  the  immensity  of  his  power, 
who  regulates  and  governs  all  these  wide-extended  mo- 
tions ;'  '€  who  hath  measured  the  waters  in  the  hollow  of 
his  hand,  and  meted  out  the  heavens  with  a  span." 

Who  turns  his  eye  on  Nature's  midnight  fare, 
But  must  inquire — What  hand  behind  the  scene, 
What  arm  almighty,  put  these  whei  ling  globes 
In  motion,  and  wound  up  the  vast  machine? 


\>  lid l  di  111  <iiiingiii\  ,  pill  uicsc  »nn  jij 

In  motion,  and  wound  up  the  vast  ma< 


It  was  a  new  star  discovered  by  Hipparehus,  the  chief 
of  the  ancient  astronomers,  that  induced  him  to  compose 
a  catalogue  of  the  fixed  stars,  that  luture  observers  might 
learn  irom  his  labours,  whether  any  of  the  known  stars 
disajp  ared,  or  new  ones  were  produced.  The  same  mo- 
tives e  gaged  the  illustrious  Tjcho  Erahe  to  form,  with 
unr  nutting  labour  and  assiduity,  another  new  catalogue 
of  the  stars. 


OF    THE    FIXED    STARS.  189 

Of  new  stars,  the  first,  of  which  we  have  a  good  ac- 
count, is  that  which  was  discovered  in  the  constellation 
of  Cassiopea,  in  the  month  of  November  of  the  year 
1572,  a  time  when  astronomy  was  sufficiently  cultivated 
to  enable  the  astronomers  to  give  the  account  with  preci- 
sion. It  remained  visible  about  sixteen  months  ;  during 
this  time,  it  kept  its  place  in  the  heavens  without  the 
least  variation.  It  had  all  the  radiance  of  the  fixed  stars, 
and  twinkled  like  them ;  *nd  was  in  all  respects  like 
Sirius,  excepting  that  it  surpassed  it  in  brightness  and 
magnitude.  It  appeared  larger  than  Jupiter,  who  was 
at  that  time  in  his  perigee  ;  and  was  scarce  less  bright 
than  Venus. 

It  was  not  by  degrees  that  it  acquired  this  diameter, 
but  shone  forth  at  once  in  its  full  size  and  brightness, 
as  if  of  instantaneous  creation.  It  continued  about 
three  weeks  in  full  and  entire  splendour,  during  which 
time  it  might  be  seen  even  at  noon  day,  by  those  who 
had  good  eyes,  and  knew  where  to  look  for  it.  Before 
it  had  been  seen  a  month,  it  became  visibly  smaller, 
and  from  thence  continued  diminishing  in  magnitude 
till  March,  1574,  when  it  entirely  disappeared.  As  it 
decreased  in  size,  it  varied  in  colour  ;  at  first,  its  light 
was  white,  and  extremely  bright ;  it  then  became  yel- 
lowish, afterwards  of  a  rudy  colour,  like  Mars ;  and 
finished  with  a  pale  livid  white,  resembling  that  of  Sa- 
turn. 

In  1596,  Fabricius  observed  a  new  star  in  the  neck 
of  the  Whale :  he  first  saw  it  in  August,  and  it  disap- 
peared in  October  of  the  same  year.  In  1637,  Phocyl- 
lides  Holwarda  observed  it  again,  and  not  knowing  that 
it  had  been  seen  before,  took  it  for  a  new  discovery  :  he 
watched  its  place  in  the  heavens,  and  saw  it  appear 
again  the  succeeding  year,  nine  months  after  its  disap- 
pearance. It  has  been  since  found  to  be  every  year  very 
regular  in  its  period,  except  that  in  1672  it  was  missed 
by  Heve!ius9  and  not  seen  again  till  1676.  Bullialdus 
having  compared  together  the  observations  which  had 
been  made  of  it  from  1638  to  1666,  determined  the 
periodical  time  between  this  star's  appearing  in  its  great- 
est brightness,  and  returning  to  it  again,  to  be  223  days  ; 


190  PROPER    MOTION    OF    THE    STARS. 

observing  further,  that  this  star  did  not  appear  at  once 
in  its  full  magnitude,  or  brightness,  but  by  degrees  ar- 
rived at  them  :  He  also  formed  an  hypothesis,  to  ac- 
count for  these  periodic  changes. 

Three  changeable  or  re-apparent  stars  have  been  dis- 
covered in  the  constellation  of  the  Swan  ;  the  first  was 
seen  by  Jansonius,  in  1600  ;  the  second  was  discovered 
in  1670;  the  third  by  Kirchius,  in  1686. 

In  the  latter  end  of  September,  1 604,  a  new  star  was 
discovered  near  the  heel  of  the  right  foot  of  Serpenta- 
rius.  There  were  in  that  part  of  the  heavens,  at  that 
time,  the  three  seperior  planets,  which  so  engaged  the 
attention  of  astronomers,  that  no  appearance  thereabouts 
could  have  long  escaped  them.  Kepler,  in  describing  it, 
says,  that  it  was  precisely  round,  without  any  kind  of 
hair,  or  tail  ;  that  it  was  exactly  like  one  of  the  stars, 
except  that  in  the  vividness  of  its  lustre,  and  the  quick- 
ness of  its  sparkling,  it  exceeded  any  thing  he  had  ever 
seen  before.  It  was  every  moment  changing  into  some 
of  the  colours  of  the  rainbow,  as  yellow,  orange,  pur- 
ple, and  red ;  though  it  was  generally  white,  when  it 
was  at  some  distance  from  the  vapours  of  the  horizon. 
Those  in  general  who  saw  it,  agreed,  that  it  was  larger 
than  any  other  fixed  star,  or  even  any  of  the  planets,  ex- 
cept Venus  :  it  preserved  its  lustre  and  size  for  about 
three  weeks ;  from  this  time  it  grew  gradually  smaller. 
Kepler  supposes,  that  it  disappeared  some  time  between 
October,  1605,  and  the  February  following,  but  on  what 
day  is  uncertain. 

Besides  these  several  re-apparent  stars,  so  well  cha- 
racterized and  established  by  the  earliest  among  the  mo- 
dern astronomers,  there  have  been  many  discovered 
since,  by  Cassini,  Maraldi,  and  others  ;  Mr.  Montanere 
speaks  of  having  observed  above  one  hunnred  changes 
among  the  fixed  stars.  / 

PROPER    MOTION    OF    THE    STARS. 

Many  stars  have  been  found  to  alter  their  position 
with  respect  to  those  to  which  they  were  adjacent,  and 
this  change  of  position  has  been  termed  the  proper  motion 
rfihe  stars. 


PROPER    MOTION    OF    THE    STARS,  191 

The  proper  motion  of  Sirius,  Castor,  Procyon,  Pol- 
lux, Regulus,  Arcturus,  and  Aquilas,  in  100  years  in 
right  ascension,  are  respectively,  1  minute,  3  seconds ; 
1  minute  28  seconds  ;  1  minute,  33  seconds ;  41  se- 
conds ;  2  minutes,  20  seconds ;  and  57  seconds.  The 
proper  motion  of  Sirius  in  declination,  in  a  century,  is 
•2  minutes ;  and  of  Arcturus  is  3  minutes,  21  seconds. ; 
both  tending  to  the  south. 

The  apparent  brightness  of  some  of  the  fixed  stars 
is  observed  to  be  periodic.  The  star  Algol,  in  Me- 
dusa's head,  has  been  observed  long  since  to  appear  of 
different  magnitudes,  at  different  times.  The  period  of 
it  has  been  lately  settled  by  J.  Goodrich,  Esq.  of  York. 
It  periodically  changes  from  the  first  to  the  fourth  mag- 
nitude ;  the  time  employed  from  one  greatest  diminu- 
tion to  the  other,  was,  in  the  year  1783,  at  a  mean,  2 
days,  20  hours,  49  minutes,  3  seconds.  The  changes 
are  thus  :  during  four  hours  it  gradually  diminishes  in 
lustre ;  during  the  succeeding  four  hours,  it  recovers 
its  first  magnitude  by  a  like  gradual  increase  ;  and  du- 
ring the  remaining  part  of  the  period,  namely,  2  days, 
12  hours,  49  minutes,  3  seconds,  it  invariably  preserves 
its  greatest  lustre ;  after  the  expiration  of  which  term, 
the  diminution  again  commences. 

The  causes  of  these  appearances  cannot  be  assigned 
at  present,  with  any  degree  of  probability  ;  perhaps  they 
have  some  analogy  to  the  spots  on  the  sun,  which  at . 
some  times  appears  in  greater  numbers  than  at  others, 
some  of  them  bigger  than  the  whole  earth  ;  or  perhaps 
they  are  owing  to  some  real  motions  of  the  stars  them- 
selves. 

There  are  several  stars  that  appear  single  to  the  naked 
eye,  which  are,  on  examination  with  a  telescope,  found  to 
consist  of  two,  three,  &c.  The  number  of  double  stars 
observed  before  the  time  of  Dr.  Herschel,  was  but  small ; 
but  this  celebrated  astronomer  has  noted  upwards  of  four 
hundred  ;  among  these,  some  that  are  double,  others 
that  are  treble,  double-double,  quadruple,  double-treble, 
and  multiple  ;  his  catalogue  gives  the  comparative  size  of 
these  stars,  their  colour  as  they  appeared  to  him,  with  se- 
veral other  very  curious  particulars. 


L     192     ] 


HERSCHEL  ON  THE  CONSTRUCTION  OF  THE  UNIVERSE, 

Before  I  leave  the  subject  of  the  fixed  stars,  I  shall 
endeavour  to  give  you  an  account  of  Dr.  Herschei's  ideas 
of  the  construction  of  the  universe.  Former  astrono- 
mers had  supposed  that  our  sun,  besides  occupying  the 
centre  of  his  own  system,  was  also  the  centre  of  the 
universe  ;  and  that  the  sidereal  heavens  mi^ht  be  properly 
represented  on  the  concave  surface  of  the  sphere ;  but 
these  are  ill  adapted,  says  the  Doctor,  for  a  delineation 
of  the  interior  parts  of  the  heavens.  Being  able  to  pe- 
netrate into  these  regions  by  means  of  large  telescopes,* 
we  may  now  consider  them  as  a  naturalist  regards  a  rich 
extent  of  ground,  or  a  chain  of  mountains,  containing 
strata  variously  inclined  and  directed,  and  composed  of 
very  different  materials.  He  gives  strong  reasons,  de- 
duced from  a  series  of  observations,  as  well  as  consi- 
derations drawn  from  analogy,  to  prove  that  the  visible 
system  of  nature,  which  we  call  the  universe,  consisting 
of  all  the  celestial  bodies,  and  many  more  than  can  be 
seen  by  the  naked  eye,  is  only  a  group  of  stars  or  suns, 
with  the^r  planets,  constituting  one  of  those  patches  call- 
ed a  nebula  ;  and  this  is,  perhaps,  not  one  ten  thou- 
sandth part  of  the  universe. 

Dr.  Herschel  found  that  his  large  telescope  completely 
resolved  the  whitish  appearance  of  Via  Lactea  into  stars. 
Having  viewed  and  gaged  this  bright  zone  in  all  direc- 
tions, he  found  it  composed  of  shining  stars,  whose 
number  constantly  increases  or  diminishes,  in  propor- 
tion to  its  greater  or  less  apparent  brightness  to  the  naked 
eye.  There  is  no  doubt,  says  he,  but  that  the  Milky  Way 
is  a  most  exensive  stratum  of  stars  of  various  sizes,  and 
that  ours  un  is  one  of  the  heavenly  bodies  belonging  to  it. 

The  portion  of  the  Milky  Way  that  he  first  observed, 
was  that  about  the  hand  and  club  of  Orion  ;  here  he 
found  an  astonishing  multitude  of  stars,  which  he  at- 


*  Dr.  Herschei's  observations,  on  which  this  theory  is  founded,  were 
made  by  a  Newtonian  reflector,  of  ~0  feet  focal  length,  and  an  aperture  of 

18  inches. 


CONSTRUCTION    OF    THE    UNIVERSE.  19$ 

tempted  to  number,  by  estimating  the  number  con- 
tained  in  the  field  of  his  telescope  at  once,  and  comput- 
ing from  a  mean  of  these  how  many  might  be  contained 
in  a  given  portion  of  the  milky  way  :  in  the  most  vacant 
places  about  that  part,  he  found  63  stars  ;  other  six  fields 
contained  1 10,  60,  70,  90,  70,  and  74  stars  ;  a  mean  of 
these  gives  7^  for  the  number  of  stars  in  each  field  ;  so 
that  allowing  J  5  minutes  for  the  diameter  of  his  field  of 
view,  a  belt  of  15  degrees  long,  and  2  degrees  broad, 
could  not  contain  less  than  50,000  stars,  large  enough 
to  be  distinctly  numbered  ;  besides  which,  he  suspected 
twice  as  many  more,  which  could  be  seen  only  now  and 
then  by  faint  glimpses,  for  want  of  sufficienc  light. 

It  is  very  probable,  that  the  great  stratum,  called  the 
milky  way,  is  that  in  which  the  sun  is  placed,  though 
not  in  the  very  centre  of  its  thickness.  This  is  gather- 
ed partly  from  the  appearance  of  the  Galaxy,  which 
seems  to  encompass  the  whole  heavens,  as  it  certainly 
must  do,  if  the  sun  be;!  within  the  same.  For,  suppose  a 
number  of  stars  arranged  between  two  parallel  planes, 
infinitely  extended  every  way,  but  at  a  given  consider- 
able distance  from  each  other,  and  calling  this  a  sidereal 
stratum,  an  eye  placed  somewhere  within  it  will  see  all 
the  stars  in  the  direction  of  the  planes  of  the  stratum 
projected  into  a  great  circle,  which  will  appear  lucid  on 
account  of  the  accumulation  of  the  stars  ;  while  the  rest 
of  the  heavens  at  the  sides  will  only  seem  to  be  scatter- 
ed over  with  constellations,  more  or  less  crowded,  ac- 
cording to  the  distance  of  the  planes,  or  number  of 
the  stars  contained  in  the  thickness  or  sides  of  the  stra- 
tum. 

If  the  eye  were  placed  without  the  stratum,  but  at  no 
very  great  distance,  the  appearance  of  the  stars  within  it 
would  assume  the  form  of  one  of  the  lesser  circles  of 
the  sphere  ;  which  would  be  more  or  less  contracted, 
according  to  the  distance  of  the  eye :  if  this  were  ex- 
ceedingly increased,  the  whole  stratum  might  at  last 
be  drawn  into  a  lucid  spot  of  any  shape,  according  to 
the  position,  length,  and  height  of  the  stratum.  What 
has  heen  instanced  in  parallel  planes,  may  be  easily  ap- 
plied to  strata  irregularly  bounded  and  running  in  va- 

W  L,  IV.  C  2 


194  HERSCHEL    ON    THE 

rious  directions ;  and  thus  any  kind  of  curvatures,  as 
well  as  various  degrees  of  brightness,  may  be  produced 
in  the  projection. 

From  appearances,  he  infers  that  the  sun  is  placed  in 
one  of  the  great  strata  of  the  fixed  stars,  and  very  pro- 
bably not  far  from  the  place  where  some  smaller  stratum 
branches  out  from  it.  Such  a  supposition  accounts  for 
all  the  phenomena  of  the  milky  way  with  great  ease 
and  simplicity,  while  every  star  in  the  stratum  will  have 
its  own  Galaxy,  with  only  such  variations  in  form  and 
lustre,  as  may  arise  from  their  particular  situation. 

There  is,  says  Dr.  Hersc/jel,  a  remarkable  clearness 
and  purity'  in  the  heavens,  when  we  look  out  of  our 
stratum  at  the  sides,  that  is,  towards  Leo,  Virgo,  and 
Coma  Berenices  on  one  side,  and  towards  Cetus  on  the 
other  ;  whereas  the  ground  of  the  heavens  becomes  trou- 
bled as  we  approach  towards  the  length  or  height  of  it. 
These  troubled  appearances  are  easily  to  be  explained  by 
ascribing  them  to  some  of  the  distant  straggling  stars, 
that  hardly  yield  light  enough  to  be  distinguished  ;  but 
when  we  look  towards  the  pole  of  our  system,  where 
the  visual  ray  does  not  graze  along  the  side,  the  strag- 
gling stars  of  course  will  be  very  few  in  number,  and 
therefore  the  ground  of  the  heavens  will  appear  much 
purer  and  more  clear. 

Dr  Herschel  points  out  the  methods  whereby  the  sun's 
place  in  the  sidereal  stratum  may  be  ascertained,  but  these 
demand  more  previous  knowledge  than  is  necessary  in 
an  introductory  lecture  like  the  present.  After  this,  he 
lays  down  some  suppositions,  on  the  subject,  taking  a 
point  of  view  at  a  very  remote  period  of  time,  and  an 
immense  distance  of  space  ;  these  for  the  same  reason 
we  shall  leave  untouched,  and  proceed  to  his  view  of  the 
heavens  from  our  own  retired  station,  in  one  of  the  pla- 
nets, attending  to  a  star  in  its  great  combination  with 
numberless  others  ;  and  in  order  to  investigate  what  will 
be  the  appearances  from  this  contracted  situation,  let  us 
begin  with  the  naked  eye. 

The  stars  of  the  first  magnitude  being  in  all  probabili- 
ty the  nearest,  will  furnish  us  with  a  step  to  begin  the 
scale.     Setting  off,  therefore,  with  the  distance  of  Sinus 


CONSTRUCTION    OF    THE    UNIVERSE.  195 

or  Arcturus,  for  instance,  as  unity,  we  shall  at  present 
suppose,  that  those  of  the  second  magnitude  are  at  dou- 
ble, those  of  the  third  at  treble  the  distance,  &c.     Tak- 
ing it  for  granted  then,  that  a  star  of  the  seventh  magni- 
tude, the  smallest  visible  to  the  naked  eye,  is  about  seven 
times  as  far  as  one  of  the  first,  it  follows,  that  an  obser- 
ver, who  is  inclosed  in  a  globular  cluster,  of  stars,  and 
not  far  from  the  centre,  will  never  be  able  by  his  naked 
eye  to  see  to  the  end  of  it  ;  for  since,  according  to  the 
foregoing  estimations,  he  can  only  extend  his  view  to 
about  seven  times  the  distance  of  Sirius,  it  cannot  be  ex- 
pected that  his  eyes  should  reach  the  borders  of  a  cluster, 
which  has  perhaps  no  less  than  50  stars  in  depth  every 
where  around  him.    The  whole  universe,  therefore,  to  an 
observer  confined  to  unassisted  vision,  will  be  comprised 
in  a  set  of  constellations  richly  ornamented  with  scattered 
stars  of  all  sizes.     Or,  if  the  united  brightness  of  a 
neighbouring  cluster  of  stars  should,  in  a  remarkably 
cloar  night,  reach  his  sight,  it  will  put  on  the  appearance 
of  a  small,  faint,  whitish  nebulous  cloud,  not  to  be  per- 
ceived without  the  greatest  attention.     Let  us  suppose 
him  placed  in  a  much  extended  stratum,  or  branching 
cluster  of  millions  of  stars  :  here  the  heavens  will   not 
only  be  richly  scattered  over  with  brilliant  constellations, 
but  a  shining  zone  or  milky  way  will  be  perceived  to  sur- 
round the  whole  sphere  of  the  heavens,  owing  to  the 
combined  light  of  the  stars  that  are  too  remote  to  be 
seen  ;   our  observer's  sight  will  be  so  confined,  that  he 
will  imagine  this  single  collection  of  stars,  though  he 
does  not  perceive  the  thousandth  part  of  them,  to  be  the 
whole  contents  of  the  heavens.     Allowing  him  now  the 
use  of  a  common  telescope,  he  begins  to  suspect  that  all 
the  milkiness  of  the  bright  path,  which  surrounds  the 
sphere,  may  be  owing  to  stars  :  he  perceives  a  few  clus- 
ters of  them  in  various  parts  of  the  heavens,  and  finds 
also  that  there  is  a  kind  of  nebulous  patches ;  but  still 
his  views  are  not  extended  to  reach  so  far  as  to  the  end 
of  the  statum  in  which  he  is  situate ;   so  that  he  looks 
upon  these  patches  as  belonging  to  that  system  which  to 
him  seems  to  comprehend  every  celestial  object.     He 
now  increases  his  power  of  vision,  and  applying  himself 


196  HERSCHEL    ON    THE 

to  a  closer  observation,  finds  that  the  milky  way  is 
indeed  no  other  than  a  collection  of  very  small  stars  : 
he  perceives  that  those  objects,  which  had  been  called 
nebulas,  are  evidently  nothing  but  clusters  of  stars ; 
their  number  increases  upon  him  ;  and  whilst  he  re- 
solves one  nebula  into  stars,  he  discovers  ten  new  ones 
that  he  cannot  resolve.  He  then  forms  the  idea  of  im- 
mense strata  of  fixed  stars,  of  clusters  of  stars,  and  of 
nebulae,  till  going  on  with  such  interesting  observa- 
tions, he  soon  finds  that  all  these  appearances  arise  from 
the  confined  situation  in  which  we  are  placed.  Con- 
fined it  may  be  justly  called,  though  contained  in  no 
smaller  a  space  than  what  appeared  before  to  be  the 
whole  region  of  fixed  stars,  but  which  now  has  assum- 
ed the  shape  of  a  crookedly  branching  nubula ;  not 
one  of  the  least,  but  probably  very  far  from  being  the 
most  considerable,  of  those  numberless  clusters  that 
enter  into  the  construction  of  the  heavens.  Dr.  Hers- 
chel  confirms  these  ideas  by  a  series  of  observations, 
and  thinks  it  will  be  found  upon  the  whole,  that  this 
view,  with  all  its  consequential  appearances,  as  seen  by 
an  eye  enclosed  in  one  of  the  nebulae,  is  no  other  than 
a  drawing  from  nature,  wherein  the  features  of  the  ori- 
ginal have  been  closely  copied  ;  and  Dr.  Herschel  hopes 
the  resemblance  will  not  be  called  a  bad  one,  when  it 
shall  be  considered  how  very  limited  must  be  the  pen- 
cil of  an  inhabitant  of  so  small  and  retired  a  spot  of 
an  infinite  system,  in  attempting  the  picture  of  so  un- 
bounded an  extent. 

In  the  most  crowded  parts  of  the  milky  way,  he  has 
had  a  field  of  view  of  588  stars,  and  these  continued 
for  many  minutes ;  so  that  in  one  quarter  of  an  hour's 
time,  no  less  than  1 16,000  stars  have  passed  through  the 
field  of  his  telescope  :  he  endeavours  to  show,  that  the 
powers  of  his  telescope  are  such,  that  it  will  not  only 
reach  the  stars  at  49?  times  the  distance  of  Sirius,  so 
as  to  distinguish  them,  but  that  it  also  shows  the  united 
lustre  of  the  accumulated  stars  that  compose  a  milky  ne- 
bulosity at  a  far  greater  distance.  From  these  considera- 
tions, it  is  highly  probable  that,  as  his  20  feet  telescope 
does  not  show  such  a  nebulosity  in  the  milky  way,*  it 


ORIGIN    OF    NEBULOUS    STRATA,  197 

goes  already  far  beyond  its  extent ;  and,  therefore,  a 
more  powerful  instrument  would  remove  all  doubt,  by 
exposing  a  milky  nebulosity  beyond  the  stratum  which 
could  then  no  longer  be  mistaken  for  the  dark  ground 
of  the  heavens. 

To  the  foregoing  arguments  we  may  add  the  follow- 
ing, drawn  from  analogy.  Dr.  Herschel  says,  that 
among  the  great  number  of  nebulae  which  he  has  alrea- 
dy seen,  amounting  to  more  than  900,  there  are  many 
in  all  probability,  equally  extensive  with  that  which  we 
inhabit ;  and  yet  they  are  all  separated  from  each  other 
by  very  considerable  intervals.  Some  indeed  there  are, 
that  seem  to  be  double  and  treble  ;  and  though  with 
most  of  them  it  may  be,  that  they  are  at  a  very  great 
distance  from  each  other,  yet  he  does  not  mean  to  say 
that  there  are  no  such  conjunctions  ;  though  there  may 
be  also  some  thinly  scattered  solitary  stars,  not  yet 
drawn  into  systems ;  their  number  cannot  be  very  con- 
siderable :  a  conjecture  that  is  abundantly  confirmed,  in 
situations  where  the  nebulas  are  near  enough  to  have 
their  stars  visible  ;  for  they  are  all  insulated,  and  gene- 
rally to  be  seen  upon  a  very  clear  and  pure  ground, 
without  any  star  near  them  that  might  be  supposed  to 
belong  to  them  :  and  though  they  may  be  often  seen 
in  beds  of  stars,  yet  from  the  size  of  these  stars,  we 
may  be  certain  that  they  are  much  nearer  to  us  than 
those  nebulae,  and  belong  undoubtedly  to  our  own  sys- 
tem. 

THE    ORIGIN    OF    NEBULOUS    STRATA. 

Dr.  Herschel  thinks  the  nebula  that  we  inhabit,  has 
fewer  marks  of  profound  antiquity  upon  it  than  the 
rest ;  having  previously  supposed  that  the  condensa- 
tion of  clusters  of  stars  is  to  be  ascribed  to  a  gradual  ap- 
proach ;  the  number  of  ages  that  must  have  passed  be- 
fore some  of  the  clusters  could  be  so  far  condensed  as 
they  are  at  present,  makes  him  naturally  ascribe  a  cer- 
tain air  of  youth  and  vigour  to  many  very  regularly  scat- 
tered regions  of  our  sidereal  system.  There  are  many 
places,  where  he  asserts  that  there  is  reason  to  believe, 


198  PLANETARY   NEBULA. 

that  the  stars,  if  we  may  judge  from  appearances,  are 
now  drawing  towards  various  secondary  centres,  and 
will,  in  time,  separate  into  different  clusters  so  as  to  oc- 
casion many  sub-divisions.  Our  system,  after  a  number 
of  ages,  may  be  divided  so,  as  to  give  rise  to  a  stratum 
of  two  or  three  hundred  nebulae. 


AN  OPENING    IN    THE    HEAVENS. 

Some  parts  of  our  system  seem  to  have  already  sus- 
tained greater  ravages  from  time  than  others  :  in  the 
body  of  the  Scorpion  there  is  an  opening  or  hole,  which 
is  probably  owing  to  this  cause  ;  it  is  four  degrees  broad. 


A    PERFORATED    NEBULA,    OR    RING    OF    STARS. 

Among  the  curiosities  of  the  heavens,  should  be  plac- 
ed a  nebula  that  has  a  regular  concentric  dark  spot  in 
the  middle,  and  is  probably  a  ring  of  stars  ;  it  is  of  an 
oval  s>hape  ;  in  the  northern  side  three  very  faint  start* 
may  be  seen,  as  also  one  or  two  in  the  southern :  the 
verticies  of  the  longer  axis  seem  less  bright,  and  not  so 
well  defined  as  the  rest. 


PLANETARY    NEBULA. 

These  are  so  named  from  a  singulariry  of  appearance, 
which  renders  it  difficult  to  class  them.  Their  light  is 
so  uniform  and  vivid,  the  diameters  so  small  and  well 
defined,  as  to  make  it  improbable  that  they  should  beN 
common  nebulae  :  if  nebulae,  they  must  be  compressed 
and  condensed  in  the  highest  degree. 

Though  the  words  condensation  and  cluster  often  occur 
in  the  foregoing  extract,  we  are  by  no  means  to  infer 
that  any  of  the  celestial  bodies,  in  our  nebula,  are  nearer 
to  one  another  than  we  are  to  Sinus,  whose  distance  is 
supposed  to  be  not  less  than  18,717,442,526  of  miles. 
The  whole  extent  of  the  nebulae,  being  in  some  places 


TELESCOPIC  APPEARANCE  OF  THE  PLANETS.     199 

near  500  times  this  distance,  must  be  such,  that  the 
light  of  a  star  placed  at  its  extreme  boundary,  supposing 
it  to  fly  with  the  velocity  of  1 2,000,000  miles  every  mi- 
nute, must  have  taken  near  3000  years  before  it  could 
reach  us.* 

These  immense  spaces,  these  numerous  hosts  of  sys- 
tematic universes,  are  probably  connected  the  one  with 
the  other.  Like  so  many  immense  circuses,  by  the  mu- 
tual contact  of  their  circumambient  spheres,  they  press 
each  other  :  these  aerial  atmospheres  being  also  connect- 
ed and  interwoven  together  by  an  infinity  of  insertions, 
constitute  a  celestial  sphere,  which  is  again  linked  with 
others,  till  by  an  infinity  of  orbs  they  obtain  a  form, 
which  is  the  origin  and  pattern  of  all  forms,  in  which  all 
the  variegated  sidereal  revolutions  harmoniously  concur 
to  one  and  the  same  end  ;  that  of  mutually  strengthening 
and  establishing  each  other,  and  forming  a  celestial  union. 


OF  THE    TELESCOPIC    APPEARANCE   OF   THE    PLANETS. 
OF    THE    SUN. 

"  The  observations  which  might  with  fulness  of  evi- 
dence confirm  the  opinion  of  planetary  worlds,  seem  to 
be  placed  out  of  our  reach,  and  we  can  scarce  hope  to 
make  our  optical  instruments  sufficiently  perfect  to  ren- 
der the  inhabitants  thereof  visible  to  us.  All,  therefore, 
that  we  can  do,  is  to  examine  whether  the  planets  be  ac- 
commodated with  those  things  which  we  are  used  to 
consider  as  necessary  to  animal  existence.  Lands,  seas, 
clouds,  vapours,  and  an  atmosphere,  or  body  of  air,  are 
objects  that  we  may  expect  to  find  on  the  face  of  an  in- 
habitable world." 

By  means  of  the  telescope,  we  are  enabled  in  some 
measure  to  ascend  into  the  celestial  region,  and  view  the 
sun,  moon,  and  stars,  as  they  would  appear  to  us  if  they 
were  brought  so  many  times  nearer  to  us  as  the  telescope 

*  The  exact  positions  of  new  planetary  nebulae,  &c.  as  discovered  by 
Dr.  Herschel,  air'  communicated  to  me  by  him,  are  placed  on  the  new  18 
inch  British  celestial  globt E.  Edit. 


200  THE    TELESCOPIC    APPEARANCE 

magnifies ;  the  light  proceeding  from  the  luminary  we 
are  looking  at  being  diminished  in  the  same  proportion. 

The  telescope  is  one  of  those  discoveries,  of  which  no 
idea  could  have  been  formed,  previous  to  the  period  in 
which  the  Supreme  Being  was  pleased  to  unveil  to  the 
human  mind  some  of  the  mysterious  powers  of  glass:  the 
importance  of  this  discovery,  and  the  extent  to  which  it 
may  be  carried,  still  lie  hid  among  the  secrets  of  infinite 
wisdom.  It  is  by  this  instrument,  more  than  by  any 
other,  that  we  have  been  led  onward  in  our  advances  to- 
wards a  perfect  knowledge  of  the  heavenly  bodies,  and 
that  astronomy  has  been  raised,  from  little  more  than  a 
catalogue  of  stars,  into  a  science. 

When  we  look  at  the  sun  through  a  telescope  even  of 
moderate  power,  the  eye  being  defended  by  a  piece  of 
coloured  or  smoked  glass,  nay,  sometimes  even  by  the 
naked  eye,  when  guarded  in  the  same  manner,  we  dis- 
cover on  his  surface  many  black,  or  rather  less  bright 
spots,  of  various  sizes  and  shapes.  Sometimes  these  spots 
will  vanish  in  a  very  short  time  after  their  first  appear- 
,ance ;  sometimes  they  travel  over  his  whole  disk,  or  vi- 
sible surface,  from  west  to  east,  when  they  disappear,  and 
in  twelve  or  thirteen  days  appear  again,  so  as  to  be  known, 
by  their  magnitude  and  figure,  to  be  those  that  had  dis- 
appeared before.  Those,  however,  which  are  of  the  long- 
est continuance,  do  not  appear  to  have  much. solidity  of 
consistence,  for  in  a  little  time  they  also  vanish,  or  become 
bright  like  the  rest  of  the  surface. 

The  spots  are  more  frequent  at  some  periods  than  at 
others;  in  some  years,  the  sun's  disk  has  for  many  months 
been  perfectly  free  from  them ;  in  others,  he  has  for  months 
been  more  or  less  obscured  !>y  spots  :  the  most  remark- 
able phenomena  of  these  spots,  as  observed  by  Schenier 
and  Hevetius,  are  as  follow  :  3 .  Every  spot,  which  has  a 
nucleus,  of  dark  part,  hath  also  an  umbra,  or  fainter 
shade,  surrounding  it.  2.  The  boundary  betwixt  the 
nucleus  and  umbra  is  always  distinct  and  well  defined. 
3.  The  increase  of  a  spot  is  gradual,  the  breadth  of  the 
nucleus  and  umbra  dilating  at  the  same  time.  4.  In  like 
manner,  the  decrease  of  a  spot  is  gradual,  the  breadth  of 
the  nucleus  and  umbra  diminishing  at  the  same  lime. 


OF    THE    PLANETS.  201 

5.  The  exterior  boundary  of  the  umbra  never  consists 
of  sharp  angles,  but  is  always  curvilinear,  how  irregular 
soever  the  outside  of  the  nucleus  may  be.  6.  The  nucleus 
of  a  spot,  whilst  on  the  decrease,  often  changes  its  figure, 
by  the  umbra  incroaching  irregularly  upon  it;  insomuch, 
that  in  a  small  space  of  time  new  incroachments  are  dis- 
cernible, whereby  the  boundary  between  the  nucleus  and 
umbra  is  perpetually  varying.  7.  It  often  happens,  that 
by  these  incroachments  the  nucleus  of  a  spot  is  divided 
into  two  or  more  nuclei.  8.  The  nuclei  of  the  spots  vanish 
before  the  umbra.  9.  Small  umbras  are  often  seen  with- 
out nuclei.  10.  A  large  umbra  is  seldom  seen  without  a 
nucleus  in  the  middle  of  it.  11.  When  a  spot,  which  con- 
sisted of  a  nucleus  and  an  umbra,  is  about  to  disappear, 
if  it  be  not  succeeded  by  a  faecula,  or  spot,  brighter  than 
the  resc  of  the  disk,  the  place  it  occupied  is  in  a  very  lit- 
tle time  not  to  be  perceived. 

In  the  Philos.  Trans,  vol.  Ixiv.  the  reader  will  find  se- 
veral curious  observations  on  these  spots  by  Professor 
Wilson  and  the  Rev.  Mr.  Wolaston.  The  latter  gentleman 
says,  he  once  saw,  with  a  twelve-inch  reflector,  a  -  spot 
burst  to  pieces,  while  he  was  looking  at  the  sun  ;  the  ap- 
pearance was  to  him  as  that  of  a  piece  of  ice,  when  dash- 
ed on  a  frozen  pond,  which  breaks  to  pieces,  and  slides 
in  various  directions. 

The  spots  are  by  no  means  confined  to  one  part  of  the 
sun's  disk,  though  we  do  not  know  that  any  have  been 
observed  about  his  polar  regions.  Though  their  direction 
is  from  east  to  west,  yet  the  paths  they  describe  in  their 
course  over  the  disk,  are  exceedingly  different,  sometimes 
being  in  straight  lines,  sometimes  in  curves  ;  at  one  time 
descending  from  the  northern  to  the  southern  p*rt  of  the 
disk,  at  other  times  ascending  from  the  southern  to  the 
northern  part. 

The  larger  spots,  most  of  which  exceed  the  whole 
earth  in  magnitude,,  last  a  considerable  time,  sometimes 
three  months  before  they  disappear,  at  which  time  they 
are  generally  converted  into  spots  exceeding  the  rest  of 
the  sun  in  brightness.  The  general  opinion  concerning 
their  nature  is,  that  they  are  volcanoes,  or  burning  moun- 
tains of  immense  size ;  and  that  when  the  eruption  is 
VOL.  IV.  2  D 


202  OF    THE    PLANETS. 

nearly  ended,  and  the  smoke  dissipated,  the  fierce  flames 
are  exposed,  and  appear  as  luminous  spots.  D.  Wilson 
supposes  them,  on  the  other  hand,  to  be  excavations  in 
the  luminous  matter,  or  atmosphere,  that  environs  the 
body  of  the  sun. 

The  diameter  of  a  spot  near  the  middle  of  the  disk,  is 
measured  by  comparing  the  time  it  takes  in  passing  over 
a  cross-hair  in  a  telescope,  with  the  time  wherein  the 
whole  disk  of  the  sun  passes  over  the  same  hair.  It  may 
also  be  measured  by  a  micrometer.  Hevelius  observed  a 
spot  that  rose  and  vanished  in  16  or  1 7  hours.  None  have 
been  observed  to  continue  longer  than  70  days.* 

OF    THE    MOON. 

When  we  look  at  the  moon  with  the  naked  eye,  we 
discern  a  great  number  of  irregular  spots  on  her  disk, 
distinguished  by  their  dark  colour  from  the  brighter  or 
more  glaring  parts ;  but  when  viewed  through  a  tele- 
scope, their  number  is  prodigiously  increased ;  and  it  is 
perceived,  that  many  of  these  appearances  are  occasioned 
by  vast  obscure  pits  or  cavities,  and  elevations  or  moun- 
tains. The  spots  in  the  moon  always  keep  their  places, 
not  being  moveable  like  those  of  the  sun.  Sometimes 
more  or  less  of  the  northern,  and  southern,  and  eastern^ 
and  western  part  of  the  disk  is  seen,  which  is  owing  to 
what  is  called  her  libration. 

These  mountains  and  cavities  are  known  to  be  such, 
from  the  shadow  they  cast.  In  the  first  and  second  quar- 
ters, when  the  light  of  the  sun  falls  obliquely  upon  them, 
the  elevated  part  casts  a  triangular  shadow  on  the  side 
opposite  to  the  sun ;  whereas,  with  respect  to  the  cavi- 


*  Dr.  Hemchel'8  opinion  of  the  spots  en  the  sun  has  been  given  in  my 
note  in  page  15  of  this  volume.  It  may  be  proper  to  add  here,  that  from  the 
changes  in  the  atmosphere  of  Jupiter  he  accounts  for  the  phenomena  of  his 
belts;  and  on  a  similar  principle  he  illustrates  the  various  appearances  of 
a  solar  spot  which  he  observed  in  the  sun  in  1779.  This  spot,  he  says,  ex- 
tended above  50,000  miles,  and  tiq.  think**  may  be  easily  jnd  satisfactorily 
explained,  if  we  allow  that  the  real  body  of  the  sun  itself  was  seen  on  this  oc- 
casion, though  we  rarely  see  more  than  its  shining  atmosphere.  This  hy- 
pothesis he  also  applies  to  the  solution  of  phenomena  exhibited  by  other 
spots,  as  observed  by  him*.*.  E.  Edit. 


t)F    THE    PLANETS.  203 

ties,  these  have  that  side  which  is  opposite  to  the  sun  illu- 
minated, and  that  which  is  next  the  sun  is  dark  and  ob- 
scure, the  same  as  would  happen  to  a  hollow  bason,  pla- 
ced on  a  table  at  some  distance  from  a  candle,  in  a  room 
where  there  was  no  other  light.  The  shadows  shorten  as 
the  sun  becomes  more  directly  opposed  to  the  anterior  face 
of  the  moon,  and  at  length  disappear  at  the  time  of  the 
full.  During  the  third  and  last  quarters,  the  shadows 
appear  again,  but  all  fall  towards  the  contrary  side  of  the 
moon,  though  still  with  the  same  distinction,  namely,  that 
the  mountains  are  dark  and  shady  on  the  side  farthest 
from  the  sun,  and  the  pits  are  dark  on  the  side  next  the 
sun. 

The  full  moon  is  a  very  pleasing  sight  through  a  tele- 
scope, and  has  a  great  variety  of  lustre  and  colour;  but 
this  is  not  the  phase  on  which  to  discover  the  mountains, 
these  are  best  seen  at  the  increase  or  decrease  ;  for,  be- 
sides the  evidence  derived  from  the  shadows,  we  may  then 
see  the  tops  of  these  mountains  catching  the  rays  of  the 
sun  before  they  reach  that  part  of  the  surface  on  which 
their  bottoms  are  placed. 

On  April  19,  1787,  Dr.  Herschel  observed  some  ap- 
pearances on  the  surface  of  the  moon,  which,  judging  by 
analogy  from  things  perceived  here  with  us,  he  thought 
he  might  term  volcanoes.  Three  of  these  he  observed  in 
different  places  of  the  dark  part  of  the  moon  ;  two  of  them 
appeared  nearly  extinct,  or  going  to  break  out;  the  third, 
as  an  actual  eruption  of  fire,  or  luminous  matter.  On  the 
20th  it  burnt  with  greater  violence,  and  might  be  computed 
to  be  about  three  miles  in  diameter  :  the  eruption  resem- 
bled a  piece  of  burning  charcoal,  covered  by  a  thin  coat 
of  white  ashes ;  all  the  adjacent  parts  of  the  volcanic 
mountain  were  faintly  illuminated  by  the  eruption,  and 
were  gradually  more  obscure  as  they  lay  at  a  greater  dis- 
tance from  the  crater.  Dr.  Herschel  had,  in  1 783,  observed 
an  eruption,  somewhat  similar  to  that  of  the  foregoing 
volcano.  Indeed  an  appearance  of  this  kind  had  been 
seen  before,  by  Don  Ul/oa,  in  an  eclipse  of  the  sun.  It 
was  a  small  bright  spot,  near  the  margin  of  the  moon, 
which  he  supposed  to  be  a  hole  with  the  sun's  light  shin- 
ing through  it. 


204  OF    THE    PLANETS. 

That  the  moon  is  surrounded  by  an  atmosphere,  is 
rendered  probable  by  many  observations  of  solar  eclip- 
ses, in  which  the  edge  or  limb  of  the  sun  was  observ- 
ed to  tremble  just  before  the  beginning.  The  planets 
are  likewise  observed  to  change  their  figure  from  round 
to  oval,  just  before  the  beginning  of  an  occultation  be- 
hind the  moon,  which  can  be  attributed  to  no  other 
cause,  than  that  their  light  is  refracted  by  being  seen 
through  the  moon's  atmosphere.  That  we  see  no 
clouds,  will  not  appear  surprising,  if  we  consider,  that 
the  lunar  days  and  nights  are  thirty  times  as  long  as 
ours  ;  it  will  be  easy  to  conceive,  that  with  them  the 
phenomena  of  vapours  may  be  very  different  from  what 
they  are  with  us  ;  perhaps  their  clouds  and  rain,  if  any, 
may  be  condensed  into  visible  quantities  only  during 
the  absence  of  the  sun,  and  of  course  when  they  must 
be  invisible  to  us.* 

Mercury  being  at  all  times  near  the  sun,  we  can  on- 
ly distinguish  by  the  telescope,  a  variation  of  his  figure, 
which  is  sometimes  that  of  a  half  moon,  sometimes  a 
little  more  or  less  than  half. 

Venus,  when  in  the  form  of  a  crescent,  and  at  her 
brightest  times,  affords  a  more  pleasing  telescopic  view 
than  any  other  of  the  heavenly  bodies  ;  her  surface  is 
diversified  with  spots,  like  those  of  the  moon  ;  by  the 
motion  of  these,  the  time  she  takes  up  in  revolving  up- 
on her  axis  is  discovered.  With  a  powerful  telescope, 
mountains,  like  those  in  the  moon  may  be  seen.f 

Mars  appears  always  round  and  full,  except  at  the 
time  of  the  quadrature,  when  its  disk  appears  like  that 
of  the  moon  about  three  days  after  the  full.     By  the 


*  Dr.  Herschd  conceives,  that  probably  all  the  planets  emit  light  in  some 
degree  from  their  circumambirMt  atmospheres,  consisting  of  vanouselastic 
fluids,  some  of  which  exhibit  a  shirting  brilliancy,  while  others  are  merely- 
transparent  ;  and  that  from  the  removal  of  this  fluid  die  dark  body  of  the 
planet  becomes  visible. 

As  a  proof  of  this,  he  alleclges  the  observation  of  a  lunar  eclipse  in  1790, 
in  which  there  could  be  no  illumination  from  the  rays  reflected  by  our  atmos- 
phere, the  focus  in  which  they  meet  being  more  than  189,000  miles  beyond 
the  moon. — E.  Edit. 

t  Dr.  Herschcl  has  observed  a  faint  illumination  in  the  unenlightened 
part  of  the  planet  Venus,  which  he  ascribes  to  some  phosphoric  quality  of 
its  atmosphere. — E.  Edit. 


OF    THE    PLANETS.  205 

spots  which  are  seen  on  its  surface,  its  diurnal  revolu- 
tion has  been  ascertained.  From  its  characteristic  rud- 
diness, and  from  other  phenomena,  it  has  been  suppos- 
ed that  its  atmosphere  is  nearly  of  the  same  density 
with  ours.  Dr.  Herschel  has  observed  two  white  lu- 
minous circles  surrounding  the  poles  of  this  planet ; 
they  are  supposed  to  arise  from  the  snow  lying  about 
those  parts. 

The  appearance  of  Jupiter  through  a  telescope,  opens 
a  vast  field  for  speculative  inquiry.  The  surface  is  not 
equally  bright,  but  is  distinguished  by  certain  bands, 
or  belts,  of  a  duskier  colour  than  the  rest  of  the  sur- 
face, running  parallel  to  each  other,  and  to  the  plane  of 
its  orbit.  They  are  not  regular  or  constant  in  their  ap- 
pearance ;  sometimes  only  one  is  seen,  at  other  times 
eight  have  been  seen  ;  their  breadth  is  likewise  variable; 
one  belt  growing  narrow  while  another  in  its  neighbour- 
hood becomes  broader,  as  if  one  had  flowed  into  the 
other ;  in  this  case  an  oblique  belt  has  been  observed  to 
lie  between  them,  as  if  for  the  purpose  of  forming  a 
communication.  Sometimes  one  or  more  spots  are 
formed  between  the  belts,  which  increase  till  the  whole 
are  united  in  one  large  dusky  band.  There  are  also  bright 
spots  to  be  discovered  on  Jupiter's  surface  ;  these  are 
rather  more  permanent  than  the  belts,  and  re-appear  af- 
ter unequal  intervals  of  time.  The  remarkable  spot, 
by  whose  motion  the  rotation  of  Jupiter  on  his  axis 
was  ascertained,  disappeared  in  1694,  and  was  not  seen 
again  till  1708,  when  it  re-appeared  exactly  in  the  same 
place,  and  has  been  occasionally  seen  ever  since.  The 
disappearance  and  re-appearance  of  the  spots  is  not  so 
wonderful  as  the  changes  that  have  been  observed  in 
the  belts ;  the  elder  Cassini  saw  one  evening  five  belts 
upon  the  planet,  but  while  he  was  viewing  them,  they 
underwent  the  most  surprizing  change.  In  an  hour 
from  their  fullest  appearance  there  remained  only  three 
out  of  five,  and  one  of  these  scarce  perceptible.  The 
most  remarkable  telescopic  appearances  of  this  planet, 
are  the  satellites,  but  these  I  have  particularly  described 
under  the  head  of  satellites. 


206  OF    THE    PLANETS. 

Though  the  great  distance  of  the  planet  Saturn,  and 
the  tenuity  of  its  light,  do  not  permit  us  to  distinguish 
the  varieties  of  its  surface  ;  yet  some  of  the  first  dis- 
coveries made  by. the  telescope  were  on  this  planet,  and 
the  ring  is  still  one  of  the  most  curious  phenomena  we 
are  acquainted  with.  There  is  not,  indeed,  any  thing 
in  the  whole  system  of  nature  more  wonderful  than  this 
ring,  which  appears  nearly  as  bright  as  any  part  of  the 
surface  of  the  planet  :  by  what  means  it  is  suspended, 
or  by  what  law  supported  ;  whether  it  be  ojily  a  bright 
but  permanent  cloud,  or  a  vast  number  of  satellites  dis- 
posed in  the  same  plane,  whose  blended  light  gives  it  to 
us  the  form  of  one  continual  body,  we  can  only  form 
crude  conjecture.  M.  Messier  has  observed  on  the 
anses  of  this  ring  several  luminous  white  twinkling 
points,  differing  in  vivacity  from  each  other. 

Sometimes  our  eye  is  in  the  plane  of  the  ring,  and 
then  it  becomes  invisible  :  as  its  plane  always  keeps  pa- 
rallel to  itself,  it  disappears  twice  in  every  revolution 
of  the  planet,  that  is,  about  once  in  fifteen  year  ;  and  he 
sometimes  appears  quite  round  for  nine  months  together. 
At  other  times  the  distance  betwixt  the  body  of  the 
planet  and  the  ring  is  very  perceptible,  insomuch  that 
Dr.  Clarke's  father  saw  a  star  through  the  opening. 
When  Saturn  appears  round,  if  our  eye  be  in  the  plane 
of  the  ring,  it  will  appear  as  a  dark  line  across  the  mid- 
dle of  the  planet's  disk  ;  if  the  eye  be  elevated  above 
the  plane,  a  shadowy  belt  will  be  visible ;  when  the 
plane  appears,  the  ring  next  the  body  is  the  brightest ; 
when  the  ring  appears  of  an  elliptical  form,  the  parts 
about  the  ends  of  the  largest  axis  are  called  ansse. 
These,  a  little  before  and  after  the  disappearing  of  the 
ring,  are  of  unequal  magnitude.  It  has  been  supposed, 
that  the  ring  has  a  rotation  round  an  axis. 

With  very  long  telescopes  two  belts  have  been  disco- 
vered on  Saturn,  which  appear  parallel  to  that  formed 
by  the  edge  of  the  ring  ;  these  are  said  to  be  perma- 
nent :  Cassiht  and  Fatio  perceived  a  bright  streak  upon 
Saturn  which  was  not  permanent,  being  visible  one  day, 
and  disappearing  the  next,  when  another  came  into  view 


OF    COMETS.  207 

near  the  edge  of  his  disk.     Besides  these  there  are  its 
five  satellites,  mentioned  under  their  proper  heads. 


OF    COiMETS. 

Comets  are  a  kind  of  stars,  appearing  at  unexpected 
times  in  the  heavens,  and  of  singular  and  various  figures, 
descending  from  far  distant  parts  of  the  system,  with 
great  rapidity,  surprizing  us  with  the  singular  appear- 
ance of  a  train,  or  tail  ;  and,  after  a  short  stay,  are  car- 
ried off  to  distant  regions,  and  disappear. 

They  were  imagined  in  ancient  times  to  be  prodigies 
hung  out  by  the  immediate  hand  of  God  in  the  heavens, 
and  intended  to  alarm  the  world,  Their  nature  being 
now  better  understood,  they  are  no  longer  terrible.  But, 
as  there  are  still  many  who  think  them  to  be  heavenly 
warnings,  portents  of  future  events,  it  may  not  be  impro- 
per to  inform  you,  that  the  Architect  of  the  universe  has 
framed  every  part  according  to  divine  order,  and  sub- 
jected all  things  to  laws  and  regulations ;  and  that  he  does 
not  hurl  at  random  stars  and  worlds,  and  disorder  the 
system  of  the  whole  glorious  frame,  to  produce  false 
apprehensions  of  distant  events,  fears  without  founda- 
tion, and  without  use.  Religion  glories  in  the  test  of 
reason,  of  knowledge,  and  of  true  wisdom  ;  it  is  every 
way  connected  with,  and  is  always  elucidated  by  them. 
From  philosophy  we  may  learn,  that  the  more  the  works 
of  the  Lord  are  understood,  the  more  he  must  be  ador- 
ed ;  and  that  his  superintendency  over  every  portion  is 
more  clearly  evinced,  and  more  fully  expressed  by  their 
unvaried  course,  than  by  ten  thousand  deviations. 

The  existence  of  a  universal  connection  betwreen  all 
the  parts  of  nature  is  now  generally  allowed.  Comets 
undoubtedly  form  a  part  of  this  great  chain  ;  but  of  the 
part  they  occupy,  and  of  the  uses  for  which  they  exist, 
we  are  equally  ignorant.  It  is  a  portion  of  science  whose 
perfection  is  reserved  for  some  distant  day,  when  these 
bodies  and  their  vast  orbits  may,  by  long  and  accurate 
observation,  be  added  to  the  known  parts  of  the  solar 
system  ;  when  astronomy  will  appear  as  a  new  science, 


208  OF    COMETS. 

after  all  our  discoveries,  great  as  we  at  present  imagine 
them  to  be. 

The  astronomy  of  comets  is  very  imperfect ;  for  but 
little  can  be  known  with  certainty,  where  but  little  can 
be  seen.  Comets  afford  few  observations  on  which  to 
ground  conjecture,  and  are  for  the  greatest  part  of  their 
course  beyond  the  reach  of  human  vision  ;  but  that 
they  are  not  meteors  in  the  air  is  plain,  because  they 
rise  and  set  in  the  same  manner  as  the  moon  and  stars, 
they  are  called  comets  from  their  having  a  long  tail 
somewhat  resembling  the  appearance  of  hair ;  some, 
however,  have  appeared  without  this  appendage,  as  well- 
defined  and  round  as  planets. 

It  is  generally  supposed,  that  they  are  planetary  bo- 
dies, making  part  of  our  system,  revolving  round  the 
sun  in  extremely  long  elliptic  curves  ;  that  as  the  orbit 
of  a  comet  is  more  or  less  eccentric,  the  distance  to 
which  they  recede  from  the  sun  will  be  greater  or  less. 
Very  great  difference  has  been  found  by  observation  in 
this  respect,  even  so  great,  that  the  sides  of  the  elliptic 
orbit  in  some  cases  degenerate  almost  into  right  lines. 
They  are  very  numerous  :  450  are  supposed  to  belong 
to  our  solar  system. 

Those  comets,  which  go  to  the  greatest  distance 
from  the  sun,  approach  the  nearest  to  him  at  their 
return. 

The  motions  in  the  heavens  are  not  all  direct,  or 
according  to  the  order  of  the  signs,  like  those  of  the 
planets.  The  number  of  those  which  move  in  a  retro- 
grade order,  is  nearly  equal  to  those  whose  motion  is 
direct. 

Their  orbits  of  most  of  them  are  inclined  in  very 
large  angles  to  the  plane  of  the  ecliptic. 

The  velocity  with  which  they  move  is  variable  in  eve- 
ry part  of  their  orbit ;  when  they  are  near  the  sun, 
they  move  with  incredible  swiftness  ;  when  very  remote 
from  him,  their  motion  is  inconceivably  slow. 

When  they  appear,  they  come  in  a  direct  line  to- 
wards the  sun,  as  if  they  were  going  to  fail  into  his 
body  :  and  after  having  disappeared  for  some  time,  in 
consequence  of  his  extreme  brightness,  they  fly  off  on 


GF    COMETS.  209 

the  other  side  as  fast  as  they  came,  continually  loosing 
their  splendour,  till  at  last  they  totally  disappear.  Their 
apparent  magnitude  is  very  different,  sometimes  seeming 
not  bigger  than  the  fixed  stars,  at  other  times  equal  in 
diameter  to  Venus.  Hevelius  observed  one  in  1652, 
which  was  not  inferior  to  the  moon  in  size,  though  not 
so  bright ;  its  light  pale  and  dim,  its  aspect  dismal. 

A  greater  number  of  comets  are  seen  in  the  hemisphere 
towards  the  sun,  than  in  the  opposite  ;  and  are  generally 
invisible  at  a  smaller  distance  than  that  of  Jupiter.  Mr. 
Brydone  observed  one  vx  Palmero,  in  July  1770,  which, 
in  24  hours,  described  an  arc  in  the  heavens  upwards  of 
50  degrees  in  length ;  so  that  if  it  was  far  distant  from 
the  sun,  it  must  have  moved  at  the  rate  of  upwards  of 
60  millions  of  miles  in  a  day. 

They  differ  also  in  form  from  the  other  planets,  con- 
sisting of  a  large  internal  body,  which  shines  with  the 
reflected  light  of  the  sun,  and  is  encompassed  with  a  very 
large  atmosphere,  apparently  of  a  finer  matter,  much  re- 
sembling that  of  the  Aurora  Borealis  ;  this  is  called  the 
head  of  the  comet,  and  the  internal  part  the  nucleus. 
When  a  comet  arrives  at  a  certain  distance  from  the 
sun,  an  exhalation  arises  from  it,  which  is  called  the 
tail. 

The  tail  is  always  directed  to  that  part  of  the  heavens 
which  is  directly  or  nearly  opposite  to  the  sun,  and  is 
greater  and  brighter,  after  the  comet  has  passed  its  peri- 
helion, than  in  its  approach  to  it ;  being  greatest  of  all 
when  it  has  just  passed  the  perihelion.  The  tail  of  the 
comet  of  1680  was  of  a  prodigious  size,  extending  from 
the  head  to  a  distance  scarcely  inferior  to  that  of  the  sun 
from  the  earth. 

No  satisfactory  knowledge  has  been  acquired  concern- 
ing the  cause  of  that  train  of  light  which  accompanies 
the  comets.  Some  philosophers  imagine,  that  it  is  the 
rarer  atmosphere  of  the  comet  impelled  by  the  sun's 
rays.  Others,  that  it  is  the  atmosphere  of  the  comet, 
rising  in  the  solar  atmosphere  by  its  specific  levity ; 
while  others  imagine,  that  it  is  a  phenomenon  of  the 
same  kind  with  the  Aurora  Borealis ;  and  that  this  earth 

vol.  iv.  e  a 


l210  OF    A    PLURALITY    OF    WORLDS. 

would  appear  like  a  comet  to  a  spectator  placed  in  ano- 
ther planet. 

The  number  of  the  comets  is  certainly  very  great,  con- 
siderably beyond  any  estimation  that  might  be  made  from 
the  observations  we  now  possess. 

There  are  some,*  who  do  not  think  the  present  astro- 
nomy of  comets  well  established ;  and  that  as  so  many 
small  ones  are  frequently  seen,  they  think  that  nothing 
can  be  determined  with  certainty,  till  some  better  marks 
are  discovered  for  distinguishing  one  from  another,  than 
any  at  present  known ;  and  that  even  the  accomplish- 
ment of  Dr.  Halley's  prediction  is  uncertain:  for  it  is 
very  singular,  that  out  of  four  years,  in  which  three  co- 
mets appeared,  the  only  one,  in  which  no  comet  was  to 
be  seen,  should  be  that  very  year  in  which  the  greatest 
astronomers  that  ever  existed  had  foretold  the  appearance 
of  one ;  and,  in  accounting  for  its  non-appearance,  Mr. 
Clairault  would  have  been  equally  supported  by  cometic 
evidence,!  whether  he  concluded  the  comet  to  have  been 
retarded  or  accelerated  by  the  action  of  Jupiter  or  Saturn : 
a  comet  appeared  in  1757,  as  well  as  in  1755,  and  had 
he  determined  the  retardation  of  the  comet  to  be  twice 
as  great  as  he  did,  another  appeared  in  1760  to  have  veri- 
fied his  calculations. 


OF    A    PLURALITY    OF    WORLDS. 

The  fixed  stars  are  generally  supposed  to  be  of  the 
same  nature  with  our  sun,  each  of  them  attended  by  pla- 
nets which  are  inhabited  by  rational  creatures,  like  this 
earth. 

Instead,  therefore,  of  one  sun,  and  one  world,  we  find, 
that  the  region  of  unbounded  space  is  peopled  with  suns, 
and  stars,  and  worlds.  This  opinion  has  been  held  and 
taught  by  many  of  the  most  celebrated  philosophers  and 
astronomers,  both  in  ancient  and  modern  times ;  in  this 
view  of  things,  our  system  resembles  a  single  individual 


*  Encyclopaedia  Britannica,  vol.  ii.  p.  765.    Second  Edition. 
f  There  does  not  indeed  seem  any  evidence  to  prove  the  return  of  the 
same  comet. 


OF    A    PLURALITY    OF    WORLDS.  211 

of  some  one  species  of  being  in  outward  nature,  diversi- 
fied from  all  its  fellow  individuals,  by  differences  unes- 
sential to  the  kind  and  species  ;  but  which  constitute  that 
beauty,  which  arises  from  uniformity  amidst  variety. 

That  the  fixed  stars  are  suns,  shining  by  their  own 
light,  is  probable,  on  account  of  their  immense  distance 
from  us ;  for,  as  it  is  impossible  that  at  these  distances 
they  could  be  seen  by  any  reflexion  of  light  from  the 
sun,  it  is  natural  to  suppose  them  endowed  with  a  power 
of  emitting  light  from  their  own  bodies.  By  comparing 
the  apparent  diameter  of  objects  at  different  distances,  it 
is  clear,  that  our  sun  would  appear  like  a  star,  were  he 
removed  to  the  distance  at  which  they  are  placed ;  and 
that  therefore  it  is  truly  reasonable  to  suppose,  that  the 
fixed  stars  are  equal,  if  not  superior  in  magnitude,  to  that 
which  is  the  centre  of  our  system;  and  that  they  are  made 
for  the  same  purposes  with  the  sun,  to  bestow  light,  heat, 
and  vegetation,  on  a  certain  number  of  planets  revolving 
round  them.* 

Of  their  immense  distance  from  us,  and  the  vastness 
of  the  space  they  occupy,  the  reader  may  form  some 
idea,  when  he  is  told,  that  numbers  amongst  them  are  at 
too  great  a  distance  to  be  adequately  expressed  by  figures, 
and  beyond  the  reach  of  admeasurement ;  and  this  will 
be  heightened,  if  he  considers,  that  the  smallest  of  the 
stars  visible  to  the  eye  are  much  more  remote  than  the 
larger  ones,  and  that  the  telescope  discovers  stars  which 
are  too  distant  to  be  perceptible  to  the  naked  eye :  that 
the  instrument,  like  our  eyes,  has  its  limits  ;  but  the  ex- 
tent of  the  heavens  has  no  bounds. 

The  fixed  stars  being  so  far  removed  from,  and  for  the 
most  part  invisible  to  us  ;  it  can  scarcely  be  conceived  by 
the  narrowest  mind,  that  they  form  a  part  of  our  system, 
or  were  created  only  to  give  a  faint  glimmering  light  to 


*  Dr.  Herschel  closes  his  conjectures  on  the  sun,  Sec.  wi.h  the  following 
general  inference.  "  It  seems  therefore,  on  the  whole,  not  impossible,  that 
in  many  cases  stars  are  united  in  such  clo^e  systems,  as  not  to  lea\e  much 
room  for  the  orbits  of  planets  or  comets,  and  that  consequently,  upon  this 
account  also,  many  stats,  unless  we  could  make  them  mere  useless  brilliant 
points,  may  themselves  be  lucid  planets,  perhaps  unattended  by  satellites." 

E.  Edit. 


212  OF    A    PLURALITY    OF    WORLDS. 

the  inhabitants  of  this  globe :  for  one  additional  moon 
would  have  afforded  us  more  light  than  the  whole  host 
of  stars  ;  such  an  opinion  is  unworthy  of  our  reason,  in- 
adequate to  our  conceptions  of  the  Deity.  It  would  be  also 
absurd  to  suppose,  that  the  Author  of  nature  had  made 
so  many  suns  without  planets,  to  be  enlightened  by  their 
light,  and  vivified  by  their  heat ;  but  more  so,  to  ima- 
gine so  many  habitable  worlds  enlightened  by  suns  with- 
out inhabitants  ;  we  may,  therefore,  safely  conclude,  that 
all  the  planets,  of  every  system,  are  inhabited. 

This  reasoning  is  still  further  strengthened,  by  consi- 
dering the  immensity  of  the  starry  heavens,  in  which  are 
innumerable  hosts  of  stars,  created  as  the  means  to  some 
great  end.  "  Every  star  may  be  the  centre  of  a  magni- 
ficent system,  attended  by  a  retinue  of  worlds,  irradiated 
by  its  beams,  and  revolving  round  by  its  active  influence." 
Thus  the  greatness  of  God  is  magnified,  and  the  gran- 
deur of  his  empire  made  manifest.  He  is  not  glorified  on 
one  earth,  or  in  one  world,  but  in  ten  thousand  times 
ten  thousand.  "  If  we  could  wing  our  way  to  the  high- 
est apparent  star,  we  should  there  see  other  skies  expand- 
ed, other  suns  that  distribute  their  inexhaustible  beams 
of  day ;  other  stars  that  gild  the  alternate  night,  and 
other,  perhaps  nobler,  systems  established  in  unknown 
profusion,  through  the  boundless  dimensions  of  space. 
Nor  does  the  dominion  of  the  Sovereign  of  all  things 
terminate  here ;  even  at  the  end  of  this  vast  tour  we 
should  find  ourselves  advanced  no  further  than  the  fron- 
tiers of  creadon,  the  commencement  of  the  great  Jeho- 
vah's kingdom.* 

This  mode  of  reasoning  applies  with  greater  force  to 
the  planets  of  our  own  system,  and  gains  additional 
strength  from  other  considerations.  For  who  would  ven- 
ture to  assert,  that  infinite  love  and  consummate  wisdom 
had  formed  such  immense  material  masses,  some  of  which 
exceed  our  earth  in  size,  convey  them  in  revolutions  round 
the  sun,  furnish  them  with  moons,  grant  them  the  alter- 
nate changes  of  night  and  day,  vicissitudes  of  seasons, 
and  all  this  only  to  emit  their  scantly  light  on  our  earth. 

*  Htrvty's  Meditations. 


OF*  A    PLURALITY    OF    WORLDS.  213 

Or  who  that  has  seen  any  engine,  a  windmill  for  in- 
stance, and  who  knows  the  use  of  it,  if  he  travel  into 
another  country,  and  there  see  an  engine  of  the  same 
sort,  will  not  reasonably  conclude  that  it  is  designed  for 
the  same  purpose  ?  So  when  we  know  that  the  use  of  this 
planet,  the  earth,  is  for  a  habitation  of  various  sorts  of 
animals,  and  we  see  other  planets  at  a  distance  from  us, 
some  bigger,  some  less  than  the  earth,  moving  periodi- 
cally round,  revolving  on  their  axes,  and  attended  with 
moons ;  is  it  not  highly  reasonable  to  conclude,  that  they 
are  all  designed  for  the  same  use  as  this  earth  is,  and  that 
they  are  habitable  worlds  like  that  we  live  in  ? 

Who  can  conceive  them 

.Unpossess'd, 

By  living  soul,  desert  and  desolate.. 
Only  to  shine,  yet  scarce  to  contribute, 
Each  orb  a  gleam  of  light  ? 

Or  that  the  Almighty,  who  has  not  left  with  us  a  drop  of 
water  unpeopled,  who  has  in  every  instance  multiplied  the 
bound  of  life,  should  leave  such  immense  bodies  desti- 
tute of  inhabitants  ?  It  is  surely  much  more  rational  to 
suppose  them  the  possession  of  human  beings,  beings 
formed  with  capacities  for  knowing,  loving,  and  serving 
their  Almighty  Creator ;  blest  and  provided  with  every 
object  conducive  to  their  happiness,  and  many  of  them 
in  a  far  greater  state  of  purity  than  the  inhabitants  of  our 
earth,  and  therefore  in  possession  of  higher  degrees  of 
bliss,  and  placed  in  situations,  furnishing  them  with  scenes 
of  joy,  equal  to  all  that  poetry  can  paint,  or  religion  pro- 
mise :  all  under  the  direction,  indulgence,  and  protection 
of  infinite  wisdom  and  goodness.* 

The  more  the  heavenly  bodies  excite  our  astonishment, 
from  their  size,  their  distances,  the  regularity  of  their  mo- 
tions, or  any  peculiarity  or  perfection  we  discover  in  those 
attractions  by  which  they  seem  retained  in  their  places, 
the  more  clear  it  is  to  any  reasoning  head,  that  they  could 
not  have  made  themselves  :  and  that  close  connexion  be- 


*  See  the  Rev.  Mr.  IVoolaston's  Reflexions 


SJ14  ON    PHYSICAL    ASTRONOMY^ 

tween  cause  and  effect,  which  the  farther  we  search  the 
more  clearly  we  discern,  though  it  has  staggered  the  faith 
of  many  a  celebrated  naturalist,  is  itself  a  proof,  if  he  had 
not  stopped  short  of  the  conclusion,  that  all  these  must 
have  been  the  contrivance  of  consummate  wisdom,  and 
guided  by  an  unerring  hand. 

Yet,  at  the  same  time,  he  who  sees  that  every  little  cor- 
ner of  this  earth  of  ours  is  replete  with  animal  life,  though 
but  one  race  on  it  seems  to  be  endowed  with  reasoning 
faculties,  cannot  but  be  led  on  to  a  conjecture  at  least, 
that  all  those  vast  bodies  he  discovers  in  the  heavens  may 
be  peopled  with  their  gradations  of  inhabitants  likewise ; 
and  that  each  of  them,  not  improbably,  contains  its  ra- 
tional beings  too,  to  acknowledge  and  adore  the  Creator 
of  them  all.  So  far  the  heathen  philosopher  may  go : 
though,  if  he  be  a  modest  inquirer  after  truth,  he  will  not 
dogmatize,  or  enter  into  any  particular  detail  of  what  is 
so  totally  out  of  his  reach. 


LECTURE  XLV. 

ON    PHYSICAL    ASTRONOMY.* 


1  HE  causes  of  the  celestial  motions  have  in  all  ages 
been  the  objects  of  philosophical  curiosity.  Men  have 
generally  conducted  their  researches  on  this  subject  upon 
principles  of  analogy.  Some  resemblances  have  been 
noticed  between  the  motions  of  the  celestial  bodies,  and 
other  motions  nearer  at  hand,  and  more  familiar  to  us ; 
and  the  same  resemblances  have  been  supposed  to  exist 
between  their  causes. 

*  Professor  Robinson's  Outlines  of  Mechanical  Philosophy,  p.  105. 


OF    PHYSICAL    ASTRONOMY.  215 

I  shall  notice  four  of  these  different  resemblances,  or 
analogies. 

1.  The  motions  of  the  heavenly  bodies  have  been 
thought  to  resemble  the  spontaneous  motions  of  intelli- 
gent beings.  Aristotle,  Leibnitz,  Tucker,  Monboddo,  and 
some  others,  both  in  ancient  and  modern  times,  have 
taught  that  the  planets  were  conducted  by  spiritual  in- 
telligent beings. 

^  Though  accounts  of  the  celestial  phenomena  may  be 
given  by  means  of  this  resemblance,  that  are  chargeable 
with  no  false  reasoning ;  yet  as  they  afford  no  explana- 
tion, they  answer  no  purpose  in  philosophy. 

2.  The  celestial  motions  have  been  thought  to  repre- 
sent the  motions  of  bodies  carried  about  centres  by  means 
of  solid  connexions. 

This  motion  suggested  to  philosophers  the  opinion, 
that  the  planets  were  attached  to  solid  orbits,  which  turn 
round  the  axis  of  revolution  :  this  opinion  has  been  falsely 
attributed  to  Aristotle.  It  is  altogether  contradictory  to 
our  ideas  of  the  etherial  matter  that  occupies  celestial 
space,  and  not  easily  reconcileable  to  the  elliptic  motion 
of  the  planets. 

3.  The  celestial  motions  have  been  thought  to  resem- 
ble the  motions  of  bodies  carried  round  by  a  circulating 
fluid.  Many  philosophers  have  supposed  the  planets  to 
:>e  earned  round  in  such  vortices.  Descartes  and  Leib- 
nitz were  at  great  pains  to  establish  this  doctrine.  More 
nodern  writers*  have  removed  the  difficulties,  and  obvi- 
ited  the  objections  made  to  this  system.  It  will  there- 
ore  be  necessary  to  lay  before  you  some  of  the  argu- 
nents  urged  in  its  favour;  in  doing  this,  I  shall  be  under 
ne  necessity  of  repeating  some  of  the  observations  that 
nave  made  before. 

These  writers  urge,  that  so  long  as  we  keep  within  the 
mits  of  natural  and  experimental  philosophy,  we  must 
ccount  for  the  motions  in  nature,  by  referring  them  to 
orporeal  causes ;  and  where  this  cannot  be  performed 


i£dS^^  Ess*y™  the  First  Principles  of 

pS^P^   **•  -*-  »*on'»  Observations  on  the  Moving  Powers 


216  ON    PHYSICAL    ASTRONOMY. 

satisfactorily,  we  must  give  them  up,  or  wait  with 
patience  for  some  better  clue  of  investigation,  or 
some  further  light  from  experience.  It  is  contrary  to 
sound  philosophy  to  amuse  ourselves  with  names  and 
qualities,  which  contradict  the  known  laws  of  mecha- 
nism, and  supercede  the  operation' of  the  elements. 

Nothing  is  intelligible  in  philosophy  but  the  action 
of  matter  upon  matter  ;  the  power  of  impulse  is  the 
only  sensible  and  experimental  cause  of  motion  ;  and 
there  is  the  strongest  presumption  from  analogy  in  fa- 
vour of  the  universal  material  mechanism  of  the  opera- 
tions of  nature.  All  other  principles  of  motion  are 
founded  on  conjecture,  and  incapable  of  proof.  If  you 
attempt  to  soar  above  this  principle  in  theory,  you  are 
always  obliged  to  descend  to  it  in  practice.  Natural 
philosopy  has  been  principally  advanced  by  the  experi- 
ments which  have  been  made  on  the  elements  ;  but 
these  experiments  prove,  that  matter  interferes  in  pro- 
ducing all  the  changes  and  motions  that  are  observed 
in  bodies  distant  from  each  other. 

Look  into,  and  observe  the  operations  in  nature  :  how 
does  the  sun  act  upon  the  fruits  of  the  earth,  but  by  the 
mediation  of  its  light?  How  do  the  clouds  water  the 
earth,  but  by  the  mediation  of  air  ?  How  does  the  che- 
mist produce  such  wonderful  changes  in  natural  bodies, 
but  by  the  mediation  of  fire  ?  In  a  word,  every  experi- 
ment, every  observation  proves,  that  in  all  cases  where 
distant  bodies  are  found  to  affect  each  other,  there  is 
always  something  to  mediate,  whether  we  do  or  do  not 
perceive  it ;  and  when  this  mediation  can  be  no  further 
traced,  natural  philosophy  is  at  an  end,  and  the  fictions 
of  imagination  begin,  which  are  oi~  equal  value,  by 
whatever  name  they  may  be  called,,  or  with  whatever 
parade  of  demonstration  they  may  be  introduced. 

It  is  very  singular,  they  observe,  that  inquirers  after 
physical  truth  should  observe  and  acknowledge  mechan- 
ism in  the  greater  part  of  nature,  and  yet  not  be  led 
thereby  to  inquire,  whether  it  be  not  universally  ex- 
tended ;  the  more  so,  as  matter  and  motion  must  have 
the  same  invariable  properties.  If  vapours  rise  mecha- 
nically, why  may  not  a  stone  descend  by  the  samelaw? 


ON    PHYSICAL    ASTRONOMY.  217 

If  fluids  circulate  in  organized  bodies  by  continued  im- 
pulse, why  may  not  a  planet  revolve  in  the  organized 
system  of  the  universe  by  the  same  cause  ? 

All  true  philosophers  agree  in  considering  the  uni- 
verse  as  a  great  machine,  so  created,  fitted,  and  dispo- 
sed by  the  power  of  God,  as  to  perform  all  the  opera- 
tions, which  are  carried  on  throughout  the  whole. 
There  is  a  connexion  and  communication  between  all 
the  distant  parts  thereof.  No  one  part  can  be  consi- 
dered as  acting  without  being  acted  upon. 

It  is  highly  unphilosophical  to  assert,  that  matter, 
considered  in  general,  or  any  part  thereof,  has  essential 
separate  qualities,  by  which  one  part  acts  upon  another. 
It  is  the  essential  property  of  no  one  wheel  in  a  machine 
to  move  its  fellow,  though,  in  consequence  of  its  being 
placed  in  the  station  it  is  fitted  for,  it  acts  upon  its  fel- 
low, because  it  is  acted  upon.  If  you  interrupt  the  con- 
tact in  a  machine,  you  destroy  the  motion  in  all  those 
parts  where  the  communication  is  destroyed. 

It  is  just  the  same  with  the  whole  system  of  nature, 
you  cannot  take  up  any  parcel  of  matter  and  say  of  it, 
this  has  essential  properties  which  empower  it  to  be  a 
natural  agent.  A  philosopher  ought  to  consider  it  as  a 
concrete,  with  a  certain  disposition  of  parts  liable  to  be 
acted  upon  by  the  subtiler  parts  of  the  machine,  which 
can  by  no  means  be  restrained  by  art  therefrom*  It 
might  be  as  justly  asserted,  that  it  is  the  essential  pro- 
perty  of  animal  substances  to  live,  as  that  it  is  the  es- 
sential property  of  the  loadstone  to  attract. 

The  promoters  of  the  opinion  now  under  considera- 
tion, urge  further,  that  every  known  operation  in  na- 
ture is  mechanical ;  and  that  in  all  experiments,  where 
the  explanation  is  clear  and  certain,  the  effects  are  pro- 
duced by  matter  acting  upon  matter  ;  and  we  are  able 
to  trace  this  mechanism  in  such  a  variety  of  instances, 
that  unless  the  world  be  governed  by  opposite  and  con- 
tradictory principles,  it  must  obtain  throughout  the 
whole. 

Thus  the  body  of  man,  which  is  the  highest  piece  of 
machinery  in  nature,  is  made  to  see,  to  hear,  and  speak, 

VOL.  iv.  2.r 


213  ON    PHYSICAL    ASTRONOMY. 

upon  mechanical  principles  ;  and  it  dies,  unless  there 
be  a  constant  impression  of  a  material  force  upon  it, 
from  the  element  of  air. 

Again,  from  the  pressure  of  air,  the  mercury  is 
made  to  rise  in  the  tube  of  the  barometer.  Hail,  snow, 
and  vapour,  are  formed  in  the  atmosphere  by  the  dif- 
ference in  its  temperature ;  the  clouds  are  sustained 
therein,  and  driven  about  to  water  the  earth  ;  plants 
grow  and  are  nourished  thereby. 

For  those  effects  where  the  cause  is  not  so  obvious, 
you  find  fire  a  more  subtile  agent,  whose  reality  is 
proved,  and  its  operations  pointed  out  both  by  observa- 
tion and  experiment.  It  may  be  transferred  from  one 
parcel  of  matter  to  another.  It  will  enter  the  pores, 
and  fill  the  interstitial  vacuities  of  all  other  substances. 
It  acts  with  a  force  and  velocity  adequate  to  all  the  ef- 
fects we  can  desire  to  ascribe  thereto.  It  gives  an  elas- 
tic force  to  air,  and  occupies  every  space  from  which 
the  air  is  exhausted.  In  some  cases  it  acts  as  light,  in 
others  as  fire  ;  light,  as  it  illuminates  and  renders  ob- 
jects visible ;  fire,  as  it  burns  and  consumes  what  it 
acts  upon. 

Thus  you  find  the  fluid  etherial  matter  of  the  hea- 
vens acting  by  impulse  on  the  solid  matter  of  the  earth, 
being  instrumental  in  every  one  of  its  productions,  and 
necessary  to  every  stated  phenomenon  of  nature. 

We  are  forced  by  the  evidence  of  every  phenomenon 
in  nature,  by  every  experiment  in  philosophy,  to  con- 
clude, that  impulse*  is  the  only  material  cause  of  mo- 
tion. All  the  properties  of  matter  are  such  as  fit  them 
to  act,  and  to  be  acted  upon  in  a  mechanical  way.  They 
are  all  such  as  can  be  adapted  to  the  known  principles 
of  mechanism  among  artists.  We  are,  therefore,  bound 
by  every  rule  of  sound  reasoning  to  consider  it  as  the 
cause  of  all  the  motion,  and  continuance  of  motion,  in 
the  material  universe.     It  is  the  one  certain  and  only 


*No  rr.er.hnnic.il  motion  cat)  subsist  without  a  plenum  ;  this  must  be, 
wherever  such  mechanical  motion  subsists.  This  is  so  necessary  a  con>e- 
quence  of  motion  being  carried  on  by  impulse,  that  it  needs  no  other  de- 
monstration. 


ON    PHYSICAL    ASTRONOMY.  219 

universal  known  cause.     Neither  the  properties  of  mat- 
ter, nor  experiment,  nor  observation,  afford  any  other. 
No  independent    motion  can  be  discovered.     It  is 
therefore  wrong  to  consider  the  motion  of  any  body  ab- 
stractedly, or  as  a  thing  by  itself.  The  system  of  nature 
is  connected  and  related  ;  and,  to  be  understood,  must 
be  considered  under  those  relations  and  connections. 
Speculations  which  carry   you  out  of  the  world,  can 
never  inform  you  how  things  are  carried  on  in  the  world. 
Matter  subsisting  as  a  part   of  the  created  world  has 
motion,  but,  if  separated  from  the  rest,  would  have  no 
more  motion  than  a  limb  divided  from  the  body  ;  and 
he  who  studies  the  nature  of  motion  by  taking  matter 
ab^  ractedly,  is  studying  motion  from  that  which  has  no 
motion  belonging  to  it. 

Another  strong  argument  adduced  in  favour  of  this 
system,  is  derived  from  the  continuance  and  permanency 
of  the  motions  observed  in  nature.  That  there  is  a 
universal  principle  of  motion  throughout  the  system  of 
things,  is  self-evident.  We  know  that  matter  moving 
can  be  the  cause  of  motion  in  matter  at  rest ;  and  we 
know  of  no  other  physical  cause  capable  of  producing 
such  motion.  Here,  therefore,  we  must  look  for  the 
causes  of  permanent  motions. 

Of  the  motion  in  different  bodies,  it  is  observable,  that 
some  retain  the  motion  they  have  acquired,  without  any 
diminution,  while  others  are  soon  reduced  to  a  srate  of 
rest.  When  a  body  retains  its  motion  without  diminu- 
tion, itfiis  moved  by  such  causes  as  would  renew  its  mo- 
tion, if  it  were  stopped.  When  a  cloud  is  flying  before 
the  sun,  the  same  wind  that  drives  it  on,  would  restore 
its  motion  if  it  could  be  stopped.  In  the  same  manner 
the  sails  of  a  wind-mill,  after  you  have  brought  them  to 
a  state  of  rest,  and  even  confined  them,  will  receive  a 
new  motion  from  the  wind,  as  soon  as  the  obstruction  is 
removed.  If  you  stop  the  motion  of  the  lungs  by  an  ef- 
fort of  the  muscles,  you  find  that  the  natural  causes 
that  act  upon  the  body  tend  to  renew  the  motion,  and 
cannot  be  hindered  from  effecting  it,  without  a  consider 
rable  effort. 


'220  ON    PHYSICAL    ASTRONOMY. 

Every  lasting  motion  is  such  a  one,  therefore ,  that  will 
be  renewed  upon  its  own  principles.  This  observation  is 
of  great  importance  towards  accounting  truly  for  the 
undecaying  motions  of  the  universe,  to  all  which  it  may 
undoubtedly  be  extended  :  so  that  if  it  were  possible  to 
stop  the  planet  Jupiter  in  his  orbit,  the  establised  causes 
that  act  upon  him,  would  renew  his  motion  without  any 
artificial  motion. 

A  body  continues  to  move  as  long  as  the  natural 
causes  of  motion  continue  to  act  thereon ;  and  rest, 
which  is  mechanical  death,  inevitably  follows,  when  the 
causes  of  motion  are  no  longer  present.  There  may  be 
subtile  cases,  in  which  it  may  be  as  hard  for  us  to  trace 
the  causes  of  motion,  as  to  show  why  life  remains  for 
some  time  in  an  animal  body  under  water  without  re- 
spiration. Still,  however,  the  general  assertion  must  be 
true,  if  every  effect  must  have  its  cause ;  for  then  to 
every  permanent  effect  there  must  be  a  permanent  cause. 

It  is  therefore  not  only  absurd,  but  contrary  to  every 
analogy  in  nature,  to  suppose  that  any  of  the  durable 
motions  of  the  celestial  bodies  depend  upon  projection 
in  a  vacum  :  because  if  you  were  to  stop  a  body  moved 
upon  this  principle,  you  have  no  means  of  renewing  its 
motion,  it  must  either  fall  into  the  sun,  and  thus  come 
to  a  point  of  rest,  or  be  dead  and  motionless  for  ever, 
without  some  miracle  to  give  it  a  new  motion ;  but  this 
being  contrary  to  the  conditions  of  every  undecaying 
motion,  which  will  be  renewed  on  its  own  principles  in 
the  ordinary  course  of  nature,  and  by  means  .already 
established,  is  not  to  be  admitted  into  philosophy. 

They  further  urge  as  a  reason  for  rejecting  the  hypo- 
thesis of  a  projectile  impulse,  that  it  obliges  its  sup- 
porters to  make  the  universe  a  vacum  :  because  those 
elements  which  are  ordained  to  act  upon  matter,  and 
keep  up  the  life  and  motion  of  the  world,  would  stand 
in  the  way,  hinder  the  freedom,  and  disturb  the  opera- 
tion of  an  imaginary  principle,  projection.  They  con- 
sider projection  not  only  as  a  hypothetical,  but  as  an 
artificial  and  unnatural  principle,  that  cannot  be  proved 
to  obtain  any  where  in  nature.  If  it  be  received,  they 
say  it  must  be  received  as  an  article  of  faith. 


ON    PHYSICAL    ASTRONOMY.  221 

Experiments  have  been  made  with  a  central-force  ma- 
chine, to  illustrate  the  doctrine  of  centripetal  and  centri- 
fugal forces.*  But  they  by  no  means  apply  to  any  case 
in  nature,  for  the  moving  body  is  always  connected  by 
a  line  to  its  centre  of  motion  ;  a  circumstance  that  ne- 
ver can  be  reconciled  to  motion  in  a  vacuum,  where  no 
connection  is  supposed  ;  nay,  is  even  objected  to  upon 
principle.  But  these  experiments  are  still  further  de- 
ficient, because  the  centrifugal  force  being  a  consequence 
of,  or  derived  from  the  artificial  revolution  of  the 
whirling  body,  cannot  be  used  as  a  cause  of  the  motion  : 
for  it  is  the  nature  of  all  causes  to  be  prior  to  their 
effects,  but  here  it  is  posterior  ;  the  body  is  never  dis- 
posed to  fly  off  in  a  tangent  till  it  has  acquired  its  revo- 
lution. This  force  can,  therefore,  never  be  applied  to 
account  for  any  of  the  celestial  motions,  because  it 
brings  us  to  this  absurdity,  that  there  is  nothing  to  ac- 
count for  the  motion,  but  the  motion  itself,  or  the  con- 
sequence of  the  motion. 

The  same  objections  apply,  and  even  further,  to  an- 
other illustration,  namely,  casting  round  a  weight  sus- 
pended in  a  sling  ;  for  the  power  of  the  sling  restrain- 
ing the  body  from  flying  off  in  a  tangent,  bears  no  ana- 
logy to  a  power  actually  drawing  the  moving  body  to- 
wards its  centre  of  motion. 

It  has  been  objected  to  this  reasoning,  that  no  body 
can  move  in  a  space  filled  with  matter,  commonly  called 
a  plenum.  But  this  entirely  depends  on  the  condition  of 
the  matter  and  the  circumstances  of  the  moving  body  : 
if  the  matter  filling  the  space  be  a  fluid,  whose  parts 
can  easily  slide  over  one  another,  they  will  be  able  to 
move  in  different  or  contrary  directions  at  the  same 
time,  and  while  the  place  of  the  whole  mass  remains 
the  same,  the  place  of  the  parts  of  which  it  is  composed 
may  be  continually  changing. 


*  The  machine  here  alluded  to,  is  the  whirling-table  ;  the  description  of 
which  I  have  given  in  vol.  iii.  p.  319,  et  seq.  Ii  is  a  machine,  in  my  opinion, 
oi  a  very  evident  and  illustrative  nature,  shewing  by  experiments  the  laws 
of  central,  &c.  forces  whatever  may  be  the  real  cause  of  motion. — E.  Edit. 


222  ON    PHYSICAL    ASTRONOMY. 

The  fulness  of  the  space  is  therefore  no  objection  to 
the  free  motion  of  the  parts  of  any  fluid  among  them- 
selves ;  neither  is  it  any  objection  to  the  motion  of  any 
solid  body  in  such  a  fluid  medium.  Though  a  vessel  be 
filled  with  water  closely  stopped,  and  the  fluid  so  com- 
pressed, that  a  very  small  point  made  to  enter  therein 
would  burst  the  containing  vessel,  yet  any  solid  will  move 
freely  therein  from  one  side  to  the  other,  or  from  the 
top  to  the  bottom  ;  because  the  parts  of  the  fluid  which 
are  displaced  before,  fall  into  that  space  behind  quitted 
by  the  body.  So  fast  as  the  body  proceeds,  just  so  fast 
do  the  parts  of  the  fluid  recede  ;  so  that  there  is  neither 
impediment  nor  vacuity.  The  same  is  true  in  other 
cases  ;  there  may  be  motion,  provided  there  be  a  circu- 
lation among  the  parts. 

When  a  solid  body  is  moved  in  a  fluid  by  any  artifi- 
cial force  or  violence,  contrary  to  the  nature  of  the  me- 
dium in  which  it  moves,  the  parts  of  the  medium,  by 
endeavoring  to  recover  their  natural  state,  will  resist 
the  motion°of  the  body  till  the  equilibrium  be  restored, 
and  the  body  at  rest.  Such  will  necessarily  be  the  case 
of  all  violent  motions ;  it  is  soon  destroyed  by  resist- 
ance though  the  time  in  which  it  is  destroyed  may  dif- 
fer from  a  variety  of  circumstances. 

But  on  the  other  hand,  if  the  motion  of  the  body 
arise  from  the  motion  of  the  medium  in  which  it  moves, 
then  the  resisting  nature  of  the  medium  is  no  longer 
an  objection  to  the  motion  of  the  body,  neither  can  it 
be,  for  it  is  the  cause  of  its  motion ;  and  it  is  absurd 
to  suppose,  that  the  cause  of  the  motion  can  resist  the 
motion  it  causes.  No  inference,  therefore,  from  the 
resistance  of  mediums  can  lead  us  to  the  necessity  of  a 
vacuum.  A  vacuum  is  only  necessary  when  a  motion 
is  proposed,  which  is  independent  of  the  action  of  eve- 
ry medium  ;  but  nature  knows  of  no  such  motion. 

A  variety  of  motions  may  be  exhibited,  for  whose 
production  the  presence  of  a  resisting  medium  is  abso« 
lutely  necessary  ;  and  they  show,  that  so  far  from  a 
vacuum  being  necessary  to  the  continuance  of  motion 
in  any  space,  the  motion  is  promoted  and  occasioned 
by   a  resisting   medium.     That  hypothetical  train   of 


ON    PHYSICAL    ASTRONOMY.  223 

reasoning  which  leads  us  to  conclude,  that  if  less  mat- 
ter  were  in  the  space,  the  motion  would  be  more  free 
and  continue  much  longer,  is  as  unphilosophical,  as  it 
would  be  if,  in  order  to  enable  a  man  to  run  faster,  we 
should  rid  him  of  the  incumbrance  of  his  boots  and 
spurs,  by  cutting  off  his  legs. 

Air  is,  you  know,  a  resisting  medium,  yet,  instead  of 
retarding  the  motion  of  the  lamp-machine,  which  I  be- 
fore showed  you,  by  its  resistance  it  preserves  that  mo- 
tion ;  and  if  the  motion  be  at  last  discontinued,  it  does 
not  arise  from  defect  or  irregularity  of  the  cause,  but 
from  the  imperfection  of  the  materials.  If  the  materials 
which  are  acted  upon  would  but  continue  in  the  same 
state,  the  motion  would  be  unretarded  as  long  as  air  and 
fire,  which  are  the  causes  thereof,  subsist  in  the  world. 
In  this  experiment  the  causes  are  not  artificial  and  vio- 
lent, as  in  the  central-force  machine,  but  such  as  are 
supplied  by  nature  itself,  in  its  regular  mode  of  action  ; 
which  both  begins  and  continues  the  motion.  What  is 
performed  by  the  agents  in  nature  in  the  one  case,  may 
certainly  be  done  in  others.  The  planets  may  be  car- 
ried round  in  their  orbits  by  the  same  means.  The 
heavens  may  be  filled  throughout  with  an  etherial  fluid, 
not  infinitely  rarefied,  unresisting,  and  impotent,  but 
dense  and  continuous. in  its  parts. 

The  writers  in  favour  of  the  mechanical  system  urge, 
that  their  opponents  have  no  notion  or  means  of  resolv- 
ing their  axioms,  or  relative  laws  of  motion,  to  mecha- 
nism, but  consider  them  merely  as  laws  ;  another  word, 
as  they  use  it,  for  ultimate,  spiritual,  unmechanical  pow- 
er. As  the  penetration  of  some  among  them  has  car- 
ried them  so  far  as  to  suppose  an  impelling  etherial 
medium  for  maintaining  attraction,  gravitation,  &c.  &c. 
it  is  rather  surprizing  that  they  could  not  perceive  that 
the  same  medium  was  necessary  for  supporting  their 
laws  of  motion,  rest,  resistance,  &c.  for  the  difficul- 
ty does  not  lie  in  accounting  for  gravitation,  or  any 
particular  kind  of  motion,  but  in  finding  powers  to 
produce  and  maintain  motion  in  general.  If  these  be 
mechanical,  it  is  easy  to  suppose,  that  the  contriver 
may  have  adjusted  the  mechanism  so  as  to  produce  the 


224  ON    PHYSICAL    ASTRONOMY. 

particular  tendencies.  But  if  they  be  unmechanical, 
you  may  call  them  laws,  properties,  or  any  other  name, 
either  with  or  without  a  meaning.  How  detrimental 
is  it  to  the  increase  of  knowledge  in  the  powers  and 
agency  of  nature,  to  have  the  most  curious  productions 
of  these  powers  reduced  to  unintelligible  laws,  charac- 
terized by  words  without  meaning,  and  which  render 
their  inventors  no  wiser  than  the  most  heedless  and 
unattentive ! 

Without  instrumental,  or  second  causes,  there  can  be 
no  regular  course  of  nature ;  and  without  a  regular 
course,  nature  could  never  be  understood.  The  order 
and  course  of  things,  and  the  experiments  we  daily 
make,  show  that  there  is  a  mind  that  governs  and  actu- 
ates this  mundane  system  as  the  proper  real  agent  and 
cause ;  the  inferior  and  instrumental  cause  seems  to  be 
fire  ;  with  respect  to  attraction,  it  cannot  produce,  and 
in  that  sense  account  for  the  phenomena,  being  itself 
one  of  the  phenomena  produced  and  to  be  accounted 
for.  What  is  said  of  forces  residing  in  bodies,  whe- 
ther attracting  or  repelling,  it  can  only  be  considered 
as  a  mathematical  hypothesis,  not  as  any  thing  real  and 
existing  in  nature. 

The  mechanical  agency  of  the  elements  accords  with 
the  descriptions  and  illusions  of  the  sacred  scriptures. 
The  heathens  were  in  some  degree  acquainted  there- 
with. When  this  doctrine  was  in  their  hands,  a  princi- 
ple of  intelligence  was  ascribed  to  the  active  elements, 
and  they  were  taken  for  the  Gods  who  govern  the 
world.  But  with  those  who  are  taught  that  the  True 
God  is  distinct  from,  and  above  the  world  of  matter, 
though  virtually  present  by  a  providential  inspection  and 
superintendance,  it  serves  only  to  enlarge  and  exalt 
their  ideas  by  setting  before  them  the  visible  evidence  of 
divine  wisdom,  which  with  so  exquisite  a  contrivance, 
and  such  simplicity  of  design,  hath  adopted  physical 
causes  to  the  production  of  their  respective  effects. 

We  have  now  to  consider,  4thly,  the  mathematical 
principles  of  philosophy.  The  celestial  motions  have 
been  thought  to  resemble  those  exhibited  to  us  in  the 
phenomena  of  magnetism  and  electricity ;    these  and 


KEPLER  S  LAWS.  22,5 

the  celestial  bodies  seem  to  act  upon  each  other  at  a 
distance,  without  any  observed  intervening  impulse. 
Accordingly,  many  philosophers,  both  ancient  and  mo- 
dern, have  imagined  that  the  planets  are  influenced  by 
causes  similar  to  those  of  more  familiar  phenome- 
na. But  these  philosophers  had  formed  no  accurate 
notions  of  the  agency  of  the  causes  of  the  motions  from 
which  they  attempted  to  derive  an  explanation  ;  neither 
had  they  examined  attentively  the  circumstances  of  the 
motions  which  they  attempted  to  explain.  At  last,  Sir 
Isaac  Newton  contented  himself  with  an  investigation  of 
the  laws  observed  in  the  agency  of  the  causes  of  the 
celestial  motions,  discovered  that  these  laws  were  the 
same  with  those  observed  in  the  agency  of  the  causes  of 
the  motion  of  common  heavy  bodies,  and  from  this  dis- 
covery gave  a  theory  of  mathematical  astronomy.  We 
are  indebted,  however,  to  Kepler  for  the  generalization 
of  facts,  which  form  the  basis  of  the  mathematical  theorw 


kepler's  laws. 

Kepler  s  first  law  is,  that  the  planets ,  in  revolving  round 
the  sun,  describe  equal  areas  in  equal  times. 

Kepler' *s  second  law  is,  that  the  orbits  described  by  the 
planets  are  ellipses,  having  the  sun  or  the  primary  planets 
in  the  focus. 

Kepler9 s  third  law  is,  that  the  squares  of  the  periodical 
times  of  the  planets  are  as  the  cubes  of  their  mean  dis- 
tances from  the  sun.  That  this,  as  the  square  of  the  time 
which  a  planet,  A,  takes  to  revolve  in  its  orbit,  is  to  the 
square  of  the  time  which  any  other  planet,  B,  takes  to 
run  through  its  orbit ;  so  is  the  cube  of  the  mean  dis- 
tance of  A  from  the  sun,  to  the  cube  of  the  mean 
distance  of  B  from  the  sun. 

OF    DEFLECTING    FORCES.* 

In  consequence  of  the  inertia  of  matter,  all  motion 
is  considered  as  equable  and  rectilineal,  as  being  in  a 


*  Professor  Robinson's  Outlines  of  Mechanical  Philosophy,  p.  34  to  107. 
VOL.  IV.  2  G 


226  OF    DEFLECTING    FORCES. 

straight  line  with  the  direction  of  the  moving  force ;  and 
as  preserving  this  direction  until  it  be  hindered  or  put  out 
of  its  way  by  some  extrinsic  cause. 

If  therefore  a  body  turn  in  a  curve,  that  curvature 
must  proceed  from  some  external  force  continually  act- 
ing upon  the  body  ;  and  whenever  that  force  ceases  to 
act,  the  body  will  move  forward  in  a  right  line,  touch- 
ing the  curve  in  that  point  where  the  body  is  at  the  in- 
stant of  time  when  the  force  ceases  to  act. 

When  you  observe  a  change  in  the  direction  of  any 
motion,  you  may  infer  the  action  of  a  force,  whose  di- 
rection crosses  that  of  the  former  motion.  This  may  be 
called  a  deflecting  force. 

The  change  of  direction  is  measured  by  the  angle  con- 
tained between  the  former  and  the  new  direction. 

When  the  motion  of  a  body  is  curvilineal,  the  deflex- 
ion is  continual,  and  you  may  infer  the  continual  action 
of  a  deflecting  force.  On  the  other  hand,  the  continual 
action  of  a  deflecting  force  produces  a  curvilineal  mo- 
tion. 

In  a  curvilineal  motion  the  change  of  direction  is  mea- 
sured by  the  angle  contained  between  the  tangents  to  the 
curve. 

A  curvilineal  motion  is  therefore  always  a  compound 
motion;  but  the  great  bodies  of  this  system,  as  the  pla- 
nets, move  round  the  sun  in  curve  lines  ;  on  these  prin- 
ciples, there  must  therefore  necessarily  be  two  powers 
acting  on  them ;  one  impelling  them  to  move  in  a  straight 
line,  the  other  deflecting  or  bending  them  continually  to- 
wards a  centre. 

You  may,  therefore,  consider  deflecting  forces  as  al- 
ways directed  to  or  from  a  point ;  in  the  first  case  they 
are  called  centripetal  forces,  in  the  second  case  they  are 
called  centrifugal  forces.  In  general,  they  are  termed 
central  forces  ;  and  the  point,  through  which  their  direc- 
tion always  passes,  is  called  the  centre  of  the  forces. 

Among  the  various  curvilinear  motions  which  may 
arise  from  the  action  of  central  forces,  there  is  a  circum- 
stance in  which  they  all  agree,  and  which  enables  the 
mathematician  to  investigate  the  forces  by  which  they 
are  produced^ 


OF    DEFLECTING    FORCES.  227 

If  a  body  move  in  a  curve  line,  ABCDEF,  plate  15, 
fig.  3,  by  means  of  a  force  always  directed  to  a  fixed 
point  S,  the  curve  is  all  in  one  plane,  and  the  areas, 
ASB,  ASC,  ASD,  described  by  the  straight  line  joining 
the  body  with  the  point  S,  are  proportional  to  the  times 
of  description ;  /'.  e.  equal  areas  are  described  in  equal 
times,  unequal  areas  in  unequal  times.  Thus  the  trian- 
gular areas  ASB,  BSC,  CSD,  &c.  described  by  the 
straight  line  joining  the  body,  with  the  point  S,  are  pro- 
portional to  the  times  of  description. 

Let  the  time  be  divided  into  equal  parts,  let  the  body 
be  acted  on  by  an  impulse  that  would  carry  it  from  A  to 
B,  in  the  first  given  particle  of  time  ;  then  in  the  second 
particle  it  would  go  an  equal  space,  and  describe  the  line 
B  c,  equal  to  the  line  A  B. 

But  when  the  body  is  arrived  at  B,  let  a  deflecting  cen- 
tripetal force  so  act  upon  it,  that  while  its  first  impulse 
would  carry  it  to  c,  the  deflecting  force  would  carry  it  to 
V;  complete  the  parallelogram  B  V  C  c,  and  it  is  evident 
from  the  doctrine  of  compound  forces,  that  the  body 
would  in  the  second  particle  of  time  describe  the  diago- 
nal B  C. 

Now,  as  C  c  is  parallel  to  S  V,  the  triangles  SBC, 
S  B  c,  are  between  the  same  parallel  lines,  and  as  such, 
are  by  geometry  proved  to  be  equal ;  for  the  same  rea- 
son the  triangles  S  C  D,  S  E  F,  are  proved  to  be  each 
equal  to  S  B  A. 

If  any  number  of  these  triangles  be  added  together, 
the  total  sums,  as  AD  S,  F  C  S,  will  be  proportional  to 
the  times  wherein  they  are  described. 

If  the  lines,  A  B,  B  C,  be  continued  round  a  centre, 
they  will  form  a  polygon,  and  if  the  sides  of  the  polygon 
be  indefinitely  increased  in  number,  and  indefinitely  de- 
creased in  length,  they  will  form  a  curve, — a  circle  or  an 
ellipsis :  and  the  proposition  will  be  true  of  these  curves, 
that  a  line  drawn  from  the  centre  to  a  body  in  the  cir- 
cumference of  the  circle,  or  from  the  focus  to  a  body  in 
the  circumference  of  the  ellipsis,  will  sweep  equal  areas 
in  equal  times. 

The  power,  therefore,  directed  towards  the  given  point 
S,  has  no  effect  on  the  magnitude  of  the  area  described 


228  OF    DEFLECTING    FORCES. 

by  the  line  supposed  to  be  drawn  from  the  body  to  that 
point.  It  may  accelerate  or  retard  the  motion  of  the  body, 
but  affects  not  the  area  or  space  described  by  the  line. 
The  line  will  still  continue  to  describe  the  same  spaces  in 
equal  times,  about  the  given  point,  as  it  would  have  done 
if  no  new  force  had  acted  on  the  body,  but  it  had  been 
permitted  to  proceed  uniformly  in  the  line  of  projection. 

As  one  impulse  towards  the  given  point  has  no  effect 
on  the  area  or  space  described  by  the  ray  or  line  from 
the  body  to  that  point,  so  any  number  of  successive  im- 
pulses directed  to  the  same  point  can  have  no  effect  on 
the  area ;  and  if  you  suppose  the  power  directed  to  that 
point,  to  act  continually,  it  will  bend  the  way  of  the  body 
in  motion  into  a  curve,  and  may  accelerate  or  retard  its 
velocity,  but  can  never  affect  the  area  described  in  a  given 
time  by  a  line  supposed  to  be  drawn  from  the  body  to 
the  given  point,  which  will  always  be  of  an  invariable 
quantity,  equal  to  that  which  would  have  been  described 
in  the  same  time,  if  the  body  had  proceeded  uniformly  in 
a  right  line  from  the  beginning  of  the  motion. 

The  converse  of  the  foregoing  proposition  shows,  that 
if  a  body,  A,  describe  a  curve  all  in  one  plane,  and  if 
there  be  a  point,  S,  so  situate  in  this  plane,  that  a  line 
drawn  therefrom  to  the  circumference  describes  propor- 
tional areas  in  proportional  times,  then  is  the  body  urged 
round  by  a  force  tending  towards  that  centre.  In  other 
words,  the  equable  increase  of  the  areas  described  by  a 
line  drawn  from  a  body  to  a  given  point,  is  an  indication 
that  the  direction  of  the  power  that  acts  upon  the  body, 
and  that  deflects  it  into  a  curve,  is  directed  to  that  point. 

By  the  same  propositions  we  may  illustrate  and  explain 
the  revolutions  of  the  primary  planets  in  elliptical  orbits, 
not  much  differing  from  circles,  round  the  sun,  which  is 
in  one  of  the  foci  of  each  ellipsis. 

Let  the  ellipsis  ABCDEFGHIKLM,  plate  15,  fig.  4, 
represent  the  orbit  of  a  planet  moving  therein  round  the 
sun  S,  according  to  the  order  of  the  letters,  the  sun,  S, 
being  in  one  of  the  foci  of  the  ellipsis  ;  let  the  time  of  its 
revolution  be  divided  into  any  number  of  equal  parts, 
suppose  twelve;  in  moving  from  A  through  BCD,  &c. 
the  planet  approaches  nearer  the  sun,  and  the  central  tei 


OF    DEFLECTING    FORCES.  229 

dency  continually  increasing  its  velocity,  it  goes  through 
greater  arcs  in  equal  times,  till  it  come  to  G  ;  from 
thence  its  motion  continually  carries  it  to  a  greater  dis- 
tance from  the  sun,  and  it  describes  in  equal  times  smaller 
and  smaller  arcs,  till  it  return  to  A,  from  whence  it  pro- 
ceeds as  before. 

Now,  the  triangular  spaces  passed  over  by  a  line  drawn 
from  the  planet  to  the  centre  of  the  sun  will  be  equal,  be- 
cause in  the  planet's  going  the  first  half  of  the  ellipsis 
from  A  to  G,  the  arcs  which  may  be  considered  as  the 
base  of  the  mixed  triangles  described  in  equal  times,  grow 
longer  and  longer,  as  the  legs  grow  shorter,  so  as  to  pre- 
serve the  equability  of  the  triangular  space  :  in  the  other 
half  or  the  ellipsis,  in  the  planet's  going  from  G  to  A,  the 
arcs  grow  shorter ;  but  this  is  compensated  by  the  greater 
length  of  the  legs. 

The  sum  of  what  has  been  proved  is  ;  1 .  That  the  areas 
or  spaces  revolving  round  an  immoveable  centre  are  pro- 
portional to  the  times  ;  and,  2.  That  if  a  body  revolving 
round  a  centre,  describe  about  it  areas  proportional  to  the 
times,  the  body  is  actuated  by  a  force  directed  to  that  centre. 

But,  by  Kepler's  first  law,  we  know,  "  that  the  pri- 
mary planets  describe  round  the  sun,  and  the  secondary 
planets  describe,  round  their  respective  primary  planets, 
areas  proportional  to  the  times."  From  hence  it  is  infer- 
red,  that  the  primary  planets  are  retained  in  their  orbits 
by  forces  which  are  always  directed  to  the  sun  ;  and  that 
the  secondary  planets  are  retained  in  their  orbits  round 
their  primary  planets  by  forces  which  are  always  directed 
to  those  primary  planets; 

Kepler's  second  law  is,  "  that  the  orbits  described  round 
the  sun,  and  round  the  primary  planets,  are  ellipses,  hav- 
ing the  sun  or  the  primary  planet  in  the  focus."  From 
hence  it  is  inferred,  that  the  accelerating  force,  by  which 
a  planet  is  retained  in  the  different  parts  of  its  elliptical 
orbit,  is  inversely  proportional  to  the  square  of  its  dis- 
tance from  the  sun,  or  from  its  primary. 

Kepler's  third  law  is,  "  that  the  square  of  the  periodic 
times  of  planets  revolving  round  common  centres,  are 
proportional  to  the  cubes  of  their  mean  distances."  From 
this  it  is  inferred,  that  the  forces  by  which  the  planets  are 


230  GRAVITATION    OF    THE    MOON 

retained  in  their  different  orbits,  are  inversely  proportion- 
al to  the  squares  of  their  distances  from  the  sun.  The 
same  reasoning  applies  to  the  satellites. 

Hence  it  is  also  inferred,  that  the  forces  by  which  dif- 
ferent planets  are  retained  in  their  different  orbits,  are  not 
forces  of  different  kinds,  but  the  same  force  operating  at 
different  distances. 

The  secondary  planets  accompany  the  primary  planets 
by  the  action  of  a  force  always  directed  to  the  sun,  and 
inversely  proportional  to  the  square  of  the  distance  from 
the  sun. 

That  the  moon  is  a  heavy  body,  and  gravitates  towards 
the  earth  in  the  same  manner  as  terrestrial  bodies.*' 

Sir  Isaac  Newton,  considering  that  the  power  of  gravity 
acts  equally  on  all  matter  that  is  in  or  near  the  surface  of  . 
the  earth,  that  it  is  not  sensibly  less  on  the  tops  of  the    . 
highest  mountains,  that  it  affects  the  air  and  reaches  up- 
ward to  the  utmost  limits  of  the  atmosphere,  was  indu- 
ced to  think  it  might  be  a  more  general  principle,  and 
extend  to  the  heavens,  so  as  to  affect  the  moon  at  least, 
which  is  the  nearest  to  us  of  all  the  bodies  in  the  system. 
He  afterwards  extended  this  principle  still  further,  and 
showed,  that  the  planets  consisted  of  the  same  gravitat-   | 
ing  substance  of  which  the  earth  is  formed. 

These  effects  of  the  power  of  gravity  upon  terrestrial 
bodies  mav  be  reduced  to  three  classes.  1 .  "When  in  con- 
sequence  thereof,  a  body  at  rest,  supported  by  the  ground, 
suspended  by  a  string,  or  by  any  other  means  kept  from 
falling,  endeavours  always  to  move.  In  such  cases  the 
effect  of  gravity  is  measured  by  the  pressure  of  the  qui- 
escent body  upon  the  obstacle  that  hinders  its  motion. 

2.  When  a  body  descends  in  a  vertical  line,  its  mo- 
tion is  then  continually  accelerated,  in  consequence  of  the 
incessant  action  of  the  power  of  gravity  ;  or  if  it  be  pro- 
jected upwards  in  the  same  right  line,  its  motion  is  conti- 
nually retarded  by  the  same  power  acting  incessantly  up- 
on it  in  a  contrary  direction.     In  such  cases  the  power 

*  Maclaurin's  Sir  Isaac  Newton's  Discoveries,  p.  214  to  265. 


TOWARDS    THE    EARTH.  231 

of  gravity  is  measured  by  the  acceleration  or  retardation 
of  the  motion  produced  in  a  given  time  by  the  power 
continued  uniformly  for  that  time.* 

3.  When  a  body  is  projected  in  any  direction  different 
from  the  vertical  line,  the  direction  of  its  motion  is  con- 
tinually varied,  and  a  curve  line  is  described  in  conse- 
quence of  the  incessant  action  of  gravity  ;  which  in  such 
cases  is  measured  by  the  flexure  or  curvature  of  the  line 
described  by  it ;  for  the  power  must  be  the  greatest  that 
deflects  the  course  of  the  body  most  from  the  tangent  or 
direction  in  which  it  was  projected. 

Effects  of  each  kind  of  the  power  of  gravity  continu- 
ally fall  under  your  observations  near  the  surface  of  the 
earth ;  for  the  same  power  which  renders  bodies  heavy 
while  they  are  at  rest,  accelerates  their  motion  when  they 
descend  perpendicularly,  and  bends  their  motion  into  a 
curve  line  when  they  are  projected  in  any  other  direction 
than  that  of  their  gravity. 

We  can  judge  only  of  the  powers  that  act  on  the  ce- 
lestial bodies  by  the  effects  of  the  last  kind  ;  we  see  bo- 
dies near  the  earth  falling  towards  it ;  but  this  is  a  proof 
of  the  moon's  gravity,  which  cannot  be  obtained  unless 
the  present  state  of  things  were  dissolved. 

When  a  body  is  projected  in  the  air,  you  do  not  see 
it  fall  in  the  perpendicular  towards  the  earth,  but  you  see 
it  falling  every  moment  from  the  tangent  to  the  curve, 
that  is,  from  the  direction  into  which  it  would  have  mov- 
ed, if  its  gravity  had  not  acted  for  that  moment. 

And  this  proof  is  obtained  of  the  moon's  gravity ;  for 
though  you  do  not  see  her  falling  directly  towards  the 
earth  in  a  right  line,  yet  you  observe  her  every  moment 
descending  towards  the  earth  from  the  right  line,  which 
was  the  direction  of  her  motion  at  the  beginning  of  that 
moment,  and  this  is  as  evident  a  proof  of  her  being  acted 
upon  by  gravity  or  some  power  similar  to  it,  as  her  recti- 
lineal descent  would  be,  if  she  were  allowed  to  fall  freely 
to  the  earth. 

If  we  were  in  possession  of  engines  of  a  sufficient  force, 
bodies  might  be  projected  from  them  so  as  not  only  to  be 

*  See  Lecture  on  Mechanics. 


232  GRAVITATION    OF    THE    MOON 


O    3v 


carried  a  vast  way  without  falling  to  the  earth,  but  so  a^ 
to  move  over  a  quarter  of  a  great  circle  thereof;  or,  ab- 
stracting from  the  resistance  of  the  air,  to  move  round 
the  earth  without  touching  it,  and  after  returning  to  the 
first  place,  commence  a  new  revolution  with  the  same 
force  which  they  first  received  from  the  engine,  and  after 
that  a  third,  and  thus  revolve  as  a  moon  or  satellite  round 
the  earth  for  ever. 

If  this  could  be  effected  near  the  earth's  surface,  it  might 
be  done  higher  in  the  air,  or  even  as  high  as  the  moon, 
could  the  engine  or  an  equivalent  power  be  carried  up 
and  made  to  act  there.  By  increasing  the  force  of  the 
power,  a  body  proportionally  larger  might  be  thus  pro- 
jected, and  by  a  power  sufficiently  great,  a  body  not  infe- 
rior to  the  moon,  might  be  at  first  put  in  motion,  and  be- 
ing perpetually  restrained  by  its  gravity  from  going  off  in 
a  straight  line  might  for  ever  revolve  about  the  earth. 

Thus  Sir  Isaac  Newton  saw,  that  the  curvilineal  motion 
of  the  moon  in  her  orbit,  and  of  any  projectile  at  the  sur- 
face of  the  earth,  were  phenomena  of  the  same  kind,  and 
might  be  explained  upon  the  same  principle  extended  from 
the  earth,  so  as  to  reach  the  moon  ;  and  that  the  moon 
was  only  a  larger  projectile  that  received  its  motion  in  the 
beginning  of  things  from  the  Almighty  Author  of  the 
universe. 

But  to  make  this  more  evident,  it  was  necessary  to  show, 
that  the  powers  which  act  on  the  moon,  and  on  projec- 
tiles near  the  earth,  and  which  bend  their  motions  in  a 
curve  line,  were  directed  to  the  same  centre,  and  agreed 
in  the  quantity  of  their  force  as  well  as  in  their  direction. 

All  we  know  of  force  relates  either  to  its  direction  or 
quantity,  and  a  constant  coincidence  or  agreement  in  these 
two  respects  is  sufficient  ground  to  conclude  them  to  be 
the  same,  or  similar  phenomena,  derived  from  the  same 
or  like  causes. 

Now  I  showed  you,  in  the  lecture  on  mechanics,  that 
the  gravity  of  heavy  bodies  is  directed  towards  the  centre 
of  the  earth  ;  and  it  appears  from  Kepler's  first  law,  as  I 
have  shown  you  in  this  lecture,  that  the  power  which  acts 
on  the  moon,  incessantly  bending  her  motion  into  a  curve, 
is  directed  towards  the  same  centre;  for  astronomers  find, 


TOWARDS    THE    EARTH.  233 

that  the  moon  does  not  describe  an  exact  circle  about 
the  earth,  but  an  ellipse,  and  that  she  approaches  to  the 
earth,  and  then  recedes  from  it  in  every  revolution,  but 
still  so  as  to  have  her  motion  accelerated  while  she  ap- 
proaches the  centre  of  the  earth,  and  retarded  as  she 
recedes  from  it,  describing  equal  areas  in  equal  times ; 
an  indication,  as  you  have  already  seen,  that  she  is  acted 
on  by  a  power  directed  accurately  or  nearly  towards  the 
centre. 

There  is,  therefore,  a  power  which  deflects  the  moon 
from  a  rectilineal  course,  and  which  like  gravity,  makes 
her  descend  towards  the  centre  of  the  earth  ;  so  that 
if  the  projectile  force  were  destroyed,  she  would  fall  to 
the  earth  in  a  direct  line  ;  and  as  this  power  acts  inces- 
santly, bending  every  moment  her  path  into  a  curve,  it 
would  make  her  descend  to  the  earth  with  an  accelerat- 
ed motion,  like  that  of  heavy  bodies  in  their  fall. 

It  remains  therefore  only  to  show,  that  the  power 
which  acts  on  the  moon,  agrees  with  gravity  in  the 
quantity  of  its  force,  as  well  as  in  other  respects.  But 
before  we  compare  them  in  this  particular,  I  must  ob- 
serve to  you,  that  the  power  which  acts  upon  the  moon, 
is  not  the  same  at  all  distances,  but  is  always  greatest 
when  she  is  nearest  the  earth. 

To  be  satisfied  of  this,  it  is  only  necessary  to  observe, 
as  before,  that  to  bend  the  motion  of  a  body  into  a  curve 
when  it  moves  with  a  greater  velocity,  requires  more 
power  than  when  it  describes  the  same  curve  with  a 
less  velocity. 

Though  what  I  have  just  asserted  is  sufficiently  ob- 
vious, it  may  appear  more  fully  by  considering  a  dia- 
gram; imagine  therefore  a  tangent,  plaU  1 5,Jig.  5,  drawn 
at  the  beginning  of  a  small  arc  described  by  the  body  ; 
and  as  this  is  the  line  which  the  body  would  have  fol- 
lowed, if  no  new  power  had  acted  upon  it,  the  effect 
of  that  power  is  estimated  by  the  depression  of  the 
other  extremity  of  the  arc  under  that  tangent :  now  it 
is  plain,  that  in  arcs  of  the  same  curvature,  the  greater 
the  arc  is,  the  farther  must  one  extremity  of  it  fall  be- 
low the  tangent  drawn  at  the  other  extremity  ;  and,  con^ 

VOL.  IV.  2  H 


234  GRAVITATION    OF    THE    MOON 

sequently,  when  a  body  describes  a  greater  arc,  it  muse 
be  acted  upon  by  a  greater  power  than  when  it  describes  a 
less  arc  in  the  same  time.  Now,  as  the  moon  approaches 
the  earth,  her  motion  is  accelerated,  being  swiftest  at  her 
least  distance,  slowest  at  her  greatest  distance  ;  and  the 
arches  she  describes  at  her  greatest  and  least  distance 
have  the  same  curvature ;  therefore  the  force  which 
acts  upon  her  at  her  least  distance,  when  her  motion  is 
swiftest,  must  be  the  greatest  force. 

It  will  not  now  be  difficult  to  see  according  to  what 
law  this  power  varies  at  her  least  and  greatest  distance 
from  the  earth.  To  render  this  easier,  let  us  assume 
a  simple  case,  and  suppose  that  her  least  distance  is  half 
that  of  her  greatest.  If  this  were  true,  the  moon  would 
move  with  double  velocity  in  her  least  distance  ;  and 
the  space  described  by  a  ray  from  her  to  the  earth  might 
be  equal  to  the  space  described  in  the  same  time  at  her 
greatest  distance  :  so  that  she  would  describe  at  her  least 
distance  an  arc  in  one  minute  equal  to  the  arc  she 
would  describe  in  two  minutes  at  her  greatest  distance, 
and  would  fall  as  much  below  the  tangent  at  the  begin- 
ning of  the  arc  in  one  minute  in  the  lower  part  of  her 
orbit,  or  the  perigee,  as  in  two  minutts  in  the  higher 
part  of  her  orbit,  or  her  apogee. 

If,  therefore,  her  projectile  force  were  destroyed  at 
her  least  distance,  she  would  fall  towards  the  earth  as 
much  in  one  minute,  as  in  two  minutes  if  her  projec- 
tile force  were  destroyed  at  her  greatest  distance. 

But  the  spaces  described  by  a  falling  body  are  as  the 
squares  of  the  times,  and  such  a  body  descends  through 
a  quadruple  space  in  double  time  ;  so  that  the  moon 
descending  freely  would  necessarily  fall  four  times  as 
far  in  two  minutes  as  in  one  minute  ;  that  is,  through 
four  times  as  much  space  in  one  minute  at  her  least  dis- 
tance, as  at  her  greatest  distance  in  the  same  time. 

But  the  forces  with  which  heavy  bodies  descend  are 
in  the  same  proportion  as  the  spaces  described,  in  con- 
sequence of  those  forces,  in  equal  small  parts  of  time  \ 
consequently,  the  force  which  acts  at  the.  least  distance 
is  quadruple  that  which  acts  at  a  greatest  distance,  when 
the  latter  is  supposed  to  be  double  the  former  ;  or  the 


TOWARDS    THE    EARTH.  235 

forces  are  as  4  to  1,  when  the  distances  are  as  1  to  2. 
The  force  therefore  which  •  acts  upon  the  moon,  and 
bends  her  into  a  curvilinear  orbit,  increases  as  the  dis- 
tance from  the  centre  of  the  earth  decreases,  so  as  to  be 
quadruple  at  half  that  distance. 

In  the  same  manner  it  is  shown,  that  if  her  least  dis- 
tance were  the  third  part  only  of  her  greatest  distance, 
her  velocity  would  be  triple  at  the  least  distance,  to 
preserve  the  equality  of  the  areas  described  by  a  ray 
drawn  from  her  to  the  centre  of  the  earth  ;  and  that 
she  would  be  acted  upon  there  by  a  power,  which  would 
have  the  same  effect  in  one  minute,  as  in  three  minutes 
at  her  greatest  distance  ;  so  that  if  she  were  allow- 
ed to  descend  freely  from  each  distance,  she  would  fall 
nine  times  as  far  from  the  least  distance  as  from  the 
greatest  in  the  same  time  ;  consequently,  the  power  it- 
s.lf  which  causes  her  to  descend  would  be  nine  times 
greater  at  the  third  part  of  the  distance,  or  the  distances 
being  as  1  to  3,  the  force  would  be  as  9  to  1,  or  in- 
versely as  the  squares  of  the  distances. 

In  the  same  manner  it  appears,  that  when  the  great- 
est and  least  distances  are  supposed  to  be  in  any  pro- 
portion of  a  greater  to  a  less  number,  the  velocities  of 
the  revolving  planet  are  in  the  inverse  ratio  of  the  same 
numbers  ;  and  that  the  powers  which  deflect  or  bend  its- 
motion,  into  a  curve,  are  in  the  inverse  ratio  of  those 
numbers. 

To  consider  this  in  general  j  let  T,  plate  15,  Jig,  5, 
represent  the  centre  of  the  earth,  ALP  the  moon's 
elliptical  orbit,  A  the  apogee,  P  the  perigee,  A  H  and 
P  K  the  tangents  at  those  points,  A  M  and  P  N  any 
small  arcs  described  by  the  moon  in  equal  times  at  those 
distances,  M  H,  N  K,  the  subtenses  of  the  angles  of  con- 
tact, terminated  by  the  tangents  in  H  and  K  ;  then  M 
H  and  N  K  will  be  equal  to  the  spaces  that  would  be 
described  by  the  moon,  if  allowed  to  fall  freely  from 
the  respective  places  A  and  P  in  equal  times  ;  and  will 
be  in  the  same  proportion  to  each  other,  as  the  powers 
which  act  upon  the  moon,  and  inflect  her  course  at  those 
places. 


236  GRAVITATION    OF    THE    MOON 


ire 


Let  A  m  be  taken  equal  to  P  N,  and  m  h  parallel 
AP  meet  the  tangent  at  A  in  h ;  now,  as  the  curvature 
of  the  ellipse  is  the  same  at  A  as  at  P,  m  h  is  equal  to 
K  N ;  and  if  the  moon  were  to  fall  freely  frcm  the 
places  P  and  A  towards  the  earth,  her  gravity  would 
have  a  greater  effect  at  P  than  at  A,  in  equal  times,  in 
proportion  as  m  h  is  greater  than  M  H.  But  m  h  is  the 
space  which  the  moon  would  describe  freely  by  her  gra- 
vity at  A,  in  the  time  which  m  h  would  be  described  by 
her  projectile  motion  at  A,  and  M  H  is  the  space  through 
which  she  would  descend  freely  by  her  gravity  at  A,  in 
the  time  in  which  A  H  would  be  described  by  her  pro- 
jectile motion  ;  and  these  spaces  being  as  the  squares 
of  the  times,  it  follows,  that  m  h  is  to  M  H,  as  the  square 
of  A  h  to  the  square  of  A  H,  or,  because  of  the  equa- 
lity of  the  areas  T  A  H,  T  P  K  ,as  the  square  of  T  P  to 
the  square  of  T  A. 

Therefore,  the  gravity  at  P  is  to  the  gravity  at  A,  as 
the  square  of  T  A  to  the  square  of  T  P ;  that  is,  the 
gravity  of  the  moon  towards  the  earth  increases  in  the 
same  proportion,  as  the  square  of  the  distance  from  the 
centre  of  the  earth  decreases. 

Sir  Isaac  Newton  shows  the  universality  of  this  law, 
in  all  her  distances,  from  the  direction  of  the  power  that 
acts  upon  her,  and  from  the  nature  of  the  ellipsis,  the 
line  which  she  describes  in  her  revolution ;  and  it  follows 
from  the  properties  of  this  curve,  that  if  you  take  small 
arcs  described  by  the  moon  in  equal  times,  the  space 
by  which  the  extremity  of  any  arc  descends  towards  the 
earth  below  its  tangent  at  the  other  extremity,  is  always 
greater  in  proportion  as  the  square  of  the  distance  from 
the  focus  is  less  ;  from  which  it  follows,  that  the  power 
which  is  proportional  to  this  space  observes  the  same 
proportion. 

The  moon's  orbit,  according  to  astronomers,  differs 
not  much  from  a  circle  of  a  radius  equal  to  60  times 
the  semidiameter  of  the  earth  ;  and  the  circumference 
of  her  orbit  is  therefore  about  60  times  the  circumfer- 
ence of  a  great  circle  of  the  earth. 

From  this  the  circumference  of  the  moon's  orbit  is 
easily  computed,  and  as  she  finishes  her  revolution  in 


TOWARDS    THE    EARTH.  237 

27  days,  7  hours,  and  43  minutes,  it  is  also  easy  to  cal- 
culate what  arcs  she  describes  in  one  minute. 

The  next  thing  is  to  compute  how  much  this  arc  of  one 
minute  is  deflected  below  a  tangent  drawn  at  the  other 
end  :  now  geometricians  prove,  that  this  space  is  nearly 
a  third  proportional  to  the  diameter  of  her  orbit,  and 
the  arc'she  describes  in  a  minute  ;  whence,  by  an  easy 
calculation,  this  space  is  found  to  be  about  16  feet  1 
inch. 

But  you  have  seen,  that  this  space  was  described  in 
consequence  of  her  gravity,  or  tendency  towards  the 
earth,  which  is  therefore  a  power,  that  at  the  distance 
of  60  semidiameters  of  the  earth,  is  able  to  make  her 
descend  in  one  minute  through  1 6  feet  1  inch. 

Now,  as  this  power  increases  as  she  approaches  the 
earth,  let  us  see  what  its  force  would  be  at  the  sur- 
face thereof;  and  for  this  purpose,  let  us  suppose  her 
to  descend  so  low  in  her  orbit  as  at  her  least  distance 
to  pass  by  the  surface  of  the  earth  ;  she  would  then  be 
60  times  nearer  to  the  centre  of  the  earth,  and  move 
with  a  velocity  60  times  greater,  that  the  areas  describ- 
ed by  a  drawn  line  from  her  to  that  centre  in  equal  times, 
might  still  continue  equal. 

The  moon,  therefore,  passing  by  the  earth  at  her  low- 
est ebb,  would  describe  an  arc  in  one  second  of  time, 
the  60th  part  of  a  minute,  equal  to  that  she  describes  in 
one  minute  at  her  present  mean  distance,  and  would 
fall  as  much  below  the  tangent  at  the  beginning  of  the 
arc  in  a  second,  as  she  falls  from  the  tangent  at  her 
mean  distance  in  a  minute  ;  that  is,  she  would,  near  the 
surface  of  the  earth,  fall  ]6  feet  1  inch  in  1  second  of 
time. 

Now  this  is  exactly  the  same  space,  through  which 
all  heavy  bodies  are  found  by  experience  to  descend 
by  their  gravity  near  the  surface  of  the  earth.  The 
moon,  therefore,  would  descend  at  the  surface  of  the 
earth  with  the  same  velocity,  and  every  way  in  the  same 
manner,  as  heavy  bodies  fall  towards  the  earth ;  and 
the  power  which  acts  upon  the  moon,  agreeing  in  di- 
rection and  force  with  the  gravity  of  heavy  bodies,  and 


238  GRAVITATION    OF    THE    PRIMARY 


acting  incessantly  every  moment,  as  their  gravity  does, 
must  be  of  the  same  kind,  and  proceed  from  the  same 
cause. 

Thus  Sir  Isacc  Newton  showed,  that  the  power  of  gra- 
vity is  extended  to  the  moon ;  that  she  is  heavy,  as 
all  bodies  belonging  to  the  earth  are  found  to  be  ;  and 
that  she  is  retained  in  her  orbit  by  the  same  cause  which 
occasions  a  stone,  a  bullet,  or  any  other  projectile,  to 
describe  a  curve  in  the  air.  If  the  moon  or  any  part  of 
her  were  brought  down  to  the  earth,  and  projected  in 
the  same  line,  and  with  the  same  velocity  as  a  terrestrial 
body,  it  would  move  in  the  same  curve.  On  the  other 
hand,  if  any  body  were  carried  from  our  earth  to  the 
distance  of  the  moon,  and  projected  in  the  same  direc- 
tion, and  with  the  same  velocity  with  which  the  moon 
is  moved,  it  would  proceed  in  the  same  orbit  which  the 
moon  describes,  and  with  the  same  velocity.  Thus  the 
moon  is  a  projectile,  and  the  motion  of  every  projec- 
tile gives  an  image  of  the  motion  of  a  satellite  or  moon. 


That  the  primary  planets  are  heavy  bodies  ^  and  gravitate 
towards  the  sun  ;  and  that  the  secondary  planets  gravi- 
tate towards  their  respective  primaries. 

Observation  proves,  that  each  of  the  primary  planets 
bend  their  path  about  the  centre  of  the  sun,  are  acce- 
lerated as  they  approach  to  him,  and  are  retarded  as 
they  recede  from  him,  always  describing  equal  areas  in 
equal  times  ;  from  whence  it  follows,  that  the-  power 
by  which  they  are  deflected  must  be  directed  to  the  sun. 
This  power  also  varies  always  in  the  same  manner  as  the 
gravity  of  the  moon  towards  the  earth. 

The  same  reasoning,  by  which  the  gravity  of  the 
moon  towards  the  earth  at  her  greatest  and  least  dis- 
tances were  compared  together,  may  be  applied  in  com- 
paring the  powers  which  act  on  any  primary  planet  at 
its  greatest  and  least  distances  from  the  sun  ;  and  it  will 
appear,  that  these  powers  increase  as  the  squares  of  the 
distances  from  the  sun  decrease. 


AND    SECONDARY    PLANETS.  239 

But  the  universality  of  this  law,  and  this  uniformity 
of  nature,  still  farther  appear,  by  comparing  the  mo- 
tions of  the  different  planets. 

The  power  which  acts  on  a  planet  that  is  nearer 
to  the  sun,  is  manifestly  greater  than  that  which  acts  on 
a  planet  more  remote,  both  because  it  moves  with  more 
velocitv,  and  because  it  moves  in  a  less  orbit,  which  has 
more  curvature,  and  of  course  the  body  requires  more 
force  to  be  deflected  from  its  rectilinear  course.  By- 
comparing  the  motions  of  the  planets,  it  is  found,  that 
the  velocity  of  a  nearer  planet  is  greater  than  that  of  one 
more  remote,  in  proportion  as  the  square-root  of  the 
number  expressing  the  greatest  distance,  to  the  square- 
root  of  the  number  expressing  the  lesser  distance ;  so 
that  if  one  planet  be  four  times  farther  from  the  sun 
than  another,  the  velocity  of  the  former  would  be  half 
the  velocity  of  the  latter,  and  the  nearer  planet  would 
describe  an  arc  in  one  minute  equal  to  the  arc  described 
by  the  former  planet  in  two  minutes ;  and  the  nearer 
planet  would  describe,  by  its  gravity,  four  times  as  much 
space  as  the  other  would  describe  in  the  same  time  ;  by 
the  laws  of  falling  bodies,  the  gravity  of  the  nearer 
planet  would  therefore  appear  to  be  quadruple,  from 
the  consideration  of  its  greater  velocity  only.  But  fur- 
ther, as  the  radius  of  the  lesser  orbit  is  supposed  to  be 
four  times  less  than  the  radius  of  the  other,  the  lesser 
orbit  must  be  four  times  more  curved,  and  the  extre- 
mity of  a  small  arc  of  the  same  length  will  be  four  times 
farther  below  the  tangent  drawn  at  the  other  extremity 
in  the  lesser  orbit  than  in  the  greater  ;  so  that  though 
the  velocities  were  equal,  the  gravity  of  the  nearer  pla- 
net would  on  this  account  only  be  found  to  be  quad- 
ruple. 

On  both  these  accounts  together,  the  greater  velocity 
of  the  nearer  planet,  and  the  greater  curvature  of  its  or- 
bit, the  deflecting  force,  or  its  gravity  towards  the  sun, 
must  be  supposed  sixteen  times  greater,  though  its  dis- 
tance from  the  sun  is  only  four  times  less  than  the  other ; 
that  is,  when  the  distances  are  as  1  to  4,  the  gravity  is 
reciprocally  as  the  squares  of  these  numbers,  or  as  2  6 
to  1 .     By  comparing  the  motions  of  all  the  planets  it 


240  GRAVITATION    OF    THE    PRIMARY 


is  found,  that  their  gravities  decrease  as  the  squares  of 
their  distances  from  the  sun  increase. 

The  same  principle  that  governs  the  motion  of  the  pla- 
nets in  the  great  solar  system,  governs  also  the  motion  of 
the  satellites  in  the  lesser  systems,  of  which  the  greater  is 
composed. 

There  is  the  same  harmony  in  their  motions  compared 
with  their  distances,  as  in  the  great  system.  Jupiter's  sa- 
tellites are  continually  bent  from  the  lines  that  are  the 
direction  of  their  motions,  each  describing  equal  areas  in 
equal  times,  by  a  ray  drawn  to  the  centre,  to  which  their 
gravity  is  therefore  directed. 

The  nearer  satellites  move  with  greater  celerity,  in  the 
same  proportion  as  the  nearer  primary  planets  move  more 
swiftly  round  the  sun  ;  and  their  gravity  therefore  varies 
according  to  the  same  law.  The  same  is  to  be  said  of 
Saturn's  satellites. 

There  is,  therefore,  a  power  that  preserves  the  sub- 
stance  of  these  planets  in  their  various  motions,  acts  at 
their  surfaces,  and  is  extended  around  them,  decreasing 
in  the  same  manner  as  that  which  is  extended  from  the 
earth  and  sun  to  all  distances. 

They  accompany  their  primary  planets  in  their  motion 
round  the  sun,  and  move  about  them  at  the  same  time, 
with  the  same  regularity  as  if  their  primaries  were  at  rest. 
It  is  as  in  a  ship,  or  in  any  space  carried  uniformly  for- 
ward, in  which  the  natural  actions  of  bodies  are  the  same 
as  if  the  space  were  at  rest,  being  no  way  affected  by  that 
motion  which  is  common  to  all  the  bodies. 

As  every  projectile,  while  it  moves  in  the  air,  gravi- 
tates towards  the  sun,  and  is  carried  along  with  the  earth 
about  the  sun,  while  its  own  motion  in  its  curve  is  as  re- 
gular as  if  the  earth  were  at  rest ;  so  the  moon,  which  is 
only  a  greater  projectile,  must  gravitate  towards  the  sun, 
and  while  it  is  carried  along  with  the  earth  about  the 
sun,  is  not  hindered  by  that  motion  from  performing  its 
monthly  revolutions  towards  the  earth.  It  is  the  same 
with  respect  to  the  other  secondary  planets. 

Thus  the  motions  in  the  great  solar  system,  and  in  the 
lesser  particular  systems  of  each  planet,  are  consistent 
with  each  other,  and  are  carried  on  in  a  regular  harmony. 


AND    SECONDARY    PLANETS,    &C.  241 

without  any  confusion  or  mutually  interfering  with  one 
another,  except  what  necessarily  arises  from  small  ine- 
qualities in  the  gravities  of  the  primary  and  secondary 
planets,  and  the  want  of  exact  parallelism  in  the  direction 
of  these  gravities. 

Observation  shows,  that  the  deflexion  of  the  moon  to 
the  earth,  and  of  the  planets  to  the  sun,  is  accompanied 
with  an  equal  and  opposite  deflexion  of  the  earth  to  the 
moon,  and  of  the  sun  to  the  planets ;  from  which  it  is 
inferred,  that  the  forces  which  produce  these  deflexions 
are  mutual,  equal,  and  opposite. 

As  the  planets  are  deflected  towards  each  other,  and 
as  these  deflexions  are  inversely  proportional  to  the  square 
of  the  distance  from  the  planet  towards  which  they  are 
inflected ;  it  follows,  that  all  the  bodies  of  the  solar  sys- 
tem turn  towards  each  other  .with  forces  which  are  in- 
versely proportional  to  the  squares  of  the  distances. 

The  curve  which  a  body  describes  determines  the  law 
of  its  gravitation,  or  the  relation  which  subsists  between 
the  intensity  of  the  gravitating  force,  and  the  distance 
from  the  point  to  which  it  gravitates. 

If  the  gravitation  of  every  particle  of  gravitating  matter 
be  supposed  to  be  the  same  in  the  same  circumstances, 
then  the  relation  which  is  observed  between  the  distances 
and  the  periodic  times,  will  determine  the  proportion  of 
gravitating  matter  in  a  planet ;  and  on  this  supposition  it 
has  been  concluded  from  the  phenomena,  that  this  pro- 
portion is  the  same  in  all.  But  as  the  above  supposition 
is  not  formed  from  any  direct  arguments,  all  that  can  be 
justly  inferred  from  this  observed  relation  is,  that  the  gra- 
vitation of  each  planet,  taken  in  cumulo,  is  proportional  to 
its  quantity  of  gravitating  matter. 


OF    THE    CENTRE    OF    THE    SOLAR    SYSTEM. 

Sir  Isaac  Newton  having  found,  that  the  celestial  bo* 
dies  all  mutually  gravitate  towards  each  other,  it  follows 
that  no  one  body  in  the  whole  system  can  be  supposed  to 
be  entirely  void  of  motion. 
VOL.  IV.  2  I 


242  CENTRE    OF    THE    SOLAR    SYSTEM. 


mly 


The  centre  of  gravity  of  the  whole  system  is  the  on 
point  therein,  which  can  be  supposed  quiescent ;  it  is  the 
only  immoveable  point,  round  which  all  the  bodies  in  the 
system  move  with  various  motions. 

On  an  accurate  examination  of  the  tendencies  of  the 
planets,  it  is  found,  that  the  centre  round  which  each 
planet  revolves,  is  not  the  centre  of  the  sun,  but  the  point 
which  is  the  common  centre  of  gravity  of  the  sun  and  pla- 
net, whose  revolution  is  considered.  Thus,  the  mass  of 
the  sun  being  to  that  of  Jupiter  as  1  to  T^6T,  and  the 
distance  of  Jupiter  from  the  sun  being  to  the  sun's  semi- 
diameter  in  a  ratio  somewhat  greater,  it  follows,  that  the 
common  centre  of  gravity  of  Jupiter  and  the  sun  is  not 
far  distant  from  the  surface  of  the  sun. 

By  the  same  method  of  reasoning  it  is  found,  that  the 
common  centre  of  gravity  of  Saturn  and  the  sun  falls 
within  the  surface  of  the  sun ;  and  also,  that  if  all  the 
planets  were  placed  on  the  same  side  of  the  sun,  the  com- 
mon centre  of  gravity  of  the  sun  and  all  the  planets,  would 
scarce  be  one  of  his  diameters  distant  from  his  centre. 

It  is  about  this  centre  of  gravity  that  the  planets  revolve; 
and  the  sun  himself  oscillates  round  this  centre  in  pro- 
portion to  the  actions  of  the  planets  exerted  on  him. 

When,  therefore,  the  motion  of  two  bodies,  whereof 
one  revolves  round  the  other,  is  considered  rigorously, 
the  central  body  should  not  be  regarded  as  fixed,  as  they 
both  revolve  round  their  common  centre  of  gravity  ;  but 
the  spaces  they  describe  round  this  common  centre  being 
in  the  inverse  ratio  of  their  masses,  the  curve  described 
by  the  body  which  is  the  greatest  mass,  is  almost  insensi- 
ble ;  for  which  reason,  the  curve  described  by  the  body, 
whose  revolution  is  sensible,  is  only  to  be  considered,  and 
the  small  motion  of  the  central  body,  which  is  regarded 
as  fixed,  is  neglected. 

The  earth  and  the  moon,  therefore,  revolve  round  their 
common  centre  of  gravity,  and  this  centre  of  gravity  re- 
volves round  the  centre  of  gravity  of  the  earth  and  sun. 
The  case  is  the  same  with  Jupiter  and  his  moons,  Saturn 
and  his  satellites,  and  with  the  sun  and  all  the  planeis. 
And  the  sun,  according  to  the  different  position  of  the 


IRREGULARITIES  PRODUCED  BY  GRAVITY.       243 

planets,*  moves  successively  on  every  side  around  the  com- 
mon centre  of  gravity  of  our  planetary  system. 

This  centre  is  tlje  point  where  all  the  bodies  of  our  pla- 
netary system  would  meet,  if  their  projectile  forces  were 
destroyed,  though  the  sun  is  in  perpetual  agitation ;  be- 
ing, as  I  have  shown  you,  so  near  it,  he  may  with  pro- 
priety be  considered  by  astronomers  as  the  centre  of  the 
solar  system. 

Gravity  produces  some  small  irregularities  in  the  motion 
of  the  planets. 

The  regularity  of  the  planetary  motions  is  disturbed 
by  their  mutual  gravitation,  each  disturbing  the  motions 
of  the  others,  with  a  force  proportional  to  its  quantity  of 
matter  directly,  and  to  the  square  of  its  distance  from 
them  inversely.  In  order  to  calculate  these  disturbances, 
it  was  necessary  for  mathematicians  previously  to  ascer- 
tain the  quantity  of  matter  in  the  sun  and  planets. 

When  a  fleet  of  ships  is  carried  away  by  a  current  that 
affects  them  equally,  it  has  no  effect  on  their  particular 
motion  amongst  themselves,  nor  is  the  motion  from  the 
current  discovered  by  them,  unless  they  have  some  body 
in  sight,  that  is  not  affected  thereby  in  the  same  manner. 
The  regularity  in  the  motions  of  a  planet,  A,  round  the 
sun,  would  not  be  disturbed  by  the  gravitation  to  a  planet 
B,  if  the  sun,  and  the  planet  A,  did  gravitate  to  B  with 
equal  forces,  and  in  parallel  directions ;  and  the  disturb- 
ance of  the  motion  A,  arises  from  the  inequality  and  obli- 
quity of  the  gravitations  of  the  sun,  and  of  A  and  of  B. 

In  consequence  of  this  disturbing  force,  the  motion  of 
the  earth  in  its  orbit  is  retarded  from  the  time  that  Jupiter 
is  in  opposition,  till  the  time  that  he  is  in  quadrature  with 
the  sun.  It  is  then  accelerated  till  he  be  in  conjunction, 
then  retarded  till  he  be  in  quadrature,  and  then  accelerated 
till  he  be  again  in  opposition. 

The  earth's  gravitation  to  the  sun  is  increased  while 
Jupiter  is  in  or  near  the  quadratures,  and  diminished 
while  he  is  in  or  near  the  conjunction  and  opposition. 

The  augmentation  of  the  earth's  gravitation  to  the  sun 
is  greatest  when  Jupiter  is  in  quadrature,  being  then  about 


244       IRREGULARITIES  PRODUCED  BY  GRAVITY. 


. 


•siwff  of  the  whole  gravitation  to  the  sun.  The  diminu- 
tion of  the  earth's  gravitation  to  the  sun  is  greatest  when 
Jupiter  is  in  opposition,  being  then  about  bW  of  the 
whole  gravitation. 

The  diminution  of  the  earth's  gravitation  to  the  sun 
when  Jupiter  is  in  conjunction,  is  about  7—000  of  the 
whole  gravitation. 

In  consequence  of  this  change  in  the  earth's  gravita* 
tion  to  the  sun,  the  line  of  the  apsides  of  the  earth's  orbit 
changes  its  place  in  the  heavens,  sometimes  advancing, 
and  sometimes  retreating  ;  but,  on  the  whole,  advancing, 
because  the  earth's  gravitation  to  the  sun  is  more  dimi- 
nished than  it  is  augmented. 

In  like  manner,  the  aphelion  of  any  inferior  planet  ad- 
vances in  consequence  of  the  gravitation  to  the  superior 
planets  ;  but  the  aphelion  of  a  superior  planet  retreats  in 
consequence  of  the  gravitation  to  the  inferior  planets. 
For  these  reasons,  and  because  Jupiter  and  Saturn  are 
much  larger  than  the  inferior  planets,  the  aphelia  of  all 
the  planets,  excepting  Saturn,  advance,  while  the  aphe- 
lion of  Saturn  retreats. 

The  accelerations  and  retardations  of  the  planets  Mer- 
cury, Venus,  the  Earth,  and  Mars,  arising  from  their 
mutual  gravitations,  and  their  gravitations  to  Jupiter, 
nearly  compensate  each  other ;  and  no  effects  of  them 
are  perceived  in  any  long  tract  of  years.  But  the  posi- 
tion of  the  aphelia  of  Jupiter  and  Saturn  is  such,  that  the 
retardations  of  Saturn  sensibly  exceed  the  accelerations  j 
so  that  the  anomalistic  period  of  Saturn  is  increasing,  at 
present,  about  a  day  in  a  century.  On  the  contrary,  the 
period  of  Jupiter  is  diminishing. 

The  disturbances  occasioned  by  the  mutual  gravita- 
tions of  the  planets  and  comets  are  considerable.  The 
comet  of  1777  has  suffered  a  remarkable  change  in  its 
motions  by  the  action  of  Jupiter. 

The  earth's  motion  round  the  sun  is  remarkably  affect- 
ed by  the  moon. 

In  consequence  of  the  mutual  gravitations  of  the  pla- 
nets, the  nodes  of  a  disturbed  planet  retreat  on  the  orbit 
of  the  disturbing  planet.  Hence  the  nodes  of  all  the  pla- 
nets retreat  on  the  ecliptic,  except  that  of  Jupiter,  which 


APPROACH  AND  RECESS  OF  TH      PLANETS.      245 

advances  by  retreating  on  the  orbit  of  Saturn,  from  which 
it  suffers  the  greatest  disturbance. 


OF  THE  APPROACH  AND   RECESS   OF   THE  PLANETS  TO 
AND  FROM  THE  SUN  IN  EVERY   REVOLUTION. 

Having  shown  you,  that  the  forces  which  produce  the 
regular  motions  of  the  planets  vastly  exceed  those  that 
disturb  them,  I  shall  explain  more  fully,  how  the  motions 
in  their  orbits  proceed  from  the  actions  of  those  powers; 
and  how  the  planet  is  made  to  ascend  and  descend  by 
turns,  while  it  revolves  about  the  centre  of  its  gravita- 
tion. 

We  have  nothing  similar  to  this  in  the  motion  of  heavy 
bodies  at  the  earth's  surface ;  but  you  must  remember, 
that  the  force,  with  which  heavy  bodies  are  projected 
from  our  most  powerful  engines,  is  inconsiderable,  com- 
pared  with  the  motions  which  their  gravity  could  gene- 
rate in  them  in  a  few  minutes ;  and  they  move  over  such 
small  spaces  when  compared  with  their  distances  from 
the  centre  of  the  earth,  that  their  gravity  is  considered  as 
acting  in  parallel  lines,  without  any  sensible  error  ;  so 
that  the  centrifugal  force  arising  from  the  rotation  about 
that  centre,  is  altogether  neglected. 

But  when  the  motion  of  a  projectile  in  the  larger  spa- 
ces is  examined,  and  traced  in  its  orbit,  it  is  necessary  to 
take  in  the  centrifugal  force,  arising  from  its  motion  of 
rotation  about  that  centre  ;  and  then  it  will  appear,  that 
there  are  indeed  some  laws  of  gravity,  which  would  make 
the  body  approach  to  the  centre  continually,  till  it  fall 
into  it,  but  that  there  are  other  laws  which  make  bodies 
to  approach,  and  Suffer  them  to  recede  from  it  by  turns. 
If  a  planet  at  B,  plate  15,  Jig.  6,  gravitate,  or  is  at- 
tracted towards  the  sun,  so  as  to  fall  from  B  to  y  in  the 
time  that  the  projectile,  force  would  have  carried  it  from 
B  to  X,  it  will  describe  the  curve,  B  Y,  by  the  combined 
action  of  these  two  forces,  in  the  same  time  that  the  pro- 
ectile  force  singly  would  have  carried  it  from  B  to  X,  or 
:he  gravitating  power  singly  have  caused  it  to  descend 
trom  B  to  y ;  and  these  two  forces  being  duly  propor- 


246 


APPROACH  AND   RECESS  OF  THE   PLANETS, 


tioned,  and  perpendicular  to  each  other,  the  planet  obey- 
ing both,  will  move  in  the  circle  B  Y  T. 

But,  if  whilst  the  projectile  force  would  carry  the  pla- 
net from  B  to  b,  the  sun's  attraction,  which  constitutes 
the  planets  gravitation,  should  bring  it  down  from  B  to 
1 ,  the  gravitating  power  would  then  be  too  strong  for  the 
projectile  force,  and  would  cause  the  planet  to  describe 
the  curve  B  C.  When  the  planet  comes  to  C,  the  gra- 
vitating power,  which  always  increases  as  the  square  of 
the  distance  from  the  sun  diminishes,  will  yet  be  strong. 
er  for  the  projectile  force ;  and  by  conspiring  in  some 
degree  therewith,  will  accelerate  the  planet's  motion  all 
the  way  from  C  to  K  ;  causing  it  to  describe  the  arcs  BC, 
C  D,  D  E,  E  F,  &c.  all  in  equal  times.  Raving  its  mo- 
tion thus  accelerated,  it  thereby  gains  so  much  centrifu- 
gal force,  or  tendency  to  fly  off,  at  K  in  the  line  K  k,  as 
overcomes  the  sun's  attraction  ;  and  the  centrifugal  force 
being  too  great  to  allow  the  planet  to  be  brought  nearer 
the  sun,  or  even  to  move  round  him  in  the  circle  K 1  m  n, 
&c.  it  goes  off,  and  ascends  in  the  curve  K  L  M  N,  &c. 
its  motion  decreasing  as  gradually  from  K  to  B,  as  it 
increased  from  B  to  K ;  because  the  sun's  attraction  now 
acts  against  the  planet's  projectile  motion,  just  as  much 
as  it  acted  with  it  before.  When  the  planet  has  got 
round  to  B,  its  projectile  force  is  as  much  diminished 
from  its  mean  state  about  G  or  N,  as  it  was  augmented 
at  K  ;  and  so,  the  sun's  attraction  being  more  than  suf- 
ficient to  keep  the  planet  from  going  off  at  B,  it  describes 
the  same  orbit  over  again,  by  virtue  of  the  same  forces  or 
powers. 

A  double  projectile  force  will  always  balance  a  quad- 
ruple power  of  gravity.  Let  the  planet  at  B  have  twice 
as  great  an  impulse  from  thence  towards  X,  as  it  had 
before  ;  that  is,  in  the  same  length  of  time  it  was  pro- 
jected from  B  to  b,  as  in  the  last  example,  let  it  now  be 
projected  from  B  to  c  ;  and  it  will  require  four  times  as 
much  gravity  to  retain  it  in  its  orbit ;  that  is,  it  must 
fall  as  far  as  from  B  to  c  :  otherwise  it  could  not  de- 
scribe B  D,  as  is  evident  by  the  figure.  But,  in  as  much 
time  as  the  planet  moves  from  B  to  C  in  the  higher 
parts  of  its  orbit,  it  moves  from  I  to  K,  or  from  K  to  L, 


TO    AND    FROM    THE    SUN.  247 

in  the  lower  part  thereof ;  because,  from  the  joint  action 
of  these  two  forces,  it  must  always  describe  equal  areas 
in  equal  times,  throughout  its  annual  course.  These 
areas  are  represented  by  the  triangles  B  S  C,  C  S  D, 
D  S  E,  E  S  F,  &c.  whose  contents  are  equal  to  one  an- 
other, quite  round  the  figure. 

As  the  planets  approach  nearer  the  sun,  and  recede 
farther  from  him,  in  every  revolution,  there  may  be  some 
difficulty  in  conceiving  the  reason  why  the  power  of  gra- 
vity, when  it  once  gets  the  better  of  the  projectile  force, 
does  not  bring  the  planets  nearer  and  nearer  the  sun  in 
every  revolution,  till  they  fall  \ipon  and  unite  with  him  ; 
or  why  the  projectile  force,  when  it  once  gets  the  bet- 
ter of  gravity,  does  not  carry  the  planets  farther  and 
farther  from  the  sun,  till  it  remove  them  quite  out  of 
the  sphere  of  his  attraction,  and  cause  them  to  go  on  in 
straight  lines  for  ever  afterwards  :  but,  by  considering 
the  effects  of  these  powers,  this  difficulty  will  be  remov- 
ed. Suppose  a  planet  at  B  to  be  carried  by  the  projec- 
tile force  as  far  as  from  B  to  b,  in  the  time  that  gravity 
would  have  brought  it  down  from  B  to  1  ;  by  these  two 
forces  it  will  describe  the  curve  C  ;  when  the  planet 
comes  down  to  K,  it  will  be  but  half  as  far  from  the  sun, 
S,  as  it  was  at  B ;  and  -therefore,  by  gravitating  four 
times  as  strongly  towards  him,  it  would  fall  from  K  to 
Y  in  the  same  length  of  time  that  it  would  have  fallen 
from  B  to  1  in  the  higher  part  of  its  orbit,  that  is,  through 
four  times  as  much  space  ;  but  its  projectile  force  is  then 
so  much  increased  at  K,  as  would  carry  it  from  K  to  k 
in  the  same  time  ;  being  double  of  what  it  was  at  B,  and 
is,  therefore,  too  strong  for  the  gravitating  power  either 
to  draw  the  planet  to  the  sun,  or  cause  it  to  go  round 
him  in  the  circle  K  1  m  n,  &c.  which  would  require  its 
falling  from  K  to  W,  through  a  greater  space  than  gravity 
can  draw  it,  whilst  the  projectile  force  is  such  as  would 
carry  it  from  K  to  k  ;  and  therefore  the  planet  ascends  in 
its  orbit  KLMN,  decreasing  in  its  velocity  for  the  causes 
already  assigned. 


[     248     ] 


THE    MOON'S  IRREGULARITIES. 

There  is  nothing  that  shows  betterthe  excellency  of 
the  Newtonian  philosophy,  or  more  clearly  demonstrates 
the  truth  of  its  principles,  than  its  so  easily  and  clearly 
accounting  for  those  many  irregularities  of  motion,  to 
which  all  the  secondary  planets,  and  the  moon  in  par- 
ticular, are  subject. 

Though  these  are  called  irregularities,  yet  they  are 
not  to  be  apprehended  as  random  or  fortuitous  ones,  bat 
such  as  are  regular  under  the  like  circumstances,  and 
subject  to  numbers  and  calculation. 

For  it  was  by  observing  the  period  of  those  lunar  ine- 
qualities, that  Dr.  Halley  was  enabled  to  foretel  an  eclipse 
of  the  sun,  with  an  exactness  little  inferior  to  the  obser- 
vation itself. 

It  hath  been  seen  before,  that  gravitation  is  a  prim 
ciple  belonging  to  all  gravitating  matter  ;  and  that  bodies, 
describing  orbits  about  another  placed  in  the  centre  of 
their  motion,  by  a  centripetal  and  projectile  force,  de- 
scribe equal  areas  in  equal  times. 

As  this  is  the  law  by  which  the  primary  planets  regu- 
late their  motions  about  the  sun,  so  likewise,  were  there 
no  sun,  by  the  same  law  would  the  moon  regulate  her 
motion  about  the  earth. 

This  tendency  of  the  moon  towards  the  sun,  then,  is 
the  cause  of  those  inequalties  in  her  motion,  which  are 
called  her  irregularities. 

These  are  commonly  reckoned  eight,  arising  from 
causes  now  to  be  mentioned. 

J .  That  variation,  whereby,  if  we  suppose  E  the  earth, 
plate  15,  fig.  7,  and  the  circle  A  B  C  D,  the  orbit  of  the 
moon,  while  the  moon  describes  the  quadrant  A  B,  that 
is,  while  she  goes  from  the  quadrature  to  the  conjunction, 
the  force  tending  towards  the  sun  at  S,  conspires  with 
the  force  tending  towards  the  earth  at  E,  and  therefore 
accelerates  her  motion.  But  while  she  goes  from  the 
conjunction  B,  to  the  next  quadrature  C,  the  force  tend- 
ing towards  the  sun  will  act  contrary  to  the  force  tend- 
ing towards  the  earth,  and  therefore  will  retard  her  mo- 
tion. 


the  moon's  irregularities.  249 

In  the  same  manner,  while  she  goes  from  the  quad- 
rature C,  to  the  next  syzygy  D,  the  same  force  tend- 
ing towards  S,  will  accelerate  her  again  ;  but  while  she 
goes  from  thence  to  the  quadrature  at  A,  it  will  again 
retard  her. 

The  moon,  therefore,  in  her  monthly  revolution  about 
the  earth,  is,  by  this  action  of  the  sun,  alternately  acce- 
lerated and  retarded. 

2.  This  force  tending  towards  the  sun  being  the  dis- 
turbing force,  or  that  force  which  prevents  the  moon 
from  describing  about  the  earth  equal  areas  in  equal 
times,  will  be  greatest  at  the  octants. 

For,  this  force  being  resolved  into  two  others,  after  Sir 
Isaac  Newton's  manner,  one  of  them  at  the  quadratures 
or  syzygies  will  be  found  to  point  from  or  towards  E, 
the  centre  of  the  earth,  directly,  and  therefore  will  not 
hinder  the  moon  from  describing  equal  areas  in  equal 
times ;  the  other,  likewise,  in  those  places  will  be  found 
to  tend  towards  the  centre  of  the  sun,  and,  therefore,  nei- 
ther of  them  will  prevent  the  moon  there  from  describ- 
ing equal  areas  in  equal  times,  /'.  e.  will  not  at  the  quad- 
ratures disturb  the  moon's  motion  at  all. 

But,  when  the  moon  is  in  the  octants,  as  at  L,  plate 
15,  Jig.  8,  this  force  being  resolved  into  two  others,  one 
of  them,  as  L  H,  will  point  directly  to  or  from  the 
centre  of  the  earth,  and  therefore  will  increase  or  dimi- 
nish the  moon's  tendancy  towards  the  earth,  but  not  hin- 
der her  from  describing  equal  areas  in  equal  times.  But 
the  other,  as  L  I,  or  H  G,  points  neither  towards  the 
centre  of  the  earth  nor  sun,  and  therefore,  in  the  oc- 
tants, prevents  her  describing  equal  areas  in  equal  times. 

But  this  being  the  mid-way  between  the  quadrature 
and  the  syzygy,  in  both  which  places  this  disturbing  force 
doth  not  prevent  the  moon  from  describing  equal  areas 
in  equal  times,  it  follows,  that  at  the  octants,  this  dis- 
turbing force  will  be  the  greatest  of  all. 

And  for  this  reason  it  hath  always  been  found  more 
difficult  to  obtain  the  moon's  place  in  the  octants 
agreeing  with  observation,  than  at  the  syzygies  or  quad- 
ratures. 

VOL.  IV.  2  K 


250  the  moon's  irregularities. 

3.  The  moon's  orbit  is  more  curved  in  the  quadra- 
tures than  in  the  syzygies. 

For,  her  motion  being  accelerated  during  her  progress 
from  the  quadratures  to  the  syzygies,  in  the  syzygies  her 
motion  will  be  quicker  than  it  ought  otherwise  to  be, 
and  therefore  her  centripetal  force  less  than  it  would 
otherwise  be.  She  will,  therefore,  at  the  syzygies  de- 
scribe the  portion  of  a  larger  curve,  which,  consequently, 
will  be  less  curved  than  a  smaller.  That  is,  instead  of 
describing  the  curve  A  B,  plaie  \5,fig+  9,  or  CD,  she 
will  describe  the  curve  E  F,  or  G  K. 

On  the  other  hand,  while  the  moon  goes  from  the 
syzygies  to  the  quadratures,  her  motion  is  continually 
retarded,  and  therefore,  at  the  quadratures,  her  motion 
will  be  slower  than  it  would  otherwise  be.  At  the  quad- 
ratures, therefore,  the  moon  will  describe  the  portion  of 
a  lesser  curve,  which  consequently  will  be  more  curved 
than  a  larger  one.  That  is,  instead  of  describing  the 
curve  A  B,  or  C  D,  she  will  describe  the  curve  E  F,  or 
GK,  plate  15, fig    10. 

Therefore,  at  the  quadratures,  the  moon's  orbit  is  more 
curved  than  at  the  syzygies. 

4.  Since  these  irregularities  in  the  moon's  motion  pro- 
ceed, as  was  said,  from  the  action  of  the  sun,  it  will 
follow,  that  where  the  action  of  the  sun  is  greatest, 
the  irregularities  arising  from  k  will  be  greatest  also. 
But  the  nearer  the  earth  is  to  the  sun,  the  greater  will  be 
the  action  of  the  sun  upon  the  moon  ;  and  the  more  she 
tends  towards  the  sun,  the  less  will  she  tend  towards  the 
earth. 

When,  therefore,  the  earth  is  at  the  perihelion  P,  plate 
15,  fig.  11,  and  consequently  at  its  least  distance  from 
the  sun,  the  action  of  the  sun  upon  the  moon  will  be 
greatest,  and  destroy  more  of  its  tendency  towards  the 
earth  than  at  any  other  distance,  as  S  D,  S  C,  S  B,  &c. 

Therefore,  when  the  earth  is  at  the  perihelion  P,  the 
moon  will  describe  a  greater  orbit  about  the  earth,  than 
when  the  earth  is  at  any  other  distance  from  the  sun, 
and  consequently  her  periodical  time  will  then  be  th? 
longest. 


% 


the  moon's  irregularities.  251 

But  the  earth  is  at  its  perihelion  in  the  winter,  and 
■consequently,  the  moon  will  then  describe  the  outermost 
circle  about  the  earth,  and  her  periodical  rime  will  be  the 
longest.     And  this  agrees  with  observation. 

For  the  same  reason,  when  the  earth  is  at  its  apheli- 
on A,  the  tendency  of  the  moon  towards  the  earth  will 
be  the  greatest,  and  consequently  her  periodical  time  the 
least.  And  in  this  case,  which  will  be  in  the  summer, 
she  will  describe  the  innermost  circle  about  the  earth. 

5.  Since  the  moon,  from  what  has  been  said,  appears 
to  describe  an  elliptical  orbit  about  the  earth  E,  plate  lo, 

Jig.  12,  in  the  focus  of  it;  and  since  her  centripetal 
force  towards  the  earth,  by  means  of  the  action  of  the 
sun,  is  continually  increasing  or  decreasing,  but  not 
equably,  that  is,  sometimes  less,  and  sometimes  more, 
than  in  the  inverse  duplicate  ratio  of  the  distance  of  the 
moon  from  the  earth ;  therefore,  in  this  case,  the  line  of 
the  moon's  apsides,  A  B,  will  be  continually  going  back- 
wards or  forwards.  That  is,  the  axis,  AB,  will  not  al- 
ways lie  in  that  situation,  but  go  backwards  into  the  si- 
tuation K  L,  or  forwards  into  the  situation  F  G. 

Since,  however,  taking  one  whole  revolution  of  the 
moon  about  the  earth,  the  action  of  the  sun  more  di- 
minishes the  tendency  of  the  moon  towards  the  earth, 
than  it  augments  it,  therefore  the  motion  of  the  apsides 
forwards  exceeds  their  motion  backwards.  Upon  the 
whole,  therefore,  the  apsides  of  the  moon's  orbit  go  for- 
wards, or  according  to  the  order  of  the  signs. 

6.  Because  the  moon  describes  an  eccentrical  orbit 
about  the  earth  at  E,  plate  15,  Jig.  13,  the  action  of 
the  sun  upon  her  sometimes  increases  her  tendency  to- 
wards the  earth,  and  sometime  diminishes  it,  i.  e.  makes 
her  gravity  towards  the  earth  increase  or  decrease  too  fast. 
If,  while  the  moon  ascends  from  her  lower  apside,  A, 
her  gravity  towards  the  earth  decrease  too  fast,  instead 
of  describing  her  semi-ellipsis  ABC,  and  coming  to 
the  higher  apside  at  C,  as  she  would  otherwise  do,  she 
will  run  out  in  the  curve  B  F  D,  and  come  to  the  higher 
apside  at  F.  But  the  curve,  A  B  F  D,  is  more  eccentric 
than  the  curve  A  B  C  D. 


252  the  moon's  irregularities. 


. 


Therefore,  when  the  gravity  of  the  moon  towards  the 
earth  decreases  too  fast,  the  eccentricity  of  her  orbit  will 
increase. 

On  the  other  hand,  when  the  moon  is  going  from  her 
higher  apside  C,  plate  15,  fig.  14,  to  her  lower,  A,  and 
her  gravity  towards  the  earth  increases  too  fast,  instead 
of  describing  the  same  ellipsis  C  D  A,  and  so  coming  to 
the  lower  apside  at  A,  she  will  approach  nearer  to  the 
earth,  and  describe  the  curve  D  F  B,  and  so  come  to  the 
lower  apside  at  F.  But  the  curve,  C  D  F  B,  will  be  less 
eccentric  than  the  curve  DABC. 

Therefore,  when  the  gravity  of  the  moon  towards  the 
earth  increases  too  fast,  the  eccentricity  of  her  orbit  will 
decrease,  and  the  orbit  itself  will  approach  nearer  to  a 
circle. 

Therefore,  the  eccentricity  of  the  moon's  orbit  will  be 
continually  varying. 

7.  In  considering  the  irregularities  of  the  moon's  mo- 
tion, wre  have  hitherto  supposed  the  plane  of  her  orbit  as 
coinciding  with  the  plane  of  the  ecliptic,  because  her  mo- 
tion would  be  affected  by  the  irregularities  hitherto  spo- 
ken of,  if  in  reality  it  did  so. 

But  it  hath  been  before  observed,  that  one  half  of  the 
moon's  orbit  A  C  B,  plate  15,  Jig.  15,  is  raised  above 
the  plane  of  the  ecliptic  A  E  B  G,  and  the  other  half, 
A  D  B,  depressed  below  it ;  and  the  points  A,  B,  where 
the  moon's  orbit  crosseth  the  plane  of  the  ecliptic,  are 
called  the  nodes,  and  the  line  A  B,  joining  these  points, 
is  called  the  line  of  the  nodes. 

But  when  this  line  of  the  nodes,  A  B,  lies  in  conjunc- 
tion with  the  sun,  S,  it  is  at  rest ;  but  in  all  other  posi- 
tions it  goes  backwards. 

When  this  line  of  the  nodes,  AB,  lies  in  the  quadra- 
ture, plate  15,  fg.  16,  with  the  sun  S,  it  goes  back- 
wards fastest  of  all. 

8.  It  has  been  formerly  observed,  that  the  orbit  of  the 
moon  is  inclined  to  the  plane  of  the  ecliptic  in  a  certain 
angle ;  but  this  angle  is  not  constantly  the  same,  but 
sometimes  greater,  and  sometimes  less,  according  to  the 
position  of  the  line  of  the  moon's  nodes  with  respect  to 
the  sun. 


CONCLUSION.  253 

■  When  the  line  of  the  nodes,  A  B,  plate- 1 5,  fig.  17% 
passes  through  the  syzygies,  the  plane  of  the  moon's  orbit 
produced  passes  through  the  centre  of  the  sun  S,  and, 
consequently,  not  being  affected  by  the  action  of  the  sun, 
is  then  at  its  greatest  state,  making  an  angle  with  the 
ecliptic  of  about  5  degrees  18  minutes. 

When  the  line  of  the  moon's  nodes,  A  B,  plate  1 5, 
fig.  18,  lies  in  a  quadrature  with  the  sun  S,  then,  suppos- 
ing the  line,  A  B  C  D,  to  represent  the  plane  of  the  eclip- 
tic, and  A  E  B  F  the  orbit  of  the  moon,  let  the  moon  be 
supposed  to  have  just  now  passed  the  ascending  node  at 
A,  and  going  to  her  conjunction  with  the  sun  at  E. 

The  moon  will  then  be  going  farther  and  farther  from 
the  plane  of  the  ecliptic  A  B  C  D,  and,  were  there  no 
action  of  the  sun,  would  come  in  conjunction  with  him 
atE. 

But,  because  of  the  action  of  the  sun,  the  moon,  in 
going  from  the  quadrature  at  A,  towards  her  conjunc- 
tion, will  be  perpetually  drawn  down  towards  the  eclip- 
tic, and  therefore  will  not  come  to  a  conjunction  of  the 
sun  at  E,  but  at  G,  making  an  angle  with  the  ecliptic, 
GAC,  less  than  E  AC. 

But  the  sun  continuing  still  to  act,  after  the  moon  has 
arrived  at  her  conjunction,  will  go  on  to  draw  her  down 
towards  the  ecliptic  ;  by  which  means  she  will  not  cross 
the  ecliptic  in  the  point  B,  her  former  node,  but  in  some 
other  point  nearer  to  the  sun,  as  K. 

But  wherever  the  moon  crosses  the  ecliptic,  is  her  node. 
Therefore,  her  node  in  the  mean  time  hath  gone  back- 
ward from  B  to  K,  plate  15,  fig.  19,  and  the  moon  hath 
described  a  semi-orbit  A  G  K,  making  a  less  angle  with 
the  ecliptic,  than  the  orbit  AE  B,  which  she  would  have 
described  had  there  been  no  action  of  the  sun  at  all. 


CONCLUSION. 

Aristotle  concludes  his  treatise,  De  Mundo,  with  ob- 
serving, that  "  to  treat  of  the  world  without  saying  any 
thing  of  its  author  would  be  impious,"  as  there  is  nothing 


254  CONCLUSION. 

we  meet  with  more  frequently  and  constantly  in  nature, 
than  the  traces  of  an  all-governing  Deity. 

The  philosopher  who  neglects  these  traces,  and  con- 
tents himself  with  the  appearances  only  of  the  material 
universe,  and  the  mechanical  laws  of  motion,  neglects 
what  is  most  excellent;  and  prefers  what  is  imperfect  to 
what  is  supremely  perfect,  finitude  to  infinity,  what  is  nar- 
row and  weak  to  what  is  unlimited  and  almighty,  and 
what  is  perishing  to  what  endures  for  ever.  Those  who 
do  not  attend  to  the  manifest  indications  of  supreme  wis- 
dom and  goodness  perpetually  appearing  before  them, 
wherever  they  turn  their  views  or  inquiries,  too  much  re- 
semble those  ancient  philosophers,  who  made  night,  mat- 
ter, and  chaos,  the  original  of  all  things. 

The  plain  argument  for  the  existence  of  the  Deity,  ob- 
vious to  all,  and  carrying  with  it  irresistible  conviction,  is 
derived  from  the  evident  contrivance  and  fitness  of  things 
for  one  another,  which  we  meet  with  throughout  all  parts 
of  the  universe.  There  is  no  need  of  nice  or  subtile  rea- 
sonings in  this  matter  ;  a  manifest  contrivance  immedi- 
ately suggests  a  contriver.  It  strikes  us  like  a  sensation  ; 
artrul  reasonings  against  it  may  puzzle  us,  but  can  never 
shake  our  belief. 

No  person,  for  instance,  that  knows  the  principles  of 
optics,  and  the  structure  of  the  eye,  can  believe  that  it  was 
formed  without  skill  in  that  science  ;  or  that  the  ear  was 
formed  without  the  knowledge  of  sounds ;  or  that  male 
and  female  in  animals  were  not  formed  for  each  other, 
and  for  continuing  the  species.  All  our  accounts  of  na- 
ture are  replete  with  instances  of  this  kind. 

The  admirable  and  beautiful  structure  of  things  for 
final  causes,  exalts  our  idea  of  the  contriver ;  the  unity 
of  the  design  shows  him  to  be  one  ;  revelation,  that  this 
one  is  Jesus  Christ.  The  great  motions  in  the  system 
performed  with  the  same  facility  as  the  least,  suggest  his 
almighty  power,  which  gave  motion  to  the  earth  and  the 
celestial  bodies,  with  equal  ease  as  to  the  minutest  parti- 
cles. The  subtilety  of  the  motions  and  actions  in  the  in- 
ternal parts  of  bodies,  shows  that  his  influence  penetrates 
the  inmost  recesses  of  things,  and  is  every  where  exert- 
ed.    The  simplicity  of  the  laws  that  prevail  in  the  uni- 


CONCLUSION.  255 

verse,  the  excellent  disposition  of  things,  in  order  to  ob- 
tain the  best  ends,  and  the  beauty  which  adorns  the  works 
of  nature,  far  superior  to  any  thing  in  art,  suggest  his  con- 
summate wisdom.  The  usefulness  of  the  whole  scheme, 
so  well  contrived  for  the  intelligent  beings  that  enjoy  it, 
with  the  internal  dispositions  and  moral  structure  in  those 
beings  themselves,  show  his  unbounded  goodness. 

These  are  arguments  which  are  sufficiently  open  to  the 
views  and  capacities  of  the  unlearned,  while  at  the  same 
time  they  acquire  new  strength  arid  lustre  from  the  disco- 
veries of  the  learned.  God  acting  and  interposing  in  the 
universe,  shows  that  he  governs  it,  as  well  as  that  he 
formed  it ;  and  the  depth  of  his  counsels,  even  in  con- 
ducting the  material  universe,  of  which  a  great  part  sur- 
passes our  knowledge,  keeps  up  an  inward  veneration 
and  awe  of  this  great  Being,  and  disposes  us  to  receive 
what  may  be  otherwise  revealed  to  us  concerning  him. 

It  has  been  justly  observed,  that  some  of  the  laws  of 
nature  now  known  to  us,  must  have  escaped  us  if  we 
had  wanted  the  sense  of  sight.  God  can  bestow  upon 
us  other  senses  of  which  we  have  at  present  no  idea,  with- 
out which  it  may  be  impossible  for  us  to  know  all  his 
works,  or  to  have  more  adequate  ideas  of  his  nature.  In 
our  present  state,  we  know  enough  to  be  convinced  of 
our  dependency  on  him,  and  of  the  duty  we  owe  to  him 
as  the  Lord  and  Disposer  of  all  things. 

Though  the  power  of  God  is  manifested  in  all  his 
works,  it  is  in  the  heavens  that  it  still  seems  to  beam  forth 
in  its  greatest  lustre.  By  his  power  acting  there,  he  di- 
rects the  courses  of  the  planets,  determines  the  circum- 
stances of  their  motions,  and  fixes  the  times  of  their  re- 
volutions. As  a  General  at  the  head  of  an  army,  he 
gives  the  signal  to  the  heavenly  bodies,  and  immediately 
they  shoot  forth,  and  proceed  in  their  proper  orbits.  It 
is  in  consequence  of  the  laws  laid  down  by  him,  that  the 
moon  goes  round  the  earth  in  a  month.  It  is  he  that 
has  combined  the  two  motions  of  the  earth,  one  by  which 
we  obtain  the  vicissitudes  of  day  and  night ;  the  other, 
by  which  the  seasons  of  the  year  are  brought  about.  He 
it  is,  who  at  the  appointed  times  sends  salutary  winds,  and 
fruitful  rains  and  dews ;  who  gathers  together  the  waters 


256  CONCLUSION. 


in  their  sources,  and  causes  them  to  flow  from  thence  lr 
the  beds  of  rivers  to  their  great  receptacle,  the  sea.  It  is 
he  who  makes  the  buds  to  open,  the  fruits  to  ripen,  and 
animals  to  be  prolific,  ordering  all  things  according  to 
their  different  nature,  regulating  their  birth,  their  growth, 
and  their  dissolution. 

Though  the  Author  of  so  many  wonders  is  invisible, 
you  cannot  on  that  account  deny  his  power,  or  doubt  his 
existence.  You  cannot  see  your  soul,  yet  the  effects  it 
produces  in  you  and  around  you,  are  sensible  proofs  of 
its  existence.  It  is  the  same  with  many  of  the  operations 
in  nature.  In  like  manner,  God  also,  though  invisible  in 
himself,  is  visible  in  all  his  works,  and  in  them  appears 
equally  strong  in  power,  admirable  in  wisdom,  eternal 
in  duration,  and  supreme  in  perfection. 

The  whole  universe  conspires  to  celebrate  his  praise, 
from  whom  it  derives  all  its  majesty  and  beauty.  The  sun 
that  shines  in  brightness  declares  the  ineffable  splendour 
of  its  Almighty  Creator.  The  moon  and  stars  proclaim  to 
an  understanding  heart  the  adorable  power  of  the  hand 
that  guides  them.  The  earth,  so  richly  stocked  with  pro- 
ductions of  higher  and  lower  rank,  with  the  various  kinds 
of  vegetable  and  animal  life,  paint  in  the  strongest  terms 
the  riches  of  the  divine  nature,  from  whom  issues  all  that 
adorns  the  earth,  improves  the  mind,  and  delights  the 
senses ;  governing  all  things  with  infinite  wisdom,  good- 
ness unlimited,  power  uncontrouled. 

That  a  Divine  Mind  presides  over  and  governs  the  uni- 
verse, is  indeed  the  natural  conclusion  drawn  by  common 
reason  from  the  evidence  of  common  sense.  For,  who 
that  sees  this  universal, frame  thus  wondrous  fair,  but 
must  infer  the  cause  of  it  to  be  full  of  wondrous  beauty  ? 
Who,  that  observes  ever  so  slightly  that  constancy  which 
is  in  the  motions  of  the  planets,  and  in  the  risings  and  set- 
tings of  the  fixed  stars,  &c.  can  possibly  imagine  the  in- 
constancy of  chance  to  be  the  mover  ?  What  man,  not 
disordered  in  his  own  mind,  can  suppose  any  other  thing 
than  mind  to  be  the  cause  of  that  everlasting  order,  which 
appears  in  the  regular  interchanges  of  the  elements,  and 
the  circling  returns  of  the  successive  seasons. 


ON    ELECTRICITY.  257 

As  far  as  I  have  conducted  you  through  various  branch- 
es of  natural  philosophy ;  as  far  as  I  have  proceeded  in 
giving  you  a  general  view  of  the  system  of  the  world, 
beauty  has  every  where  struck  your  eye,  and  engaged 
you  to  proceed  and  scrutinize  further  the  operations  in 
nature.  The  more  accurate  your  scrutiny,  the  more  you 
will  discover  of  regularity,  symmetry,  and  order,  in  the 
•constitution  of  nature's  frame  ;  the  further  you  penetrate 
into  her  deep  recesses,  dividing  and  subdividing,  opening 
and  unfolding,  the  minutest  part  of  every  visible  form, 
still  the  more  you  will  find  of  beauty  within  beauty,  and 
find  every  order  to  contain  a  variety  of  other  orders. 


LECTURE  XLVL 


ON    ELECTRICITY, 


MR.  STILLINGFLEET  has  well  observed,  that  if 
the  whole  scene  of  nature  were  laid  open  to  our  view,  were 
we  permitted  to  behold  the  connexions  and  dependencies 
of  every  thing  on  every  other,  and  to  trace  the  economy 
of  nature  through  the  smaller,  as  well  as  the  greater  parts 
of  this  globe,  we  should  probably  find,  that  the  Great 
Architect  had  contrived  his  works  in  such  a  manner,  that 
we  cannot  properly  be  said  to  be  unconcerned  in  any  one 
of  them  ;  and  therefore,  those  studies,  which  seem  upon 
a  slight  view  to  be  quite  useless,  may  in  the  end  appear 
of  no  small  importance  to  mankind. 

If  you  look  back  into  the  history  of  arts  and  sciences, 

you  will  be  convinced,  that  men  are  apt  to  judge  too 

hastily  of  things  of  this  nature  ;  you  will  there  find,  that 

he  who  gave  curiosity  to  his  creature,  man,  gave  it  for 

VOL.  IV.  2  L 


258  ON    ELECTRICITY. 

good  and  great  purposes ;  and  that  he  rewards  with 
useful  discoveries  what  in  the  first  instance  are  con- 
demned as  trifling  or  minute  researches. 

But  it  is  true,  that  these  discoveries  are  not  always 
made  by  the  searcher,  or  his  cotemporaries,  or  even 
by  the  immediate  succeeding  generation  ;  but  there  can 
be  no  doubt,  but  that  advantages  of  one  kind  or  other 
always  accrue  to  mankind  from  an  investigation  of  the 
operations  in  nature.  Some  men  are  born  to  observe 
and  record  what  perhaps  by  itself  is  perfectly  useless, 
but  yet  of  some  importance  to  another,  who  follows  and 
goes  a  step  further ;  then  another  succeeds  ;  and  thus 
by  degrees,  till  at  last  one  of  superior  genius  comes, 
who,  laying  all  that  have  been  done  before  his  time  toge- 
ther, brings  on  a  new  face  of  things,  improves,  adorns, 
and  exalts  human  society. 

All  those  speculations  concerning  lines  and  numbers 
so  ardently  pursued,  so  exquisitely  conducted  by  the 
Grecians,  what  did  they  aim  at ;  what  did  they  produce 
for  ages  ?  A  little  arithmetic,  and  the  first  elements  of 
geometry,  were  all  they  had  need  of.  This  Plato  as- 
serts ;  and  though,  as  being  himself  an  able  mathe- 
matician, and  remarkably  fond  of  those  sciences,  he 
recommends  the  study  of  them,  yet  he  makes  use  of 
motives  that  have  no  relation  to  the  common  purposes 
of  life. 

When  Kepler,  from  a  blind  and  strong  impulse, 
merely  to  find  analogies  in  nature,  discovered  that  fa- 
mous law  between  the  distance  of  the  several  planets 
from  the  sun,  and  the  periods  in  which  they  complete 
their  revolutions,  of  what  importance  was  it  to  him  or 
to  the  world  ? 

Again,  when  Galileo,  pushed  on  by  the"same  irresisti- 
ble curiosity,  found  out  the  law  by  which  bodies  fall  to 
the  earth,  did  he,  or  could  he  foresee,  that  any  good  could 
come  from  his  ingenious  theorems ;  or  was  there  any 
immediate  use  made  of  them  ? 

Yet,  had  not  the  Greeks  pushed  their  abstract  specula- 
tions so  far  ;  had  not  Kepler  and  Galileo  made  the  above- 
mentioned  discoveries,  we  never  could  have  seen  the 


ON    ELECTRICITY.  259 

greatest  work  that  ever  came  from  the  hands  of  man,  Sir 
banc  Newton's  Principia. 

Some  obscure  person,  whose  name  is  not  so  much 
as  known,  diverting  himself  idly,  (as  a  by  stander  would 
have  thought,)  with  trying  experiments  on  a  seemingly 
contemptible  piece  of  stone,  found  out  a  guide  for  mari- 
ners on  the  ocean  ;  and  such  a  guide,  as  no  science,  how- 
ever subtile  and  sublime  its  speculations  may  be,  however 
wonderful  its  conclusions,  could  ever  have  attained.  It 
is  the  same  with  electricity.  Who  could  have  supposed, 
on  seeing  a  person  amusing  himself  with  the  effect  of 
excited  amber  on  light  bodies,  that  this  was  one  of  the 
the  first  links  in  a  science  that  should  teach  men  how  to 
disarm  the  clouds  of  lightening,  divest  the  storm  of  its 
terrors,  and  give  life  and  power  to  the  animal  frame. 

Other  instances  might  be  produced  to  prove,  that 
bare  curiosity,  in  one  age,  is  the  source  of  the  greatest 
utility  in  another  ;  and  what  has  been  frequently  said  of 
the  chemists,  may,  perhaps,  be  applied  to  every  other 
kind  of  virtuosi.  They  hunt,  perhaps,  after  chimeras 
and  impossibilities,  and  find  something  really  valuable 
by  the  bye.  We  are  but  instruments  under  the  Su- 
preme Director,  and  do  not  know,  in  many  cases,  what 
is  of  most  importance  for  us  to  search  after ;  but  we 
may  be  sure  of  one  thing,  that  if  we  study  and  follow 
nature,  whatever  paths  we  are  led  into,  we  shall  at  last 
arrive  at  something  valuable  to  ourselves  and  others, 
but  of  what  kind,  we  must  be  content  to  be  ignorant. 

The  nature  of  aqueous  vapours,  of  fire,  and  the 
electrical  fluid,  will  clearly  prove  to  you,  that  a  num- 
ber of  substances  may  act  in  nature  without  being 
known  to  us  ;  and  that  it  is  our  ignorance  of  their  ex- 
istence, which  envelopes  in  obscurity  so  many  pheno- 
mena. 

If  it  were  not  for  the  visible  diminution  of  water  when 
its  surface  is  exposed,  and  for  the  hygroscopical  ap- 
pearances, we  should  not  have  known  that  aqueous  va- 
pours existed  in  the  atmosphere.  Notwithstanding  all 
these  phenomena,  there  are  still  those  who  do  not  ad- 
mit their  existence.  It  is  not  difficult,  however,  to  show, 
that  the  effects  produced  by  this  fluid,  while  in  an  im« 


260  ON    ELECTRICITY. 

perceptible  state,  are  incomparably  greater  than  the  im- 
mediate symptoms  of  its  existence. 

Again,  without  heat,  an  effect  which  is  only  produc- 
ed by  fire  when  it  is  disengaged  or  free,  we  should  have 
been  ignorant  of  the  existence  of  fire  :  yet  how  great 
and  various  are  the  effects  it  produces  in  the  combined 
or  latent  state  !  Heat  is  a  symptom  of  the  presence  of 
this  fluid,  and  of  its  degree  of  density,  when  free  and 
disengaged  ;  but  if  you  seek  to  follow  it  in  the  phe- 
nomena of  nature,  you  find,  that  when  it  escapes  from 
observation,  it  is  acting  the  most  important  of  parts. 
It  is  the  same  with  light,  the  companion  of  fire.  If  it 
were  not  for  the  impression  it  makes  upon  our  eyes, 
we  should  be  ignorant  of  the  greatest  and  most  imme- 
diate agent  of  all  terrestrial  phenomena. 

Thus  you  see,  that  there  are  substances  of  the  greatest 
importance  for  modifying  those  which  are  more  grossly 
perceptible,  and  of  which,  in  the  mean  time,  we  have 
little  or  no  knowledge,  though  they  are  producing  the 
greatest  effects. 

But  still  further,  the  motions  of  light  occasioned  by 
the  electrical  fluid,  when  its  natural  equilibrium  is  dis- 
turbed, are  the  only  signs  which  give  us  notice  of  its 
existence.  All  electrical  phenomena  concur  in  proving 
the  existence  of  a  certain  fluid,  possessed  of  certain 
characters,  capable  of  particular  modifications,  and  dis- 
seminated over  the  whole  surface  of  the  globe  ;  the  why 
or  wherefore  is  still  unknown,  we  are  still  ignorant  of 
its  functions.  But  we  are  at  the  same  time  ignorant  of 
the  cause  of  so  many  phenomena  in  nature,  that  we 
ought  not  to  despair  of  being  able  to  discover  those 
with  which  it  is  connected,  and  how  it  influences  them 
by  its  composition  and  decomposition. 

It  results  from  the  observations  I  have  now  made, 
that  the  known  expansible  fluids  have  two  kinds  of  pro- 
perties ;  one  whereby  they  manifest  themselves  to  one  or 
more  of  our  senses,  the  other  by  which  they  act  imper- 
ceptibly in  a  number  of  phenomena.  It  is  not  then  ne- 
cessary, either  as  a  proof  of  the  existence  of  a  sub- 
stance, or  of  its  being  a  principal  agent  in  phenomena, 
that  it  should  be  manifested  to  our  senses.     But  it  is 


ELECTRICAL    APPEARANCES.  261 

essential  in  nature,  as  soon  as  you  consider  physical  ob- 
jects, that  to  every  phenomenon  there  be  a  cause  ;  and 
the  only  method  of  assigning  a  reasonable  one,  where 
they  are  not  immediately  discoverable,  is  analogy.  When 
therefore  certain  phenomena,  whose  cause  is  hidden,  are 
analogous  to  other  phenomena  that  we  attribute  to  the 
intervention  of  some  substance,  we  are  naturally  led  to 
some  substance  as  a  cause  of  the  first  mentioned  phe- 
nomena ;  and  nothing  will  oppose  their  admission,  if 
they  explain  what  cannot  be  explained  without,  and  if 
there  be  nothing  which  renders  the  existence  of  the 
substance  obscure. 

Here  I  cannot  refrain  from  observing,  that  the  French 
philosophers,  after  extending  the  influence  of  electrici- 
ty over  all  nature,  now  pass  it  by  as  if  unworthy  of  no- 
tice ;  its  name  is  not  even  to  be  found  dans  le  tableau  de 
la  novelle  nomenclature. 

Philosophy  owes  much  to  the  assistance  it  has  receiv- 
ed from  mathematicians  ;  but  this  only  happens  when 
they  apply  themselves  to  the  study  of  phenomena  ; 
when  neglecting  these,  calculations  are  made  to  serve 
a  hypothesis  ;  the  more  elegant  and  beautiful  they  are, 
the  more  detrimental  they  become  to  science.  It  it  thus, 
that  Mpinus^  by  a  mathematical  theory  of  electricity, 
has  closed  the  door  on  all  our  reserches  into  the  nature 
and  operations  of  this  fluid. 


ELECTRICAL    APPEARANCES. 

From  these  preliminary  observations,  which  I  con- 
sidered, as  necessary  to  excite  your  attention  to  the 
electric  fluid,  and  to  lead  you  to  look  out  for,  and  to 
trace  its  connexion  with  other  agents  in  nature  ;  I  shall 
proceed  to  point  out  some  of  the  most  striking  electri- 
cal appearances.  The  knowledge  obtained  of  electrici- 
ty, like  most  other  articles  of  science,  has  arisen  from 
very  small  beginnings,  and  by  very  slow  degrees  to  its 
present  height.  It  had  been  known  for  ages,  that 
amber,  jett,  and  other  bodies,  would,  upon  rubbing, 
attract  and  repel  light  bodies,  as  hairs,  feathers,  down3 


262  ELECTRICAL    APPEARANCES. 

dust,  &c.  and  as  this  property  was  most  conspicuous 
in  amber,  which  in  the  Greek  is  called  electron,  the  pe- 
culiar power  of  that  body  was  termed  electricity.  Up- 
on further  enquiry,  it  was  found,  that  not  amber  only, 
but  several  other  substances  had  the  same  properties 
in  a  high  degree  ;  that  glass,  resinous  substances,  silk, 
dry  wood,  &c.  have  the  same  properties  ;  and  that  any 
of  these  when  dry,  and  rubbed  for  a  short  time,  would 
attract  light  substances. 

I  rub  this  stick  of  sealing  wax  with  soft  flannel,  and 
you  see  that  it  attracts  any  light  substances,  as  hairs, 
feathers,  &c.  that  I  bring  under  it ;  rub  a  dry  glass 
tube  with  dry  silk,  and  you  will  find  it  produce  the  same 
effect.  Let  us  now  darken  the  room,  rub  the  glass  tube 
again,  and  you  will  see  sparks  of  fire  follow  your  hand  ; 
present  your  knuckle  to  the  tube,  and  these  sparks  will 
be  formed  into  pencils  or  brushes  of  light,  attended 
with  a  crackling  noise  like  that  of  a  green  leaf  in  the 
fire. 

The  friction  has,  in  these  instances,  manifested  to 
the  senses  the  existence  of  a  substance  that  was  before 
imperceptible.  The  body  that  is  made  by  friction  to 
exhibit  these  appearances,  is  said  to  be  excited.  The 
appearances  are  termed  signs  of  electricity. 

Here  is  a  fine  downy  feather  tied  to  a  silk  string  ;  I 
electrify  it  strongly  by  touching  it  with  the  excited  glass 
tube,  and  it  immediately  flies  from,  or  is  repelled  by 
the  glass  tube.  I  now  present  an  excited  stick  of  seal- 
ing-wax, and  the  feather  immediately  flies  towards  it. 
Thus  you  see,  that  what  is  attracted  by  excited  wax,  is 
repelled  by  excited  glass.  This  experiment  gave  rise 
to  a  very  important  distinction  in  electricity,  implying  a 
contrariety  of  agency  therein,  and  one  power  or  agent 
by  the  glass  was  denominated  vitreous,  the  other  resinov.s 
electricity.*  Further  discoveries  showed,  that  glass  or 
wax  would,  according  to  the  circumstances  in  which 
they  were  situate,  produce  either  power. 


*  Now  generally  called  by  electricians,  fioaitive  and  negative  electricity. 
E.  Edit/ 


PRINCIPLES   OF    ELECTRICITY    &C.  26$ 

Our  two  next  experiments  lead  us  also  to  another  very- 
important  distinction  in  this  branch  of  science.  I  sus- 
pend a  brass  ball  by  a  wire  from  the  end  of  the  glass 
tube  opposite  to  my  hand,  and  excite  the  tube  as  before  ; 
as  soon  as  the  tube  is  excited,  you  will  find,  that  the 
ball  has  acquired  all  the  electric  properties  of  the  tube  ; 
it  will,  like  it,  attract  light  bodies,  and  give  the  spark. 
Let  us  now  suspend  the  ball  by  a  silk  string,  and  ex- 
cite the  tube  as  before  ;  you  may  now  rub  as  long  as 
you  please,  but  the  ball  will  exhibit  no  signs  of  electri- 
city. Here  then  we  have  two  substances,  through  one 
of  which,  the  wire,  the  electric  properties  may  be  con- 
veyed ;  whereas  the  other,  that  is,  the  silk,  prevents 
their  passing  to  the  ball.  The  wire  is  therefore  called 
a  conductor  of  electricity.  The  silk  is  termed  a  non- 
conductor. 

Or,  in  more  general  terms,  all  those  bodies,  through 
which  the  electrical  fluid  is  transmitted  freely,  are  term- 
ed conductors.  Those  bodies,  through  which  it  does  not 
pass  so  freely,  are  called  non-conductors, 

A  body  resting  entirely  upon  non-conductors  is  said 
to  be  insulated.  Thus,  in  the  last  experiment,  the  ball 
was  insulated,  because  it  was  suspended  by  a  silk  string, 
which  is  a  non-conductor.  Insulation  prevents  the  dis- 
sipation of  the  electrical  appearances. 


THE    PRINCIPLES    OF    ELECTRICITY    DEDUCED    FROM 
EXPERIMENTS    ON    ATTRACTION    AND    REPULSION. 

I  have  already  shown  you,  that  a  light  body  electri- 
fied by  excited  glass,  is  repelled  thereby,  but  will  be  at- 
tracted by  excited  wax  ;  and  that,  on  the  other  hand, 
if  it  be  electrified  by  excited  wax,  it  will  be  repelled 
thereby,  but  will  be  attracted  by  excited  glass.  This 
observation  you  must  keep  in  mind,  without  it  you  can 
never  understand  the  operations  of  electricity.  The 
greater  part  of  the  experiments  in  this  science,  and  the 
whole  of  the  reasoning  thereon,  depend  on  a  reference 
to  these  facts. 


264  PRINCIPLES    OF    ELECTRICITY 

For  the  following  experiments,  I  make  use  of  small 
light  balls  formed  out  of  the  pith  of  elder ;  they  are 
suspended  by  fine  linen  threads  from  small  cylinders  of 
wood,  and  are  insulated  upon  a  common  wine-glass,  that 
is  dry,  free  from  dust,  fibres  of  down,  &c. 

I  electrify  two  balls,  thus  suspended  by  excited  glass, 
and  you  see  they  repel  each  other.  I  destroy  this  electri- 
city by  touching  them  with  my  hand.  I  again  electrify 
them,  but  with  excited  wax,  and  they  again  repel  each 
other.  I  bring  the  balls  electrified  by  wax  towards  those 
electrified  by  glass,  and  they  immediately  fly  towards  each 
other. 

From  these  experiments  you  will  infer,  1.  That  bodies 
electrified  vitreously  repel  each  other.  2.  That  bodies  elec- 
trified resinously  repel  each  other.  3.  That  bodies  electri- 
fied with  contrary  powers  attract  each  other. 

As  those  light  substances  which  possess  the  same  elec- 
tric power  repel  each  other,  it  will  always  be  easy  for  you 
to  discover  with  what  power  they  are  electrified.  If  they 
be  repelled  by  excited  glass,  they  possess  the  vitreous  elec- 
tricity ;  if  they  be  attracted  thereby,  they  are  resinously 
electrified ;  on  the  contrary,  those  attracted  by  excited 
wax,  are  vitreously,  and  those  repelled  thereby,  resi- 
nously electrified.  In  ascertaining  the  nature  of  the 
electric  power,  you  must  avoid  bringing  the  bodies  to  be 
tried  near  each  other  suddenly ;  or  one  strongly  electri- 
fied too  near  one  that  is  weakly ;  as  it  may  render  the 
experiment  doubtful,  for  reasons  that  will  soon  be  appa- 
rent. 

Before  I  go  any  further,  it  may  be  proper  to  point  out 
to  you  the  leading  features  of  that  theory  of  electricity, 
Mr.  Eeles's,*  which  I  adopt  in  these  lectures.  I  consider, 


*  See  Edes's  Philosophical  Essays  in  several  letters  to  the  Royal  Society, 
London,  1771.  On  a  comparison  of  this  work  with  the  greater  part  of  the 
modern  writers  on  electricity,  yon  will  find,  that  they  have  been  gradually 
giving  up  the  most  essential  distinctions  oftheFranklinian  theory,  and  adopt- 
ing those  of  Mr.  lieles.  See  Wilson,  Hcnly,  Gray,  Milner,  Brooke,  Peart, 
Read,  Sec.  Sec.  And  you  will  find  him  laying  down  principles,  and  making 
experiments,  that  have  within  a  few  years  been  brought  forward  as  new; 
some  have  indeed  been  rejected  at  first,  because  deemed  contradictory  to 
a  favourite  theory,  but  which  have  since  been  fully  acknowledged.    . 


DEDUCED  FROM  EXPERIMENTS.       265 

with  him,  all  those  electrical  operations  that  are  manifest- 
ed to  the  senses,  as  occasioned  by  two  distinct,  positive, 
and  active  powers,  which  equally  and  strongly  attract  and 
condense  each  other  ;  but  when  by  any  circumstance  they 
are  rendered  unequal  to  each  other,  the  increased  power 
expands  into  an  atmosphere. 

These  two  powers  exist  together  in  all  bodies  ;  in  their 
natural  state  they  are  always  conjoined  ;  the  electric  signs, 
or  what  we  call  electricity,  are  only  rendered  sensible  to 
us  by  the  separation  of  these  powers.  In  other  words, 
though  the  electric  matter  is  acting  the  most  important 
part  among  the  operations  in  nature,  in  its  united,  and  to 
us,  latent  and  invisible  state,  yet  it  becomes  no  object  to 
our  senses,  till  its  powers  are  separated  and  rendered  un- 
equal. 

When  the  powers  are  separated  and  brought  into  ac- 
tion, the  increased  power  expands,  and  forms  what  may 
be  termed  an  electrical  atmosphere.  If  any  body  be  im- 
merged  in  this  atmosphere,  the  powers  thereof  are  sepa- 
rated, and  that  which  is  of  the  same  kind  with  the  atmos- 
phere is  repelled,  while  the  contrary  power  is  attracted : 
as  long  as  the  body  remains  immerged  therein,  the  powers 
remain  separated.  It  is  however  to  be  observed,  that  in 
exciting  electrics,  the  powers  are  never  entirely  separated. 
The  diminished  power  acts  inward  to  the  electric,  while 
the  increased  power  acts  outward  with  an  extensive  at- 
mosphere. 

I  hold  an  excited  glass  tube  over  this  metallic  cylinder, 
placed  on  a  dry  wine-glass,  as  an  insulating  stand,  plate 
1,  jig.  1,  (Electricity),  but  at  a  certain  distance  from  it, 
which  distance  will  depend  upon  the  power  of  the  glass  ; 
it  repels  the  vitreous  electricity  of  the  tube  into  the  balls, 
which  will  diverge  with  vitreous  electricity,  and  will  of 
course  recede  from  excited  glass.  I  remove  the  excited 
glass  from  over  the  balls,  and  they  close.  A  temporary 
separation  of  the  electric  matter  inherent  in  the  cylinder, 
is  in  this  instance  produced  by  the  influence  of  the  ex- 
cited glass ;  as  soon  as  this  influence  is  removed,  the 
powers  unite,  and  the  balls  close. 

I  now  place  two  cylinders,  with  their  ends  in  contact 
with  each  other,  plate  1,  jig.  2,  and  hold  the  excited 
VOL.  IV.  2  M 


266  PRINCIPLES    OF    ELECTRICITY. 

tube  over  the  end  A ;  each  pair  of  balls  diverges.  While 
they  are  in  this  state,  separate  them  one  from  the  other, 
and  you  will  find  the  balls  of  A  to  be  electrified  vitreously, 
and  these  of  B  resinously  ;  proving,  that  while  the  body 
remained  immerged  in  the  atmosphere,  the  electric  pow- 
ers thereof  were  separated,  one  being  at  each  end.  Bring 
the  tubes  together  again,  and  the  balls  immediately  close; 
proving,  1.  That  the  separated  powers  attract  each  other. 
2.  That  when  united,  they  condense  each  other,  and  that 
all  electric  signs  are  immediately  lost:  3.  The  co-exist- 
ence of  the  two  powers  in  the  cylinders. 

Again  electrify  the  balls  equally,  but  with  the  same 
powers,  then  bring-the  ends  of  the  cylinders  together,  and 
the  divergence  of  the  balls  will  not  be  altered  ;  which 
shows,  that  equal  atmospheres  of  the  same  kind  do  not 
act  on  each  other. 

Hold  an  excited  glass  tube  over  the  cylinder,  plate  1, 
fig.  4,  and  at  the  same  time  keep  one  of  your  fingers  in 
contact  with  the  opposite  end  of  the  cylinder,  remove  the 
glass  tube  and  finger  together,  and  the  balls  will  diverge 
with  resinous  electricity ;  for,  on  trying,  you  will  find 
them  fly  towards  excited  wax,  and  recede  from  excited 
glass.  The  vitreous  power  is  repelled  by  the  excited 
tube,  and  passes  into  the  finger,  which,  in  exchange, 
communicates  resinous  electricity  to  the  cylinder. 

The  tendency  of  an  electric  atmosphere  to  produce  the 
contrary  electricity,  in  the  bodies  contiguous  to  it,  is  plea- 
singly illustrated  by  the  following  experiment.  In  this, 
there  are  four  cylinders,  A,  B,  C,  D,  plate  1,  Jig.  3; 
excited  glass  held  over  A,  repels  the  vitreous  power  into 
B^  and  draws  the  resinous  into  A  ;  in  the  same  manner, 
B  repels  the  vitreous  power  of  C  into  D,  and  draws  the 
resinous  into  C ;  separate  B  and  D  from  A  and  C,  just 
before  the  excited  glass  is  removed,  and  you  will  find  A 
and  C  possessing  the  resinous,  B  and  D  the  vitreous  elec- 
tricity ;  as  you  will  find,  by  bringing  the  excited  glass  to- 
wards the  balls,  those  at  A  and  C  will  move  towards  the 
glass,  those  at  B  and  D  will  recede  from  it. 

You  saw  in  a  former  experiment,  where  the  balls  were 
equally  electrified  with  contrary  powers,  that  on  bringing 
the  cylinders  together,  the  powers  united,  and  all  electri- 


OF    THE    ELECTRICAL    MACHINE.  267 

cal  signs  vanished.  But  if  one  be  electrified  more  than  the 
other,  that  which  is  least  so,  loses  all  its  electricity  after 
contact,  and  the  two  remain  electrified,  with  the  excess 
of  the  electricity  of  that  which  was  strongest. 

From  these  experiments  it  appears,  that  the  increased 
power  expands  itself,  and  acts  outwards,  and  that  in  pro- 
portion to  the  subtraction  of  the  other  power  ;  and  that 
this  is  the  sphere  of  the  expanded  power,  which  is  called 
an  electric  atmosphere. 

It  appears  further,  that  no  substance  seems  to  be  elec- 
trified, while  the  powers  are  equal  in  or  on  that  body ; 
but  in  proportion  as  there  is  a  greater  quantity  of  one 
power,  than  there  is  of  the  other,  then  the  increased  power 
acts  outwards  from  that  body,  and  the  body  will  be  elec- 
trified with  that  power,  and  will  repel  any  other  body 
electrified  with  the  same  power  ;  but  will  attract  any  sub- 
stance electrified  with  the  contrary  power  ;  and  after  con- 
tact between  them,  all  electrical  signs  vanish,  if  they  were 
equally  electrified ;  but  if  unequally,  both  will  remain 
electrified  with  the  excess  of  the  strongest  power. 

These  positions  will  be  confirmed  by  other  experiments, 
in  which  you  will  see  the  contrary  directions  of  the  two 
powers. 

OF  THE   ELECTRICAL    MACHINE,    plate   1,  fig.  5,  AND 
ITS    MODE    OF    ACTION. 

By  turning  the  handle  of  the  machine,  and  of  course 
the  glass  cylinder  which  moves  with  it,  electricity  is  pro- 
duced ;  and  this  we  shall  find,  as  before,  of  two  kinds, 
each  strongly  attractive  of  the  other,  though  repulsive  of 
a  similar  kind  ;  when  united,  the  expansive  power  they 
before  exerted,  is  condensed,  and  all  electric  signs  va- 
nish. 

To  render  these  positions  clear,  I  insert  a  wire,  B,  in 
the  cushion,  and  another,  C,  in  the  conductor ;  each  of 
these  is  furnished  with  a  brass  ball  at  top,  and  each  of 
them  has  a  sliding  wire  with  balls  on  its  end,  that  it  may 
be  set  at  any  convenient  distance  from  the  other.  On  turn- 
ing the  cylinder,  you  observe,  1.  That  I  can  obtain  an 
electric  spark  from  the  balls  of  either  wire  on  presenting 


268  OF    THE    ELECTRICAL    MACHINE, 


my  knuckle  thereto.  2.  That  a  strong  spark  will  pass 
from  one  ball  to  the  other.  3.  That  on  holding  a  cork 
ball  suspended  by  silk,  between  the  two  brass  balls,  it  is 
alternately  attracted  and  repelled  from  one  to  the  other. 
4.  Electrify  a  pair  of  insulated  pith  or  cork  balls  by  the 
cushion,  and  you  will  find  them  to  possess  the  resinous 
electricity ;  electrify  them  by  the  conductor,  and  they 
will  possess  the  vitreous  power.  5.  Join  the  balls  toge- 
ther, and  all  electrical  signs  vanish. 

On  the  other  hand,  if  you  place  both  wires  either  on 
the  conductor,  or  the  cushion,  you  will  find,  that  no 
spark  will  pass  between  them,  that  the  cork  ball  remains 
stationary,  being  neither  attracted  nor  repelled  by  the 
balls,  and  this  because  they  both  possess  the  same  kind 
of  electricity. 

From  these  experiments  we  infer,  that  the  conductor 
and  the  cushion  are  electrified  with  different  powers;  that 
pne  attracts  what  the  other  repels,  and  that  when  they 
are  united,  they  exhibit  no  signs  of  electricity :  that  on 
the  separation  of  the  powers  by  excitation,  one  power  at- 
taches itself  to  the  excited  electric,  the  other  to  the  rub- 
ber. 

The  whole  variety  of  electrical  experiments  appear  to 
be  nothing  more  than  different  modes  of  destroying  or 
restoring  an  equilibrium.  By  destroying  the  equilibrium, 
two  positive  powers  are  at  the  same  time  produced.  By 
restoring  the  equilibrium,  all  things  return  to  their  natu- 
ral state,  and  every  appearance  of  electricity  ceases.  The 
two  powers  are  so  connected,  that  one  can  never  be  exhi- 
bited without  producing  the  other.  It  is  probable,  that  in 
the  general  operations  of  nature,  this  fluid  always  acts  in 
its  united  form,  that  in  which  it  is  to  our  senses  latent 
and  invisible. 

On  turning  the  cylinder  and  separating  it  from  the 
silk,  the  electric  powers  are  separated,  the  cylinder  gives 
its  resinous  power  to  the  cushion  in  exchange  for  the  vi- 
treous ;  the  conductor  in  like  manner  exchanges  its  pow- 
ers with  the  cylinder ;  for,  as  long  as  the  cushion  com- 
municates with  the  table  by  a  chain,  and  you  continue 
turning  the  cylinder,  you  will  find  the  conductor  strongly 
electrified  with  the  vitreous  power.  Take  the  chain  from 


AND    ITS    MODE    OF    ACTION.  269 

the  cushion,  and  suspend  it  from  the  conductor;  on 
turning  the  cylinder  you  will  find  the  cushion  strongly 
electrified  with  the  resinous  power.  Connect  the  cushion 
and  conductor  by  a  chain,  and  the  powers  re-unite  al- 
most as  soon  as  they  are  separated,  and  the  electrical 
signs  disappear. 

We  now  see,  why  conducting  substances  cannot  be 
electrified  unless  they  are  insulated.  It  is  because  the  two 
powers  join  instantaneously  in  the  non-conductor,  and 
can  therefore  exert  no  sensible  action. 

When  I  turn  the  cylinder  slowly,  only  a  small  quan- 
tity of  the  fluid  is  excited,  and  it  does  not  fly  far  in  the 
form  of  a  spark  ;  but  when  I  turn  somewhat  faster,  and 
make  the  black  silk  adhere  to  the  glass,  the  quantity  of 
excited  electricity  is  considerably  increased.  The  flash 
or  spark  passes  through  a  greater  space,  and  assumes  a 
crooked  or  zig-zag  direction,  resembling  the  flashes  of 
lightning.  The  brilliancy  of  the  spark  depends  much  on 
the  pressure  of  the  atmosphere  ;  for  the  spark  which  ex- 
plodes in  air  is  vivid  like  lightning ;  but  if  the  same  be 
tried  in  an  exhausted  receiver,  instead  of  a  spark  and  ex- 
plosion, you  have  only  a  silent,  faint,  diluted  stream. 

Before  I  proceed  to  other  experiments,  I  shall  explain 
our  machine  more  fully,  and  show  you  how  to  excite  it 
powerfully.  The  parts  of  the  machine,  which  fall  more 
immediately  under  your  attention,  are,  1 .  The  electric, 
or  the  glass  cylinder  which  is  to  be  excited.  2.  The  me- 
chanical contrivances  by  which  it  is  put  in  motion.  3.  The 
cushion  and  its  appendages.  4.  The  conductor,  or  con- 
ductors. 

The  glass  cylinder  of  the  machine  before  you,  plate  1 , 
Jig.  5,  is  put  in  motion  by  a  simple  winch  A.  This  is 
less  liable  to  be  out  of  order,  than  those  that  are  turned 
with  a  multiplying  wheel  and  pulley,  and  will  also  ena- 
ble you  to  excite  the  machine  more  powerfully.*  The 
cylinder,  F  G I K,  is  supported  by  two  strong  perpendi- 


*  There  are  some  instances  in  which,  1  think,  a  multiplying  wheel  and 
pulley  are  preferable.  They  are  less  awkward  and  tiresome  to  beginners 
than  a  handle.  I  think  those  machines  the  most  complete,  which  are  capa- 
ble of  being  used  occasionally  either  way E.  Edit. 


270  OF    THE    ELECTRICAL    MACHINE, 

cular  pieces  D  E.  The  axis  of  one  cap  of  the  cylinder 
moves  in  a  small  hole  at  the  upper  part  of  one  of  the  sup- 
ports. The  opposite  axis  passes  through  the  upper  part 
of  the  other  support.  To  this  axis  the  winch  or  handle  is 
fitted.  The  cushion  is  supported  and  insulated  by  a 
glass  pillar  ;  the  lower  part  of  this  pillar  is  fitted  into  a 
wooden  socket,  to  which  a  regulating  screw,  H,  is 
adapted,  to  increase  or  diminish  the  pressure  of  the 
cushion  against  the  cylinder.  A  piece  of  silk  comes 
from  the  under  edge  of  the  cushion,  and  lies  on  the 
cylinder,  passing  between  it  and  the  cushion,  and  pro- 
ceeding till  it  nearly  meets  the  collecting  points  of  the 
conductor.  The  more  strongly  this  silk  is  made  to  ad- 
here to  the  cylinder,  the  stronger  is  the  degree  of  ex- 
citation. Before  the  cylinder,  or  opposite  to  the  cushion, 
is  a  metallic  tube  Y  Z,  supported  by  a  glass  pillar  L  M, 
with  a  set  of  points  in  the  front,  called  the  collector,  to 
collect  the  electricity  from  the  cylinder.  This  tube  is 
sometimes  called  the  prime  conductor,  often  only  the  con- 
ductor.*  For  the  more  conveniently  trying  the  experi- 
ments on  the  two  powers,  and  exhibiting  the  different 
states  of  the  cushion  and  conductor,  there  are  two  wires, 
B,  C,  to  be  fixed  occasionally,  the  one  to  the  conductor, 
the  other  to  the  cushion  ;  on  the  upper  part  of  these, 
are  balls  furnished  with  sliding  wires,  that  they  may  be 
set  at  various  distances  from  each  other. 

Before  the  electrical  machine  is  put  in  motion,  exa- 
mine those  parts  which  are  liable  to  wear  either  from 
the  friction  of  one  surface  against  another,  or  to  be  in- 
jured by  the  dirt,  that  may  insinuate  itself  between  the 
rubbing  surfaces.  When  any  grating  or  disagreeable 
noise  is  heard,  the  place  from  whence  it  proceeds  must 
be  discovered,  wiped  clean,  and  rubbed  over  with  a 
small  quantity  of  tallow ;  a  little  sweet  oil  or  tallow 
should  also  be  occasionally  applied  to  the  axis  of  the 
cylinder. 


*  The  conductor  exhibited  in  the  figure  is  of  the  T  shape,  which  is  a 
-convenient,  but  not  an  essential  form;  they  are  more  frequently  made  cy- 
lindi'iccilj  with  the  collecting  points  placed  on  the ' side. ....K.  Edit. 


AND    ITS    MODE    OF    ACTION.  271 

The  screws  that  belong  to  the  frame  should  be  ex- 
amined, and  if  they  be  loose,  they  should  be  tightened. 

The  different  working  parts  of  the  machine  having 
been  looked  into,  and  put  in  order,  the  glass  cylinder, 
and  the  pillars  which  support  the  cushion  and  conduc- 
tor, should  be  carefully  wiped  with  a  dry  old  silk  hand- 
kerchief, to  free  them  from  the  moisture  which  glass 
attracts  from  the  air,  being  particularly  attentive  to  leave 
no  moisture  on  the  ends  of  the  cylinder,  as  any  damp 
on  these  parts  carries  off  the  electric  fluid,  and  lessens 
the  force  of  the  machine :  in  damp  weather  it  will  be 
proper  to  dry  the  whole  machine,  by  placing  it  before, 
but  also  at  some  distance  from,  the  fire. 

Take  care  that  no  dust,  loose  threads,  or  filaments, 
adhere  to  the  cylinder,  its  frame,  the  conductors,  or 
their  insulating  pillars  ;  because  these  will  gradually 
dissipate  the  electric  fluid,  and  prevent  the  machine 
from  acting  powerfully. 

Rub  the  glass  cylinder  first  with  a  clean,  coarse,  dry, 
warm  cloth,  or  a  piece  of  wash-leather,  and  then  with 
a  piece  of  dry,  warm,  soft  silk  ;  do  the  same  to  all  the 
glass  insulating  pillars  of  the  machine  and  apparatus  ; 
these  glass  pillars  that  are  varnished  must  be  rubbed 
more  lightly  than  the  cylinder. 

A  hot  iron  may,  in  some  cases,  be  placed  on  the  foot 
of  the  conductor,  to  evaporate  the  moisture,  which 
would  otherwise  injure  the  experiments. 

To  excite  your  machine ',  clean  the  cylinder  and  wipe 
the  silk. 

Grease  the  cylinder  by  turning  it  against  a  greasy 
leather,  till  it  be  uniformly  obscured.  The  tallow  of  a 
candle  may  be  used. 

Turn  the  cylinder  till  the  silk  flap  has  wiped  off  sa 
much  of  the  grease,  as  to  render  it  semi-transparent. 

Put  some  amalgam  on  a  piece  of  leather,  and  spread 
it  well,  so  that  it  may  be  uniformly  bright  ;  apply  this 
against  the  turning  cylinder  ;  the  friction  will  immedi- 
ately increase,  and  the  leather  must  not  be  removed  un- 
till  it  ceases  to  become  greater. 

Remove  the  leather,  and  the  action  of  the  machine 
will  be  very  strong. 


272      MOMENTUM    OF    THE    ELECTRICAL    FLUID. 

The  pressure  of  the  cushion  cannot  be  too  small, 
when  the  excitation  is  properly  made. 

The  amalgam  is  that  of  Dr.  Higgins,  composed  of 
zinc  and  mercury ;  if  a  little  mercury  be  added  to 
melted  zinc,  it  renders  it  easily  pulverable,  and  more 
mercury  may  be  added  to  the  powder,  to  make  a  very 
soft  amalgam.  It  is  apt  to  crystallize  by  repose,  which 
seems  in  some  measure  to  be  prevented  by  triturating 
it  with  a  small  proportion  of  grease  :  and  it  is  always 
of  advantage  to  triturate  it  before  using. 

A  very  strong  excitation  may  be  produced  by  ap- 
plying the  amalgamed  leather  to  a  clean  cylinder,  with 
a  clean  silk  ;  but  it  soon  goes  off,  and  is  not  so  strong 
as  the  foregoing,  which  lasts  several  days.* 


OF    THE    MOMENTUM    OF    THE    ELECTRICAL    FLUID. 

The  great  strength  and  velocity  displayed  by  the 
electrical  fluid  in  its  motions,  is  an  object  well  worthy 
your  investigation  :  and  if  it  be  granted  (and  I  think  I 
shall  be  able  to  prove  it  to  you)  that  the  electric  matter 
is  the  same  with  the  solar  fluid,  then  will  the  ultimate 
cause  of  its  momentum  be  the  power  by  which  the  light 
of  the  sun  is  propagated,  the  pressure  of  which  being 
equal  all  round  upon  all  bodies,  it  can  neither  move 
them  one  way  nor  another.  But  if,  by  means  of  any 
other  power,  this  pressure  be  lessened  upon  any  parti- 
cular part,  the  current  of  matter  will  set  forwards  to- 
wards that  place,  with  a  force  proportioned  to  the  dimi- 
nution of  the  pressure.  Thus,  in  the  common  experi- 
ments of  the  air-pump,  when  the  air  is  exhausted  from 
the  receiver,  the  pressure  of  the  superincumbent  atmos- 
phere is  directed  towards  every  part  of  the  glass  ;  so  that 


*  In  summer,  a  machine  and  apparatus,  kept  in  a  room  free  from  dust, 
will  require  merely  wiping  with  a  dry  cloth.  In  winter,  and  if  the  appa- 
ratus be  damp  and  foul,  carefully  wiping  it  all  over,  and  a  general  small 
degree  of  warming  by  the  fire,  will  be  necessary.  Supposing  the  silk  used 
clean  and  perfect,  a  small  application  of  amalgam  to  the  cylinder  or  rub- 
ber will  occasion  the  cylinder  to  be  strongly  excited.  Streams  of  fire,  or 
a  crackling  noise  at  the  knuckle,  when  presented  to  the  cylinder,  will  be  a 
sure  proo£  of  the  machine  being  in  the  best  order. — E.  Edit, 


ATTRACTION    AND    REPULSION.  275 

if  it  be  of  a  flat  square  shape,  and  not  very  strong,  it 
will  certainly  be  broken.  Now,  there  is  reason  to  sup- 
pose, that  after  the  air  is  exhausted  from  the  receiver, 
it  is  full  of  another  subtile  fluid  of  the  same  nature 
with  the  electric.  If  this  could  also  be  extracted  from 
the  receiver,  the  pressure  on  its  sides  would  be  much 
greater,  because  not  only  the  atmosphere,  but  the  whole 
surrounding  ether,  would  be  urged  towards  that  place ; 
and  it  is  not  probable,  that  this  pressure  could  be  resist- 
ed by  any  finite  force  whatever. 

The  momentum,  therefore,  of  the  electrical  fluid  de- 
pends on  two  causes  ;  the  pressure  of  the  atmosphere 
upon  the  electric  matter,  and  the  pressure  of  one  part 
of  this  matter  upon  another,  which  is  extended  through- 
out the  immensity  of  space.  The  force  and  velocity  of 
the  fluid  depend  therefore,  in  a  great  measure,  on 
that  which  surrounds  us.  There  is  a  certain  state  of 
this  fluid,  that  we  violate  by  our  experiments  ;  when 
this  violation  is  small,  the  powers  of  nature  operate 
gently  in  restoring  the  disorder  we  have  introduced ; 
but,  when  any  considerable  deviation  is  occasioned,  the 
same  powers  restore  the  original  constitution  with  ex- 
treme violence. 


EXPERIMENTS    ON     ELECTRICAL    ATTRACTION    AND 
REPULSION. 

To  the  top  of  this  wire,  three  large  downy  feathers 
are  affixed  by  three  linen  threads.  I  insert  the  lower 
end  of  the  wire  into  the  prime  conductor  ;  upon  turn- 
ing the  cylinder,  the  plumage  expands  every  way,  the 
threads  also  recede  as  far  as  possible  from  each  other. 
If  I  place  my  finger  near  the  feathers,  all  the  plumulas 
bend  towards  it ;  if  I  move  my  finger  this  way  or  that, 
they  all  move  after  it  as  if  alive  ;  I  put  my  hand  on  the 
conductor,  immediately  the  threads  lose  their  diver- 
gence, the  plumulse  collapse,  and  fall  close  together ;  I 
take  my  hand  away,  the  threads  diverge,  and  the  fea- 
thers expand  as  before. 

VOL.  IV.  2  N 


274  EXPERIMENTS    ON    ELECTRICAL 

1  cannot  explain  to  you  the  mechanism  which  occa- 
sions the  threads  to  diverge ;  but  I  can  state  those  facts 
which  must  concur  to  occasion  it.  We  know  that  those 
light  bodies,  which  possess  the  same  kind  of  electricity, 
separate  from,  or  repel  each  other  ;  the  finger  commu- 
nicates to  them  the  contrary  power  ;  towards  this,  there- 
fere,  they  are  impelled  by  their  nature,  in  order  to  re- 
store an  equilibrium  which  our  operations  "have  destroy- 
ed. By  putting  my  hand  on  the  conductor,  the  powers 
are  immediately  exchanged  and  united,  and  the  electri- 
cal effects  cease. 

I  place  this  cork  ball,  suspended  by  a  silk,  so  that 
it  may  be  even  with  the  conductor,  and  at  about  six  in- 
ches  from  it.  I  turn  the  machine,  but  the  cork  remains 
quiet ;  touch  it  with  the  end  of  the  wire  in  your  hand, 
and  the  vitreous  power  of  the  ball  is  driven  into  \ou, 
and  an  equal  quantity  of  the  resinous  is  communicated 
to  the  ball,  which  will  then  therefore  fly  with  great  ra-  , 
pidity  towards  the  conductor  ;  direct  the  pointed  end  of 
the  wire  towards  the  ball,  and  it  will  keep  it  fixed  to  the 
conductor,  by  continually  supplying  it  with  the  resinous 
power  -j  remove  the  wire,  and  the  ball  parting  with  its 
resinous  power  to  the  conductor,  in  exchange  for  the 
vitreous,  of  which  the  conductor  has  the  greatest  quan- 
tity, it  becomes  electrified  therewith,  and  repelled  from 
the  conductor. 

Analogous  to  the  foregoing  experiment  is  the  follow- 
ing, with  a  piece  of  linen  thread,  which,  from  the  viva- 
city of  its  motions  is  termed  the  animated  thread.  For 
this  purpose  I  present  a  fine  thread  towards  the  electri- 
fied conductor,  and  it  will  fly  backwards  and  forwards 
in  a  very  pleasing  manner,  according  as  it  conveys  the 
vitreous  power  to  the  hand,  or  the  resinous  to  the  con- 
ductor, to  which  it  will  sometimes  be  affixed,  for  the 
same  reason  as  the  ball  in  the  preceding  experiment. 
Let  a  thread  hang  from  the  conductor,  and  present 
another  towards  it,  they  will  attract  and  join  each  other : 
present  any  non-conducting  substance,  as  a  brass  ball, 
near  the  two  threads  ;  the  lower  one,  or  that  held  by 
the  hand,  will  fly  from  the  ball,  while  that  affixed  to 
the  conductor  flies  towards  it.    The  vitreous  atmosphere 


ATTRACTION    AND    REPULSION.  275 

of  the  conductor  repels  the  vitreous  power  of  the  ball 
into  the  hand,  and  draws  the  resinous  power  into  it ; 
the  ball  being  therefore  resinously  electrified,  attracts 
the  upper  thread,  but  repels  the  lower  one,  which  is  in 
the  same  state  with  itself,  as  acted  on  by  the  same  causes. 
In  this  experiment,  the  afflux  and  efflux  of  the  two  pow- 
ers is,  as  it  were,  visible  to  the  senses.  You  will  find, 
that  a  contrariety  of  power  must  always  precede,  and 
is  absolutely  necessary  to  all  electrical  attraction,  and, 
indeed,  to  every  communication  of  electricity. 

I  suspend  a  small  copper  plate  from  the  conductor  ; 
underneath  this,  and  at  a  small  distance  from  it,  is  a 
larger  copper  plate  which  rests  upon  a  proper  stand,  on 
the  lower  plate  I  put  a  leaf  of  gold,  turn  the  cylinder, 
the  leaf  rises  upon  the  plate,  and  expands  itself  into  a 
perfect  plane,  with  one  corner  opposite  the  upper,  the 
other  corner  opposite  the  under  plate,  moving  quickly 
upwards  and  downwards  between  both ;  I  lower  the 
under  plate  by  degrees,  the  motion  of  the  leaf  has  now 
ceased,  and  it  remains  suspended  in  the  air  between  the 
two  plates ;  darken  the  room,  and  you  will  find  the 
leaf  supported,  as  it  were,  by  pillars  of  fire  ;  now,  as 
no  substance  can  be  thus  supported  in  equilibrio,  but 
by  the  joint  action  of  two  forces  acting  in  opposite  di- 
rections, we  have  a  clear  proof  that  there  must  be  two 
forces  thus  acting  in  the  present  instance. 

Place  some  small  paper  figures  of  men,  women,  &c. 
on  the  lower  plate,  plate  1,  fig,  7  ;  turn  the  cylinder, 
and  you  see  the  images  rise  up,  moving  from  one  plate 
to  the  other.  They  generally  move  in  an  erect  posi- 
tion, sometimes  leaping  one  upon  another,  and  moving 
in  such  a  variety  of  postures,  as  to  afford  much  enter- 
tainment. The  dance  between  them  has  so  droll  an  ap- 
pearance, if  well-conducted,  that  there  are  few  who  can 
look  upon  it  without  laughing.  I  have  before  observed 
to  you,  that  there  are  two  powers  in  electricity  ;  now, 
the  heads  of  the  puppets  are  electrified  with  the  one 
power,  and  the  feet  with  the  other;  they  are  therefore 
repelled  at  both  ends,  and  never  come  in  contact,  un- 
less the  lower  part  of  one  touch  the  higher  part  of  the 
other,  and  then  they  approach  and  stick  together. 


276  EXPERIMENTS    ON    ELECTRICAL 

The  foregoing  experiment  is  not  only  amusing,  but 
instructive  :  you  will  find  that  a  very  minute  alteration 
in  their  figure  will  make  the  images  dance  between  the 
plates,  or  remain  fixed  to  the  upper  or  under  plate  ;  for 
this  end,  the  upper  part  should  be  always  so  much  larger 
than  the  lower  part,  as  to  contain  a  part  of  the  power 
going  in,  as  much  greater  than  what  goes  out,  as  will 
be  equal  to  the  gravity  of  the  paper  :  with  a  little  prac- 
tice, you  will  be  able  to  make  one  of  them  dance  for 
some  minutes  without  touching  either  the  top  or  bottom 
plate. 

To  further  illustrate  the  affluence  and  effluence  of  the 
two  powers,  dry  the  head  of  one  of  the  images,  and  the 
power  thrown  out  from  the  conductor  cannot  enter  that 
puppet  so  freely,  as  the  contrary  power  from  the  lower 
plate  enters  the  feet,  which  are  not  so  dry  ;  the  image 
will  therefore  ascend  to  the  upper  plate,  and  remain 
there  :  reverse  the  experiment,  by  drying  the  feet  and 
wetting  the  head,  and  the  image  will  remain  fixed  to 
the  lower  plate.  These,  as  well  as  many  other  ex- 
periments, will  prove  to  you,  that  it  is  not  the  mere 
component  parts  of  the  body  that  are  acted  on  in  elec- 
trical experiments,  but  that  it  is  the  different  states  of 
the  electrical  powers  inherent  or  adhesive  to  the  body 
which  occasions  the  effects  ;  and  that,  strictly  speaking, 
it  is  the  opposite  powers  only  that  attract  each  other, 
and  that  no  substance  is  ever  attracted,  until  it  have 
acquired  a  contrary  electricity. 

If  the  two  powers  cannot  be  put  in  action,  the  expe- 
riment will  not  succeed  ;  for,  if  you  place  your  images 
on  a  clean  dry  pane  of  glass,  and  hold  this  under  the  up- 
per platef,  first  removing  the  lower  plate  and  its  stand, 
you  will  find  that  the  images  will  not  be  put  in  motion, 
notwithstanding  you  continue  to  turn  the  machine. 
Glass  does  not  transmit  the  two  electricities,  and  there- 
fore no  contrariety  in  the  electric  state  of  the  image  can 
be  occasioned,  and  consequently  it  will  not  move  back- 
wards and  forwards  between  the  two  plates.  But,  if 
any  means  be  used  to  cause  an  exchange  in  the  powers, 
as  by  holding  your  finger  under  the  glass  plate,  they 
will  be  driven  backwards  and  forwards  as  before. 


ATTRACTION    AND    REPULSION.  277 

Here  is  a  small  apparatus,  consisting  of  three  bells 
with  two  clappers  between  them,  plate  1,  jig.  6  ;  they 
are  suspended  from  a  straight  piece  of  brass,  the  two 
outer  ones  by  small  brass  chains,  the  middle  bell  and 
the  clappers  are  suspended  on  silk ;  from  the  middle 
bell  there  is  a  chain  which  goes  down  to  the  table  ;  I 
hang  the  bells  on  the  conductor,  and  turn  the  machine, 
and  the  clappers  fly  from  bell  to  bell,  affording  you  a 
pleasing  peal  by  electricity.  The  power  from  the  con- 
ductor is  conveyed  down  the  chains  to  the  exterior 
bells ;  by  means  of  the  chain,  the  exterior  bells  repel 
the  same  power  with  which  they  are  electrified  from 
the  bell  or  clapper,  which,  on  the  powers  being  thus  se- 
parated, are  driven  to  the  outer  bell  by  the  contrary 
power  which  sets  in  from  the  table,  &c.  through  the 
middle  bell  ;  the  ball  becoming  electrified  with  the 
same  power  as  the  middle  bell,  is  driven  back,  and  will 
continue  going  from  one  to  the  other,  as  long  as  the 
outside  bells  are  kept  in  an  electrified  state  by  the  ma- 
chine. 

If  you  take  hold  of  the  silk  cord  which  is  tied  to  the 
lower  end  of  the  chain  that  comes  from  the  middle  bell, 
and  thereby  raise  that  chain  from  the  table,  the  ring, 
ing  will  immediately  stop  ;  for,  silk  being  a  non-con* 
ductor,  prevents  the  afflux  and  efflux  of  the  fluids.* 

As  the  apparent  attraction  and  repulsion  of  all  light 
bodies  depend  on  the  afflux  and  efflux  of  the  separated 
powers,  I  shall  not,  in  showing  you  every  experiment, 
enter  into  a  detail  of  these  circumstances,  hoping  that 
what  I  have  already  said  will  render  that  point  suffi- 
ciently clear.  I  turn  the  machine  with  one  hand,  and 
hold  the  other  about  three  or  four  inches  from  the  end 
of  the  conductor  ;  drop  a  small  lock  of  cotton  upon 
the  hand  near  the  conductor,  and  the  cotton  immedi- 
ately jumps  from  my  hand  to  the  conductor  and  back 
again,  stretching  itself  out  both  ways  into  an  extended 


*  Eight  bells,  forming  the  octave,  are  sometimes  placed  on  a  round  board 
with  a  clapper  and  fly,  and  set  in  motion  also  by  the  electric  fluid,  which 
afford  a  pleasing  musical  effect.  — E,  Edit. 


278  METHODS    OF    IMITATING    THE 


form,  and  moving  so  quick  that  you  will  scarcely  be 
able  to  perceive  its  form. 

Here  is  a  small  toy,  somewhat  resembling  a  hog ; 
I  have  coated  it  with  ermine,  in  the  hairs  of  which  I 
have  inserted  a  few  pieces  of  cotton  pulled  out,  so  as 
to  be  of  a  considerable  length  ;  place  this  upon  the  con- 
ductor, I  turn  the  machine,  and  the  hairs  of  the  ermine 
diverge,  and  the  pieces  of  cotton  are  discharged  and 
driven  some  feet  from  the  conductor.  This  apparatus, 
by  thus  discharging  its  quills,  may  be  called  with  pro- 
priety the  electrical  porcupine  * 

Few  branches  of  philosophy  afford  so  much  enter- 
tainment as  electricity  ;  here  the  useful  and  the  agree- 
able are  intimately  blended,  and  while  you  are  investi- 
gating science,  you  are  entertained  by  the  variety  and 
beauty  of  the  experiments.  It  was  the  strong  attractive 
and  repulsive  powers  exhibited  by  electricity,  that  first 
engaged  the  attention  of  natural  philosophers  ;  by  these 
they  were  led  on  to  pursue  the  subject,  as  it  were  by 
enchantment,  and  have  been  richly  rewarded  by  dis- 
coveries both  interesting  and  important. 

A  few  more  of  the  leading  experiments,  which  have 
been  so  advantageous  to  science,  will  not  be  .  uninter- 
esting. 

METHODS    OF    IMITATING  THE  PLANETARY  MOTIONS. 

Rackstrow's  orrery  consists  of  small  glass  balls  blown 
very  thin  ;  they  are  placed  on  a  wooden  board,  and 
environed  with  circles  of  brass  wire  insulated  with  seal- 
ing-wax, or  glass,  of  such  a  height  that  the  centre  of 
the  balls  may  be  nearly  parallel  to  the  wire  circles. 
One  of  these  circles  may  represent  the  orbit  of  Saturn, 
another  that  of  Jupiter,  &c.  the  circles  being  connect- 
ed with  the  conductor  of  the  machine  by  a  wire,  and 
a  glass  sphere  placed  between  each,  the  spheres  will  per- 
form their  revolutions  round  their  orbits,  and  at  the 
same  time  acquire  a  rotation  round  their  axes. 


*  Communicated  by  Mr.  IVisset* 


PLANETARY    MOTIONS.  279 

When  the  machine  is  set  in  motion,  the  balls  will  be 
first  attracted  to  the  brass  circles,  by  which  means  the 
point  that  touches  the  brass  circle  will  become  electrified, 
and  be  immediately  repelled ;  other  parts  will  in  the  same 
manner  be  attracted  and  repelled,  by  which  means  the 
glass  ball  acquires  a  kind  of  spinning  motion  on  its  axis, 
at  the  same  time  it  must  have  a  progressive  motion  round 
the  circle. 

Provide  a  ball  of  cork  about  three  quarters  of  an  inch 
in  diameter,  hollowed  out  in  the  internal  part  by  cutting 
it  in  two  hemispheres,  scooping  out  the  insides,  and  then 
joining  them  together  with  paste.  Having  attached  this 
to  a  silk  thread,  between  three  and  four  feet  in  length, 
suspend  it  in  such  a  manner  that  it  may  just  touch  the 
knob  of  an  electric  jar,  the  outside  of  which  communi- 
cates with  the  ground.  On  the  first  contact,  it  will  be 
repelled  to  a  considerable  distance,  and  after  making  se- 
veral vibrations,  will  remain  stationary  ;  but  if  a  candle 
be  placed  at  some  distance  behind  it,  so  that  the  ball  may 
be  between  it  and  the  bottle,  the  ball  will  instantly  begin 
to  move,  and  will  turn  round  the  knob  of  the  jar,  mov- 
ing in  a  kind  of  ellipsis,  as  long  as  there  is  any  electricity 
in  the  bottle.  This  experiment  is  very  striking  though 
the  motions  are  far  from  being  regular  ;  but  it  is  remark- 
able, that  they  always  affect  the  elliptical  rather  than  the 
circular  form. 

Cut  a  piece  of  India  paper  in  the  shape  of  an  isosceles 
triangle,  which  may  be  about  two  inches  long  and  two 
tenths  of  an  inch  in  breadth ;  then  erect  a  brass  ball  of 
two  or  three  inches  diameter  on  a  brass  wire  one-sixth  of 
an  inch  in  thickness,  and  two  feet  six  inches  long,  on  the 
prime  conductor ;  electrify  the  conductor,  and  then  bring 
the  obtuse  end  of  the  piece  of  paper  within  the  atmosphere 
of  the  ball ;  let  it  go,  and  it  will  revolve  round  the  ball, 
turning  often  on  its  own  axis  at  the  same  time. 


TUMBLER    AND    BALLS. 

Put  a  pointed  wire  into  one  of  the  holes  which  are  at 
the  end  of  the  conductor,  YZ,  plate  1,  Jig.  1,  hold  a 


28Q  ELECTRICAL    FLUID 

dry  glass  tumbler  over  the  point,  then  electrify  the  con- 
ductor and  turn  the  tumbler  round,  that  the  whole  inte- 
rior surface  may  receive  the  fluid  from  the  point ;  place  a 
few  pith  balls  on  a  metallic  plate  or  the  table,  and  cover 
them  with  this  glass  tumbler  :  the  balls  will  immediately 
begin  to  have  a  rapid  motion  upwards  and  downwards, 
as  if  they  were  animated,  and  will  continue  to  move  for  a 
long  time. 

ELECTRICAL    FLUID    UNIVERSALLY    DISSEMINATED, 
AND    IN    CONTINUAL    ACTION. 

That  the  electrical  fluid  is  universally  disseminated  and 
in  continual  action,  has  long  been  the  opinion  of  those 
who  have  paid  attention  to  it.  To  prove  this  to  others, 
various  instruments  have  been  contrived  to  detect  the 
smallest  variations,  and  discover  the  minutest  signs  of  its 
existence ;  these  have  been  generally  named  electrome- 
ters ;  and  among  these,  that  described  by  the  Rev.  Mr. 
Bennett  of  Wirksworth,  may  be  considered  the  first,  as 
being  by  far  more  sensible  than  any  of  the  rest.  This  is 
one  of  them,  plate  2,  fig,  1 . 

The  foot,  A,  is  made  of  brass,  and  about  three  inches 
high,  that  you  may  move  the  instrument  by  without  touch- 
ing the  glass  ;  the  cylindrical  glass,  B,  in  which  the  gold 
leaf  is  suspended,  is  about  five  inches  high,  and  one  in 
diameter  ;  the  cap,  C,  is  made  of  brass,  and  flat  on  the 
top,  that  the  various  substances  whose  electricity  is  to  be 
examined  may  be  conveniently  placed  thereon.  The  dia- 
meter of  the  cap  is  larger  than  that  of  the  glass,  and  its 
rim  is  about  an  inch  deep,  hanging  parallel  to  the  glass, 
in  order  to  keep  ir  clean  and  dry  ;  within  this  is  another 
circular  ring  that  goes  over  the  glass,  and  is  lined  with  a 
soft  substance  to  make  it  fit  close  within  this  rim  ;  at 
the  centre  of  the  cap  a  tube  is  fixed,  wherein  the  peg  is 
placed  to  which  the  two  slips  of  gold  leaf  or  silver  are 
fastened. 

If  there  were  no  glass,  the  gold  leaf  would  be  so  agi- 
tated by  the  least  motion  of  the  air,  that  it  would  be  en- 
tirely useless.  To  prevent  the  gold  leaf  from  being  at- 
tracted and  torn  by  flying  to  the  glass,  two  pieces  of  tin 


UNIVERSALLY    DISSEMINATED.  281 

foil  are  fastened,  with  varnish  on  the  opposite  sides  of  the 
glass,  where  it  may  be  expected  to  strike  these  slips  and 
carry  off  the  superfluous  electricity,  and  increase  the  sen- 
sibility of  the  instrument. 

The  experiments  made  with  this  instrument,  not  only 
show  that  the  electrical  fluid  is  universally  disseminated, 
but  that  the  smallest  motions  in  nature  disturb  its  natural 
equilibrium,  and  separate  the  two  powers,  and  thus  mani- 
fest; it  to  our  senses.  That  this  fluid  is  the  etherial  medi- 
um, or  element  of  fire,  connected  with  some  material 
substance,  can  scarcely  now  be  doubted :  if  so,  all  the 
oscillations  in  nature  put  it  in  action,  or,  what  is  more 
probable,  it  is  the  cause  of  those  oscillations.  Mr.  Ben- 
net's  electrometer  will  prove  to  you,  that  all  solution  of 
continuity  excites  electricity:  and  I  believe  there  is  scarcely 
any  instance  where  its  action  is  manifested,  but  what  may 
be  traced  to  this  source.  In  other  words,  every  thing  that 
will  increase  one  power,  or  lessen  the  other,  produces 
electric  signs. 

Not  to  interrupt  too  much  the  progress  of  our  lectures, 
I  shall  relate  to  you  some  of  Mr.  Bennet's  experiments. 

1.  Powdered  chalk  was  put  into  a  pair  of  bellows,  and 
blown  upon  the  cap,  C,  of  the  electrometer ;  the  stream 
of  chalk  produced  vitreous  electricity,  when  the  nozzle  of 
the  bellows  was  only  six  inches  distant  from  the  cap  ;  but 
the  same  stream  electrified  it  with  the  resinous  power, 
when  at  the  distance  of  three  feet.  In  this  experiment, 
the  quality  of  the  electricity  seems  to  be  changed  by  dis- 
persing or  widening  the  stream,  and  making  it  pass 
through  a  longer  tract  of  air  ;  it  is  also  changed  by  pass- 
ing the  stream  through  a  bunch  of  fine  wires,  silks,  or 
feathers,  placed  in  the  bellows:  it  is  resinous  when  blown 
from  a  pair  of  bellows,  the  iron  pipe  being  taken  off  to 
enlarge  the  stream.  This  last  experiment  seems  to  answer 
best  in  damp  weather.  The  vitreous  electricity  generally 
remains  ;  but  in  the  resinous,  the  gold  leaf  collapses  as 
soon  as  the  cloud  of  chalk  has  passed. 

2.  A  piece  of  chalk  drawn  over  a  brush,  or  powdered 
chalk  put  into  a  brush,  and  projected  on  the  cover,  gave 
resinous  electricity.    The  electricity  was  not  permanent. 

vol.  IV.  2  o 


282  THE   FRANKLIN1AN   THEORY. 

3.  Powdered  chalk  blown  with  the  mouth,  or  a  pair  of 
bellows,  from  a  plate  placed  upon  the  cover,  gave  a  per- 
manent vitreous  electricity.  If  a  brush  be  placed  upon  the 
cover,  and  a  piece  of  chalk  be  drawn  over  it,  when  the 
hand  is  withdrawn,  the  leaf  gold  gradually  expands  with 
vitreous  electricity,  as  the  cloud  of  chalk  disperses.* 


OF    THE    FRANKLINIAN    THEORY. 

It  was  not  my  intention  at  first  to  have  particularly  no- 
ticed the  defects  of  this  theory ;  but,  as  some  late  writers 
have  endeavoured  to  conceal  its  errors,  either  by  giving 
up  some  of  the  most  essential  parts,  or  by  endeavouring 
to  bend  facts  to  accommodate  them  to  this  theory,  it  be- 
came necessary  to  point  out  a  few  of  its  defects  and  incon- 
sistencies. Many  parts  thereof,  I  conceive,  would  never 
have  been  accredited,  if  it  had  not  been  necessary  for 
party  purposes,  to  establish  the  author's  reputation  as  a 
philosopher.! 

From  hence  we  may  learn,  that  all  new  discoveries 
should  be  admitted  with  caution,  for  they  are  seldom  ac- 
curate and  free  from  errors ;  we  are  too  often  apt  to  be 
led  away  by  glimmerings  of  light,  or  even  false  views  of 
objects,  which  are  often  of  worse  consequence  than  a  to- 
tal want  of  knowledge. 

I  shall  enumerate  the  leading  principles  of  the  Frank- 
linian  system,  those  which  have  always  been  considered  in 
that  light  by  the  best  writers  and  ablest  advocates  in  favour 
of  this  system ;  and  we  may,  therefore,  justly  conclude, 
that  whosoever  gives  up  any  of  these,  so  far  abandons  the 
principles  on  which  it  is  founded. 

1.  That  the  operations  of  electricity  depend  on  the 
action  of  a  simple  homogeneous  fluid. 


*  A  curious  and  useful  machine,  called  a  doubter,  has  lately  been  invent- 
ed by  the  Rev.  Mr.  Bennet.  It  is  an  instrument  by  which  the  smallest 
quantity  of  positive  or  negative  electricity,  or  the  vitreous  and  resinoufj 
may  be  continually  doubled  till  perceivable  by  common  electrometers,  or  bv 
visible  sparks.  It  has  in  its  mode  of  action  been  improved  by  Mr.  Nichol- 
son* For  a  description  of  which,  see  our  Author's  Essays  on  Electricity, 
4th  Edition,  p.  414.....E.  Edit. 

t  Oil  this  head,  the  anecdotes  to  be  related  are  numerous  and  curious. 


THE    FRANKLINIAN    THEORY.  283 

2.  That  the  electric  matter  violently  repels  itself,  but 
attracts  all  other  matter. 

3.  That  glass  and  all  other  electrics,  though  they  con- 
tain a  great  quantity  of  electric  matter,  are  nevertheless 
impermeable  thereto. 

4.  That  by  the  excitation  of  an  electric,  the  equilibri- 
um of  the  contained  fluid  is  broken,  and  one  body  be- 
comes overloaded  with  electricity,  while  the  other  is  de- 
prived of  its  natural  share. 

5.  Electricity  is  positive  when  a  body  has  more  than 
its  natural  share ;  the  electricity  is  negative,  when  a  body 
has  less  than  its  natural  share. 

With  respect  to  the  first  position,  you  will  find,  that 
its  friends  can  bring  no  experimental  proof  to  show  the 
homogeneity  of  this  fluid,  or  its  actions;  but,  on  the  con- 
trary, they  are  forced  by  experiment  to  acknowledge  a 
contrariety  of  state  in  every  operation. 

The  second  position  is  not  only  destitute  of  proof,  but 
contradictory  to  all  experiments ;  for  the  electric  fluid 
never  attracts  matter  as  such,  but  only  on  account  of  the 
state  of  the  electric  matter  therein.  It  is  not  repulsive  of 
itself;  the  appearances,  on  which  this  idea  is  grounded^ 
are  owing  to  the  resistance  of  the  air.  Both  attraction 
and  repulsion  cease  where  the  powers  can  unite  without 
this  resistance. 

In  the  course  of  these  lectures,  you  will  see  many  ex- 
periments that  prove  the  permeability  of  glass.  The  no- 
tion of  its  impermeability  is  altogether  hypothetical,  for 
it  is  not  supported  by  any  one  determinate  experiment, 
and  is  contrary  to  every  electrical  appearance.  You  will 
find  the  ideas  of  the  Franklinians  concerning  it  quite  con- 
tradictory, some  allowing  that  its  influence  acts  through 
glass,  yet  maintaining  that  it  is  impermeable  thereto; 
others  allowing  certain  kinds  of  glass  to  be  permeable. 
Indeed,  you  may  gather  from  their  writings,  that  the  best, 
and  the  worst  vitrified  glass,  that  cold  and  warm  glass 
are  all  more  or  less  permeable.* 


•  Lyon's  Remarks  on  the  leading  Proofs  of  the  Franklinian  Theory 
Milner'a  Experiments  and  Observations  on  Electricity,  &c,  &c. 


284  THE    FRANKLINIAN    THEORY. 

The  fourth  and  fifth  principles  may  be  considered  as 
one,  for  they  are  so  intimately  connected,  as  not  to  be 
separated  :  whatever  weakens  the  proofs  of  the  one,  di- 
minishes those  of  the  other.  The  whole  Franklinian  hy- 
pothesis falls  to  the  ground,  if  the  supporters  thereof  can- 
not prove,  that  positive  electricity  is  a  superabundant 
quantity,  an  accumulation  of  the  electric  matter  in  the 
body  positively  electrified,  and  negative  electricity  a  de- 
privation of  the  quantity  of  this  matter  natural  to  a  body. 

Now,  in  the  first  place,  we  have  strong  reasons  to  sup- 
pose, that  every  electric  appearance  is  occasioned  by  the 
fluid  being  in  a  divided  and  weakened  state ;  but  putting 
this  consideration  out  of  the  question,  let  us  ask  the  sup- 
porters of  the  Franklinian  system  for  a  proof  of  this  po- 
sition, and,  strange  to  tell,  you  will  find  it  destitute  there- 
of.  You  will  find  them  only  reasoning  in  a  circle,  prov- 
ing the  thing  itself;  a  method  from  which  no  conclusion 
can  be  drawn.  Thus,  for  instance,  a  body  that  is  posi- 
tively electrified,  attracts  one  that  is  negatively  electrifi- 
ed, because  the  first  has  too  much,  and  the  other  too 
little  electricity.  Demand  how  they  prove  one  has  too 
much,  and  the  other  too  little  of  this  fluid,  and  they  an- 
swer, because  they  attract  each  other ! 

According  to  their  principles,  the  electrical  fluid  is  as 
active  when  redundant,  as  when  deficient ;  and  yet  when 
it  is  in  an  intermediate  state,  it  is  inactive.  "  Two  light 
bodies  suspended  in  contact,  in  their  natural  state  show 
no  signs  of  electricity ;  take  away  part  of  their  electric 
fluid,  and  they  repel  each  other ;  take  away  still  more, 
and  the  power  of  repulsion  increases  ;  so  that  the  more  a 
body  is  deprived  of  its  electric  fluid,  the  more  active  and 
extensive  is  its  electric  action."* 

Another  mode  by  which  they  endeavour  to  support 
their  system  is,  by  showing  that  the  electric  fluid  always 
moves  in  one  direction,  that  is,  from  the  positive  to  thfi 
negative.  Now  if  it  can  be  proved,  as  I  think  it  has  al- 
ready been  in  a  great  degree,  that  there  are  two  powers 
acting  in  contrary  directions  ;  the  negative  electricity  will 


*  Peart  on  Electric  Atmospheres. 


OF    THE    ELECTRIC    SPARK,    &C.  285 

turn  out  to  be  a  positive  active  power,  and  the  Frankli- 
nian  hypothesis  will  fall  to  the  ground,  being  destitute  of 
any  proof.  I  shall  hereafter  show  you,  that  in  the  dis- 
charge of  the  Leyden  phial,  there  is  not  only  a  power 
acting  from  the  inside  to  the  outside,  but  also  at  the  same 
instant  a  power  acting  from  the  outside  to  the  inside. 
Whosoever  allows  two  currents  acting  in  opposite  direc- 
tions, whatever  may  be  his  pretences,  gives  up  the  Frank- 
linian  theory,  and  confesses  himself  unable  to  maintain 
it  on  the  original  principles  laid  down  by  the  author,  and 
vindicated  by  Canton,  Le  Roy,  Priestly,  Becket,  Henly, 
Beccaria,  Cavallo,  &c.  &c. 

Further  proofs  of  the  inconsistency  and  weakness  of 
this  theory  will  be  shown  in  the  course  of  these  lectures  ; 
but  they  are  so  numerous,  that  to  expose  them  all,  would 
occupy  too  much  of  our  time ;  one  or  two  more  I  shall 
mention  here  ;  thus  you  will  find  Dr.  Gray,  in  the  Philo- 
sophical Transactions,  proving  Dr.  Franklin* s  account  of 
the  charge  and  discharge  of  the  Leyden  jar  to  be  errone- 
ous ;  yet  endeavouring  to  support  the  weakest  part  there- 
of. Mr.  Brooke,  in  his  Miscellaneous  Experiments,  has 
shown,  what  Mr.  Eeles  had  shown  years  before,  and  that 
by  reasoning,  a  priori  from  his  theory,  contrary  to  the 
ideas  of  the  best  judges  and  friends  of  Franklin's  theory, 
that  during  the  time  of  charging  a  Leyden  jar,  both  inside 
and  outside  have  the  same  kind  of  electricity.  Mr.  Read 
has  demonstrably  proved,  by  a  method  previously  point- 
ed out  by  Mr.  Eeles,  that  in  the  discharge  of  the  Leyden 
phial,  a  vacuum  forming  a  part  of  the  circuit,  the  electric 
matter  moves  in  contrary  directions;  yet  such  is  the  force 
of  philosophic  authority,  that  both  Mr.  Brooke  and  Mr. 
Read  endeavour  to  bend  these  facts  to  support  a  theory, 
with  which  they  are  utterly  irreconcileable. 

OF  THE    ELECTRIC    SPARK,  AND   OF   THE   INFLUENCE 
OF    POINTS. 

I  bring  the  knuckle  of  my  hand  near  the  conductor, 
and  a  spark  with  the  appearance  of  fire  passes  between  the 
conductor  and  my  hand,  and  I  feel  a  sensation  somewhat 
resembling  a  stroke  from  the  end  of  a  small  wire.     I  re- 


^ 


286  OF    THE    ELECTRIC    SPARK,    AND 

move  my  knuckle  further  from  the  conductor,  and  the 
spark  is  longer,  forming  several  curves  in  its  passage, 
having  the  exact  appearance  of  a  flash  of  lightning. 
In  this  experiment,  as  much  of  one  power  passes  from 
the  finger  to  the  conductor,  as  of  the  other  from  the 
conductor  to  the  finger.  No  spark  will  pass  unless 
there  can  be  this  interchange  of  power  ;  and  the  spark 
is  always  from  those  parts  where  the  exchange  can  be 
most  readily  effected. 

Where  the  two  powers  can  be  easily  changed,  which 
is  the  case  with  pointed  metallic  bodies,  the  equilibrium 
is  restored  silently,  and  the  conductor  is  of  course  gra- 
dually divested  of  its  electric  appearances  :  but  where 
the  surface  is  large,  and  a  contrary  state  is  not  so  easily 
produced,  the  electricities  are  as  it  were  compressed, 
and  do  not  escape  till  they  have  acquired  a  power  to 
overcome  the  intervening  space  of  air,  when  it  explodes, 
and  the  spark  is  vivid  like  lightning. 

As  soon  as  I  present  a  needle,  or  any  other  fine 
pointed  substance,  to  an  electrified  body,  the  electric 
fluid  is  urged  there  with  great  velocity,  and  the  electri- 
city is  said  to  be  drawn  off.  This  drawing  off,  however, 
does  not  extend  to  any*  great  distance,  not  even  all 
around  the  electrified  body,  if  you  keep  turning  the 
machine  at  the  same  time  that  you  present  the  point. 
To  prove  this,  place  the  wire,  to  the  end  of  which  a 
number  of  fine  threads  are  fastened,  in  one  of  the  holes 
on  the  top  of  the  conductor ;  turn  the  machine,  the 
threads  on  the  wire  diverge,  and  spread  out  like  rays 
proceeding  from  a  centre  -,  now  present  a  point  towards 
one  side  of  the  conductor,  but  at  some  distance  from  it, 
and  you  see  the  threads  on  one  side  loose  their  diver- 
gence and  hang  down,  while  those  on  the  other  side  con- 
tinue to  diverge. 

Indeed  a  point  never  acts  beyond  the  electric  at- 
mospheres, nor  does  it  act  upon  them  any  further  than  it 
is  immerged  therein,  and  then  only  so  far  as  it  can  draw 
the  resinous  power  through  them,  and  part  with  so  much 
of  the  vitreous  to  them.  Suspend  a  piece  of  down,  or  a 
small  ball,  by  silk,  so  that  it  may  hang  against  the  side 
of  the  conductor  *,  when  you  turn  the  machine,  it  will 


INFLUENCE    OF    POINTS.  287 

be  electrified,  and  fly  to  the  extreme  part  of  the  con- 
ductor's  atmosphere  ;  now  stop  turning,  and  bring  a 
point  towards  the  outside  of  the  down,  and  instead  of 
the  down  being  driven  in  towards  the  conductor,  it  will 
fly  to  the  point,  till  it  has  exchanged  powers  with  the 
point ;  then  it  will  fly  to  the  conductor,  and  be  electri- 
fied, and  again  repelled  ;  when  it  comes  to  a  certain  dis- 
tance from  the  point,  it  will  fly  towards  it,  and  be  elec- 
trified thereby,  and  so  on,  as  long  as  the  conductor  re- 
mains electrified. 

When  the  down  is  on  the  verge  of  the  electric  atmos- 
phere, immerge  your  point  in  the  atmosphere,  and  you 
will  see  the  down  approach  the  conductor  in  pro- 
portion to  the  immersion  of  the  point,  and  this  is  as  of- 
ten as  you  move  the  point  forward  to  the  conductor, 
but  no  further  ;  so  that  the  point  acts  only  while  in 
contact  with  the  electric  atmosphere. 

While  the  machine  is  turning,  and  the  point  immerg- 
ed  in  the  electric  atmosphere,  there  is  a  strong  stream  of 
the  resinous  power  flowing  in  from  the  point  to  the 
conductor,  and  that  in  proportion  to  the  vitreous  pow- 
er carried  off  by  the  point.  If  this  stream  meet  an  elec- 
trified cork  ball,  or  piece  of  down,  it  will  change  their 
powers,  and  electrify  them  with  the  resinous  power, 
by  which  means  they  are  attracted  to  the  conductor, 
and  will  be  fixed  there  by  the  continual  stream  of  the  re- 
sinous power  ;  draw  back  your  hand  to  lessen  the  resi- 
nous stream,  and  you  will  see  the  down  move  from  the 
conductor  by  degrees,  and  remain  between  the  two 
powers,  without  being  forced  to  the  conductor,  or  being 
able  to  fly  far  therefrom.  The  foregoing  experiments 
are  most  decisive  with  a  weak  electricity. 

That  the  spark  or  passage  of  the  electrical  fluid, 
from  the  prime  conductor  to  any  conducting  substance, 
depends  upon  the  greater  or  less  degree  of  difficulty  in 
producing  the  contrary  current,  is  further  evinced  by 
placing  a  point  at  the  end  of  a  piece  of  sealing-wax,  and 
at  a  small  distance  from  that  part  of  the  metal  in 
contact  with  the  sealing-wax,  paste  a  small  round  piece 
of  tin-foil,  at  a  little  distance  from  this  another  piece 
&c.  put  your  finger  upon  one  of  the  pieces  of  tin  foil 


288  OF    THE    ELECTRIC    SPARK,    AND 


that  is  farthest  from  the  metallic  point,  and  present  the 
point  towards  the  conductor,  and  you  will  find  that  it 
does  not  act  near  so  powerfully,  nor  at  so  great  a  dis- 
tance as  in  the  former  case  ;  and  if  you  approach  it  suf- 
ficiently  near  the  conductor,  a  spark  will  pass  between 
it  and  the  conductor.  Connect  your  fingers  immedi- 
ately with  the  metal,  and  you  will  not  be  able  to  obtain 
a  spark,  and  the  electric  appearances  of  the  conductor 
will  be  sooner  destroyed  by  the  quicker  interchange  of 
the  contrary  powers. 

As  the  spark,  which  explodes,  and  is  bright  in  the 
air,  becomes  silent,  faint,  and  diluted  in  vacuo  ;  so,  on 
the  other  hand,  the  electricity,  that  would  pass  imper- 
ceptibly in  air,  may  be  made  to  explode,  and  become 
bright,  by  passing  it  through  mediums  more  resisting 
than  air. 

I  place  a  metallic  vessel  nearly  filled  with  common 
oil  on  the  conductor  ;  I  shall  immerge  therein  a  point, 
from  which,  in  the  open  air,  I  can  scarcely  obtain  any 
visible  appearance,  and  you  see  that,  under  these  cir- 
cumstances, strong  sparks  pass  between  the  point  and  the 
bottom  of  the  vessel,  and  the  oil  is  thrown  into  a  vio- 
lent ebulition,  by  the  afflux  and  efflux  lof  the  two  elec- 
tricities. 

Here  is  a  pointed  wire  suspended  vertically  from  the 
conductor,  the  point  being  downwards,  from  which  I 
can  obtain  no  spark,  though  the  machine  is  acting  pow- 
erfully. I  immerge  it  in  a  small  bottle  of  oil,  and  put 
my  thumb  opposite  the  point ;  the  spark  is  loud,  the 
oil  is  curiously  agitated,  and,  if  you  examine  the  bottle, 
you  will  find  it  perforated. 

Round  this  glass  tube,  plate  2,  Jig.  2,  at  small  but 
equal  distances  from  each  other,  pieces  of  tin-foil  are 
pasted  in  a  spiral  form  from  end  to  end,  hence  it  is  called 
the  spiral  tube  ;  this  tube  is  inclosed  in  a  larger  one, 
fitted  with  brass  caps  at  each  end,  which  are  connected 
with  the  tin-foil  of  the  inner  tube.  Hold. one  end  in 
the  hand,  and  apply  the  other  near  enough  to  the  elec- 
trified prime  conductor  to  take  sparks  from  it,  a  beauti- 
ful and  lucid  spot  will  then  be  seen  at  each  separation  of 
the  tin-foil  y  these  multiply,  as  it  were,  the  spark  taken 


INFLUENCE    OF    POINTS.  289 

from  the  conductor  ;  for  if  there  were  no  break  in  the 
tin-foil,  the  electric  fire  would  pass  off  unperceived.* 

Here  are  several  spiral  tubes,  plate  2,  Jig.  4,  placed 
round  a  board,  a  glass  pillar  is  fixed  to  the  centre  of  the 
board,,  on  the  top  of  this  pillar  is  a  brass  cap,  carrying 
a  fine  steel  point,  to  support  a  wire  furnished  at  each, 
end  with  a  brass  ball,  and  nicely  balanced.  I  place  this 
under  a  ball  proceeding  from  the  conductor,  so  that  a 
continued  spark  from  this  ball  to  the  centre  of  the  sus- 
pended wire,  gives  this  wire  a  rotatory  motion,  and  the- 
balls  in  their  revolution  will  give  a  spark  to  each  spiral 
tube,  which,  in  its  passage  from  one  spot  to  the  other, 
forms  a  most  beautiful  species  of  illumination. 

Take  this  piece  of  silvered  leather,  and  put  it  round 
your  head,  and  then  stand  upon  the  stool  with  glass 
feet,  connecting  yourself  with  the  conductor  by  a  chain. 
If,  while  I  turn  the  machine,  any  one  pass  their  knuc- 
kles near  the  hoop  of  leather  moving  them  round  it, 
the  leather  will  be  beautifully  illuminated,  and  brisk 
flashes  of  electric  lightning  will  pass  between  the 
knuckles  and  conductor.  This  experiment  has  been 
termed  the  diadem  of  beatification^ 

Spirits  of  wine  may  be  easily  fired  by  the  electric 
spark  ;  to  insure  success  in  making  the  experiment,  it 
is  best  either  to  heat  the  metallic  ladle  into  which  the 
spirits  are  to  be  placed,  or  else  just  to  fire  the  spirits, 
and  blow  them  put,  a  few  seconds  before  they  are  elec- 
trified. This  experiment  may  be  performed  two  ways  : 
1.  By  placing  the  ladle  with  the  spirits  on  the  con- 
ductor, and  then  taking  a  spark  through  the  spirits, 
which  will  set  them  on  fire.  Or,  2.  If  a  person  stand 
on  the  insulated  stool,  plate  2,  Jig.  6,  and  hold  in  one 
hand  a  chain  or  wire  from  the  electrified  conductor, 


*  These  sort  of  experiments  must  be  exhibited  in  a  perfect  darkened 
room,  and  then  the  effect  is  incomparably  brilliant  and  pleasing.  Pieces  of 
tin-foil  placed  on  slips  of  glass  with  their  intervals,  forming  names  and  va- 
rious figures,  will  have  very  pleasing  effects  when  exhibited  by  the  electric 
spark.—E.  Edit. 

t  For  a  further  and  greater  variety  of  experiments  on  these  principles, 
see  my  Essay  on  Electricity,  last  edition....E.  Edit. 

VOL.  IV.  2  P 


290  OF    MOTIONS    PRODUCED    BY 

and  in  the  other  a  spoon  with  the  spirits  of  wine,  and 
another  person  on  the  floor  bring  his  knuckle,  or  a 
brass  ball,  quickly  to  the  surface  of  the  spirits,  they  will 
be  instantly  in  a  flame.  You  may  vary  this  experiment 
thus  :  3.  Let  the  electrified  person  on  the  stool  hold  the 
Spirits  as  before,  while  another  person,  standing  also 
on  an  insulated  stool,  holds  in  his  hand  an  iron  poker, 
one  end  of  which  is  made  red-hot ;  he  may  then  apply 
the  hot  end  to  the  spirits,  and  even  immerge  it  in  them, 
without  firing  them  ;  but,  if  uninsulated,  he  may  set  the 
spirits  on  fire,  with  either  the  hot  or  cold  end.  The 
spirits  could  not  be  kindled  while  the  person  was  insulat- 
ed, because  the  electrical  powers  could  not  in  that  case 
be  separated  ;  and  hot  iron,  immersed  in  spirits,  will 
very  seldom  or  never  set  them  on  fire. 

You  must  have  already  observed,  from  what  you 
have  seen,  that  when  the  quantity  of  electricity  is  small, 
it  is  incapable  of  striking  at  a  considerable  distance, 
and  the  direction  of  the  spark  appears  straight ;  but 
when  it  is  strong,  and  capable  of  striking  at  a  greater 
distance,  it  assumes  a  crooked  zig-zag  direction.  In 
every  electrified  conductor,  the  electricity  always  es- 
capes from  that  part  of  the  surface,  where  the  powers 
are  most  separated.  The  spark  is  of  a  different  colour 
according  to  the  density  ;  when  it  is  rare,  it  appears 
of  a  bluish  colour  ;  when  more  dense  it  is  purple ; 
when  highly  condensed,  it  is  clear  and  white  like  the 
light  of  the  sun.  The  middle  part  of  an  electric  spark, 
where  the  two  powers  meet,  often  appears  diluted,  and 
of  a  red  or  violet  colour,  the  ends  being  more  vivid  and 
white  ;  when  very  strong,  it  will  branch  out  and  di- 
vide into  many  parts. 


OF    MOTIONS    PRODUCED    BY    THE  ELECTRIC  STREAM. 

Whenever  there  is  an  efflux  of  one  power  of  elec- 
tricity there  is  also  an  afflux  of  the  other  power,  if  any 
conducting  substance  be  placed  so  near  and  in  such  cir- 
cumstances, as  that  it  can  be  drawn  therefrom. 


THE    ELECTRIC    STREAM.  291 

Here  is  a  brass  cross,  plate  2,  fig.  5,  supported  on  a 
point  like  a  compass- needle,  with  each  of  its  points  bent 
the  same  way  ;  place  this  upon  the  conductor,  and  as 
soon  as  I  turn  the  machine,  it  turns  with  great  rapidity, 
but  always  from  the  points,  because  the  electric  fire, 
flying  off  from  the  points,  acts  forcibly  on  the  air,  and 
is  consequently  re-acted  upon,  which  occasions  the  mo- 
tion. Take  the  fly  and  its  point,  and  hold  it  in  your 
hand  under  the  conductor,  and  it  will  turn  in  the  same 
manner,  by  a  stream  of  electricity  of  a  contrary  power 
to  that  thrown  off  from  the  conductor,  which  is  drawn 
in  from  you,  and  delivered  from  the  points  of  the  fly 
to  the  conductor.  Now  insulate  the  fly,  and  place  it 
at  the  same  distance  from  the  conductor,  and  it  will 
not  move,  because  no  electricity  can  be  drawn  through 
it ;  but  hold  a  pin  near  it,  and  the  fly  will  immediately 
begin  to  turn,  as  it  draws  a  sufficient  quantity  of  elec- 
tricity from  you  through  the  pin. 

On  this  principle,  those  who  are  desirous  of  blend- 
ing agreeable  entertainment  with  philosophy,  may  con- 
trive  a  variety  of  curious  machines,  whose  motions  may 
be  produced  by  the  electrified  stream,  which  will  afford 
much  entertainment  to  those  who  can  relish  domestic  in- 
nocent amusement;  and  by  these,  science  will  be  be- 
nefited ;  for,  to  render  any  science  familiar,  is  to  render 
it  prevalent,  and  the  more  it  prevails  in  practice,  the 
more  likely  it  is  to  produce  useful  discoveries. 

If  small  boats,  or  little  swans,  &c.  be  made  of  cork 
or  light  wood,  they  may  be  attracted,  and  made  to  swim 
in  any  direction,  by  applying  a  finger  towards  them  ;  a 
fine  needle  stuck  into  the  end  of  the  boats,  in  the  man- 
ner of  a  bowsprit,  will  cause  them  to  be  repelled  from 
the  hand  held  over  it,  and  they  may  be  steered  by  it, 
stern-foremost,  to  what  point  of  the  compass  you  please. 
The  boats  may  have  the  addition  of  sails  to  them,  and 
may  then  be  made  to  move  briskly  before  an  electrical 
gale,  from  the  point  of  a  wire  held  in  the  hand. 

The  operator  in  these  tricks  would  certainly  be  looked 
upon  as  a  magician,  if  thee  lectrical  machine  were  kept 
out  of  sight.  But  a  more  striking  sight  would  be  a 
number   of  these  boats,  with  each  of  them  a  twirling 


292  SUBDIVISION    OP    FLUIDS. 

fly,  about  an  inch  in  length,  fixed  to  the  top  of  the  mast ; 
the  hand  held  over  them  would  set  them  all  in  motion  ; 
in  the  dark  they  would  appear  as  so  many  rings  of  fire, 
moving  in  various  courses,  and  following  the  hand  in 
any  direction.* 


OF    THE    DIFFUSION    AND    SUBDIVISION    OF    FLUIDS    BY 
ELECTRICITY. 

From  experiments  made  by  Abbe  Nollet,  it  appears, 
that  electricity  augments  the  natural  evaporation  of  most 
fluids,  particularly  of  those  which  have  the  greatest  ten- 
dency to  evaporate  ;  that,  in  this  respect,  it  acts  most 
powerfully  upon  the  fluids  when  they  are  contained  in 
metal  vessels  ;  but  it  never  makes  any  fluids  evaporate 
through  the  pores  either  of  metal  or  glass.  When  fluids, 
that  are  passing  through  capillary  tubes,  are  electrified, 
the  stream  is  subdivided ;  and  if  the  tube  be  less  than 
one-tenth  of  an  inch  in  diameter,  their  motion  is  general- 
ly accelerated. 

I  suspend  this  metal  pail,  to  the  bottom  of  which  a 
capillary  tube  is  adapted,  to  the  conductor  ;  before  I  turn 
the  cylinder,  the  tube  carries  off  the  water  only  by  inter- 
rupted drops  ;  but  on  turning  the  cylinder,  and  electri- 
fying the  water,  the  dropping  from  the  tube  is  changed 
into  a  continued  stream.  On  applying  my  finger  to  the 
conductor,  the  electricity  is  interrupted,  and  the  water 
again  only  descends  in  drops  :  my  finger  taken  away, 
the  water  runs  in  a  diverging  stream  :  darken  the  room, 
and  you  perceive  a  fiery  stream  descend  from  the  tube. 
This  experiment  has  been  termed  the  electrical  jet  de  feu. 

Insulate  two  pails  with  capillary  tubes  ;  connect  one 
with  the  cushion,  the  other  with  the  conductor ;  turn 
the  machine,  and  the  water,  which  is  dispersed  into  very 
minute  particles,  when  they  are  near  enough,  is  brought 
together  by  the  effort  of  the  two  powers  to  join  each 


*  Becket  on  Electricity. 


OF    THE    LEDEN    PHIAL.  293 

other  ;  the  drops  coalesce  and  come  down  like  a  heavy 
shower  of  rain. 

I  place  a  drop  of  water  on  the  conductor,  and  turn  the 
machine.  On  presenting  my  knuckle  towards  this  drop, 
long  zig-zag  sparks  are  obtained  from  the  drop  of  wa- 
ter ;  the  drop  takes  a  conical  figure  ;  my  knuckle  is 
wetted.  The  spark  was  considerably  longer  than  could 
be  obtained  from  the  conductor  without  the  water. 

Fasten  a  piece  cf  good  sealing-wax  to  the  ball  on  the 
end  of  the  conductor,  but  place  it  in  such  a  manner  that 
it  may  be  easily  set  on  fire  by  a  taper  ;  set  it  on  fire  while 
I  turn  the  machine  ;  the  wax  becomes  pointed,  and  shoots 
out  an  almost  invisible  thread  to  a  considerable  distance. 
If  you  receive  the  filaments  on  a  sheet  of  paper,  the  pa- 
per will  be  covered  in  a  very  curious  manner  by  the  elec- 
trified wax  threads ;  the  wax  flying  to  those  places  where 
it  can  unite  with  the  contrary  power. 


LECTURE  XLVII. 


OF    THE    LEYDEN    PHIAL. 


jL)R.  Preistley  has  well  observed,  that  electricity 
has  one  advantage  over  most  other  branches  of  natural 
philosophy :  it  furnishes  matter  of  entertainment  for  all 
persons  promiscuously,  while  it  is  also  a  subject  of  impor- 
tant speculation  for  the  most  philosophic  minds.  Neither 
the  air-pump,  nor  the  orrery,  nor  any  experiments  in 
hydrostatics,  optics,  magnetism,  &c.  ever  brought  toge- 
ther so  many,  or  so  great  concourses  of  people,  as  those 
of  electricity  have  singly  done. 


294  OF    THE    LEYDEN    PHIAL. 

If  you  only  consider  what  it  is  in  objects  that  makes 
them  capable  of  exciting  that  pleasing  astonishment,  which 
has  such  charms  for  all  mankind,  you  will  not  wonder 
at  the  eagerness  with  which  persons  of  both  sexes,  and 
of  every  age  and  condition,  run  to  see  electrical  experi- 
ments. For  here  you  see  the  course  of  nature  overturned 
to  all  appearance,  and  by  causes  seemingly  inconsider- 
able. 

For  it  exhibits  to  you  bodies  rising  and  falling,  moving 
this  way  and  that,  and  suspended  by  others  contrary  to 
the  principles  of  gravitation,  and  this  by  powers  which 
have  been  put  in  action  only  by  a  very  slight  friction. 
Here  you  may  see  a  piece  of  cold  metal,  or  even  water  or 
ice,  emitting  strong  sparks  of  fire,  so  as  to  be  able  to  kin- 
dle many  inflammable  substances.  Nor  will  you  find  any 
thing  more  astonishing  than  what  I  am  going  to  exhibit 
to  you.  You  will  find  a  common  glass  jar,  after  a  little 
preparation,  capable  of  giving  a  person  such  a  violent 
sensation,  as  nothing  else  in  nature  can  give ;  and  that 
the  discharge  of  the  bottle  is  attended  with  an  explosion 
like  thunder,  and  a  flash  like  lightning. 

Before  I  enter  into  the  theory  of  charged  glass,  I  shall 
show  you  in  what  manner  it  is  charged  and  discharged. 
This  jar  is  coated  on  the  outside  and  lined  on  the  inside 
with  tin-foil,  to  about  two  inches  short  of  the  top,  which 
is  stopped  with  a  piece  of  wood,  see  plate  1,  Jig.  12.  A 
wire  passes  through  the  wooden  top,  and  is  connected 
underneath  with  two  other  wires,  which  are  bent  so  as  to 
touch  the  inside  coating  of  the  jar;  a  smooth  ball  is  fixed 
on  the  top  of  the  wire. 

To  discharge  the  jar  without  receiving  what  is  called  the 
shock.  For  this  purpose  two  instruments  have  been  con- 
trived, one  called  the  common  discharging  rod,  plate  1, 
Jig.  8,  which  is  nothing  more  than  a  semicircular  brass 
wire,  furnished  with  two  brass  balls,  one  at  the  end  of 
each  wire.  The  other,  which  is  of  very  extensive  use  in 
electrical  experiments,  is  called  the  jointed  discharging 
rod,  Jig.  9;  it  is  furnished  with  a  glass  handle;  the  wires 
are  moveable,  and  may  be  set  to  any  given  distance  by 
means  of  the  joint ;  the  ends,  to  which  the  balls  are  screw- 
ed, are  pointed. 


OF    THE    LEYDEN    PHIAL.  295 

Place  the  jar  on  the  table,  so  that  the  ball  on  the  top 
of  its  wire  may  be  about  one-eighth  of  an  inch  from  the 
ball  and  wire  placed  in  the  prime  conductor  Y  Z,  plate 
19  fig.  5.  Turn  the  machine,  and  sparks  will  fly  from 
the  ball  of  the  conductor  to  the  ball  of  the  jar :  continue 
turning  as  long  as  you  perceive  the  fire  pass  between  the 
conductor  and  ball  of  the  jar  ;  when  it  ceases,  you  may 
leave  off  turning,  and  consider  the  jar  as  charged.  This 
done,  take  hold  of  the  discharger  by  the  middle,  and  ap- 
ply one  knob  first  to  the  outside  coating  near  the  bottom, 
and  keeping  it  there,  put  the  other  to  the  ball  of  the  jar, 
and  it  will  be  discharged  of  its  fire  with  a  loud  snap,  but 
the  person  who  holds  the  discharger  feels  nothing  from 
the  discharge.* 

Now  charge  the  jar,  and  touch  the  outside  coating  with 
one  hand,  and  then  bring  the  other  to  the  ball  of  the  jar, 
you  will  then  act  the  part  of  the  wire  discharger,  and  re- 
ceive a  shock  ;  it  has  affected  you  through  your  arms  and 
breast,  and  the  phial  is  discharged.  You  may  easily  con- 
trive, by  way  of  recreation,  to  render  the  surprize  occa- 
sioned by  this  experiment  more  entertaining,  by  connect- 
ing a  chain  with  the  outside  coating,  and  concealing  it 
under  a  carpet,  at  the  same  time  connecting  another  with 
the  top,f  placing  it  in  such  a  manner,  that  a  person  may 
put  his  hand  upon  it  without  suspicion,  at  the  same  time 
that  his  feet  are  upon  the  other  wire ;  but  great  care 
should  be  taken,  that  these  shocks  be  not  too  strong, 
and  that  they  be  not  given  to  all  persons  indiscrimi- 
nately. 

When  a  single  person  receives  a  shock,  the  company 
is  diverted  at  his  sole  expense ;  but  all  contribute  their 
share  to  the  entertainment,  and  all  partake  of  it  alike, 
when  the  whole  company  form  a  circle  by  joining  hands, 
the  person  at  one  extremity  of  the  circle  touching  the 
outside-coating,  while  he,  who  is  at  the  other  extremity, 


•  With  young  beginners  this  should  be  particularly  attended  to,  as  they 
will  thus  avoid  any  disagreeable  effect  of  a  shock.  No  shock  can  be  receiv- 
ed but  from  a  charged  jar,  or  from  a  considerable  portion  of  a  glass  sur- 
face covered  with  tin-foil E.  Edit. 

t  This  may  be  conveniently  done  by  what  is  called  a  medical  electrome- 
ter, either  fitted  to  the  ball  and  wire  of  the  jar,  or  to  the  conductor. 


296  OF    THE    THEORY    OF 

touches  the  ball  of  the  jar.  All  the  persons  who  form 
this  circle  being  struck  at  the  same  time,  and  "with  the 
same  degree  of  force,  it  is  pleasant  to  see  them  all  start 
at  the  same  moment,  to  hear  them  compare  their  sensa- 
tions, and  observe  the  very  different  accounts  they  give. 

It  is  often  convenient,  sometimes  necessary,  to  know 
the  state  of  a  jar  with  respect  to  the  charge  ;  Mr.  Henly's 
quadrant-electrometer  is  the  best  instrument  yet  known 
for  this  purpose.  It  consists,  plate  1,  Jig,  17,  of  a  per- 
pendicular stem  formed  at  top  like  a  ball,  and  furnished 
at  its  lower  end  with  a  brass  ferril  and  pin,  by  which  it 
may  be  fixed  in  one  of  the  holes  of  the  conductor,  or  at 
the  top  of  a  Leyden  bottle.  To  the  upper  part  of  the  stem, 
a  graduated  ivory  semicircle  is  fixed,  about  the  middle  of. 
which  is  a  brass  arm  or  cock,  to  support  the  axis  of  the 
index.  The  index  consists  of  a  very  slender  stick,  which 
reaches  from  the  centre  of  the  graduated  arc  to  the  brass 
ferril ;  and  to  its  lower  extremity  is  fastened  a  small  pith 
ball  nicely  turned  in  the  lathe.  When  this  electrometer 
is  in  a  perpendicular  position,  and  not  electrified,  the  in- 
dex hangs  parallel  to  the  pillar  ;  but  when  it  is  electrified, 
the  index  recedes  more  or  less  according  to  the  quantity 
of  electricity. 

OF  THE  THEORY  OF  THE  LEYDEN  BOTTLE. 

I  shall  now  endeavour  to  explain  to  you  the  theory  of 
this  mysterious  bottle ;  and  you  will  here  see,  that  the 
electric  powers,  when  in  equilibrio,  do  really  condense 
each  other  ;  and  that  one  power  always  expands  in  pro- 
portion as  the  action  of  the  other  is  withdrawn,  or  in  pro- 
portion to  the  increase  of  one  power,  and  the  diminution 
of  the  other ;  and  that  when  the  bottle  is  charged,  it  is 
equally  electrified  on  both  sides,  but  with  different  powers 
of  electricity  ;  and  when  a  communication  is  made  by  a 
conductor,  the  increased  power  on  the  outside  flies  in,  and 
the  increased  power  within  flies  out,  to  make  the  powers 
equal  within  and  without. 

Place  a  Leyden  bottle  upon  the  insulated  stand,  form 
a  communication  between  it  and  the  conductor,  give  the 
machine  a  few  turns,  and  both  sides  of  the  bottle  will  be 


THE    LEYDEN    BOTTLE.  297 

electrified  with  the  vitreous  power,  as  you  may  easily 
prove,  by  touching  them  with  down  or  a  small  ball  sus- 
pended by  silk  ;  for,  when  this  is  electrified  by  touching 
the  outside,  it  will  be  also  repelled  by  the  ball  which  com- 
municates with  the  inside. 

Place  an  insulated  bottle  so  that  the  ball  may  commu- 
nicate with  the  conductor ;  let  a  wire  also  be  connected 
with  the  coating,  so  as  to  form  a  communication  with  the 
table.  Now  turn  the  machine,  and,  1,  On  applying  a 
cork  ball,  you  will  not  find  any  signs  of  electricity  in  the 
coating,  but  you  will  find  the  ball,  or  inside,  electrified 
with  the  vitreous  power.  2.  Remove  the  wire  commu- 
nicating with  the  table,  and  you  will  find  the  coating  also 
electrified  with  the  vitreous  power ;  and  this  as  often  as 
you  remove  the  wire,  till  the  bottle  be  fully  charged. 
3.  When  the  bottle  is  fully  charged,  remove  its  commu- 
nication both  with  the  conductor  and  table,  touch  the 
coating,  and  the  cork  ball  will  remain  suspended  by  it 
without  any  sign  of  being  electrified  ;  then  touch  the 
knob  of  the  bottle  with  your  hand,  the  cork  ball  will  be 
strongly  repelled  from  the  coating,  and  be  electrified  with 
the  resinous  power.  4.  Take  another  cork  ball  suspended 
by  silk,  and  touch  the  knob  of  the  bottle  therewith,  and 
the  cork  ball  will  be  electrified  with  the  vitreous  power 
and  repelled.  5.  Now  touch  the  coating  with  your  finger, 
and  the  cork  ball  will  be  repelled  much  farther  by  the 
ball ;  but  that  which  is  repelled  from  the  coating,  now 
flies  towards  it,  and  remains  at  rest,  till  you  touch  the 
knob  of  the  bottle  with  your  finger  ;  it  will  then  be  elec- 
trified as  at  first,  and  be  violently  repelled  ;  the  ball  which 
was  electrified  by  the  knob  of  the  bottle  will  now  fly  to- 
wards it.  This  change  in  the  extent  of  the  atmosphere 
of  the  different  powers,  takes  place  almost  instantaneously 
as  often  as  you  touch  the  ball  or  coating. 

Or  you  may  connect  the  knob  of  the  bottle  wdth  the 
conductor  by  a  wire,  and  suspend  a  cork  ball  to  touch 
the  conductor ;  then  touch  the  coating,  and  the  ball  will 
be  repelfed  from  the  conductor,  while  that  next  the  coat- 
ing is  attracted ;  touch  the  knob  of  the  bottle,  and  the 
ball  will  be  repelled  from  the  coating  and  attracted  by  the 

VOL.  IV.  2  Q 


298  THEORY    OF 

conductor,  and  so  on,  as  often  as  you  touch  the  knob  or 
coating. 

From  hence  it  seems  plainly  to  appear,  1.  That  the 
bottle  is  electrified  with  the  vitreous  power  on  the  inside, 
and  the  resinous  on  the  outside.  2.  That  when  the  equili- 
brium of  these  powers  is  destroyed  by  lessening  the  quan- 
tity of  one,  the  extreme  part  of  the  other  expands  itself 
into  an  extensive  atmosphere;  but  the  atmosphere  of  the 
lessened  power  is  condensed,  as  appears  by  the  cork  balls 
falling  close  to  the  conductor  and  coating.  3.  It  remains 
to  be  shown,  how  these  powers  came  to  be  thus  situate 
on  the  inside  and  outside  of  the  bottle,  or  why  they  do  not 
mix  through  the  glass  where  they  seem  to  have  the  great- 
est tendency  to  unite.  Here  it  will  be  necessary  to  con- 
sider the  separation  of  these  powers  between  the  globe 
and  the  cushion,  for  all  the  other  phenomena  are  only  con- 
sequences of  the  separation  that  takes  place  between  these. 
Now,  the  cylinder  parts  with  its  resinous  power  to  the 
cushion,  in  exchange  for  the  vitreous ;  the  conductor,  in 
like  manner  to  the  globe,  and  the  inside  of  the  bottle  to 
the  conductor ;  and  so  the  exchange  would  go  on  with 
the  next  conducting  substance,  but  that  the  bottle  gives 
Some  obstruction  to  the  passage  of  the  electrical  powers ; 
by  which  means,  the  vitreous  power,  which  passes  through 
the  glass  to  the  conckicting  substance  upon  the  outside  of 
the  bottle,  is  carried  off,  together  with  the  vitreous  power 
of  the  coating,  along  the  wire  which  communicates  with 
the  table,  in  exchange  for  an  equal  quantity  of  the  resi- 
nous power  brought  back  by  the  wire  to  the  coating  of 
the  bottle ;  till  at  length,  the  resinous  power  on  the  out- 
side is  able  to  counterbalance  the  vitreous  power  on  the 
inside,  and  thus  affords  an  opportunity  for  drawing  off 
the  resinous  power  on  the  inside  of  the  bottle  to  the  con- 
ductor ;  so  that  the  bottle  remains  a  partition  between 
the  two  powers,  and  they  cannot  change  place  through 
the  peculiarly  constructed  pores  of  the  glass,  while  their 
surfaces  are  opposed  in  such  quantities. 

For,  when  the  junction  is  made  in  the  open  air,  or 
when  their  surfaces  are  opposed  in  any  quantity,  it  is  not 
done  without  violence,  occasioning  a  loud  noise  and  a 
flash  of  fire,  while  bursting  through  to  meet  each  other; 


THE    LEYDEN    BOTTLE.  299 

for,  wherever  the  different  powers  unite  in  any  quantity, 
they  are  much  condensed. 

The  violent  convulsion  felt  through  the  body  by  com- 
pleting a  circle  with  the  hands,  is  only  occasioned  by  the 
different  powers  passing  in  opposition  through  the  same 
nerves.  For,  if  one  person  touch  the  coating,  and  an- 
other  the  top  of  the  bottle,  the  bottle  will  be  discharged 
without  giving  either  of  them  the  shock.  Now,  it  is  very 
clear,  that  as  much  fire  passed  through  either  of  them, 
as  if  each  had  singly  discharged  the  bottle.  But  in  this 
case,  the  fire  is  diffused  through  all  parts  of  the  body, 
and  the  fire  brought  in  is  drawn  from  all  parts  of  the 
body ;  and,  consequently,  the  nerve  cannot  be  so  much 
shocked  as  in  the  former  case,  when  all  the  fire  passes  in 
opposition  through  the  same  nerves. 

EXPERIMENTS    ILLUSTRATING  THE    THEORY  OF  THE 
LEYDEN  PHIAL. 

Charge  an  insulated  bottle,  remove  it  from  the  con- 
ductor, and  let  a  cork  ball  suspended  by  silktiang  against 
the  outside  of  the  bottle ;  touch  the  outside  or  coating 
with  your  finger,  the  ball  will  not  be  affected  ;  but,  touch 
the  knob  of  the  bottle,  and  the  ball  immediately  flies  off, 
strongly  electrified  with  the  resinous  power ;  and  thus 
you  may  go  on  for  a  considerable  time,  altering  the  ba- 
lance of  the  powers  within  and  withoutside  the  bottle, 
by  alternately  touching  the  top  and  the  bottom  of  the 
bottle.  The  defenders  of  Franklin's  system  will  hardly 
say,  it  is  the  return  of  the  positive  electricity  which  elec- 
trifies the  ball  negatively.  The  fact  is,  that  when  you 
touch  the  top,  you  take  a  spark  of  the  vitreous  power 
from  the  inside,  and,  in  exchange,  give  as  much  of  the 
resinous  power  thereto  ;  by  these  means,  the  force  of  the 
vitreous  power  within  the  bottle  is  lessened,  which  leaves 
the  resinous  power  on  the  outside  in  greater  quantity  than 
the  vitreous  withinside,  and  consequently  at  liberty  to 
exchange  with  any  non-electric  in  contact  with  it ;  and 
thus  the  ball  becomes  electrified  with  the  resinous  power. 

Charge  a  bottle  fully,  and  remove  the  wire  from  the 
table,  and  make  the  coating  communicate  with  the  con- 


300  EXPERIMENTS    ILLUSTRATING    THE 

ductor  instead  of  the  knob,  and  then  turn  the  machine, 
and  the  resinous  power  with  which  the  coating  is  electri- 
fied becomes  covered  with  the  vitreous  power,  and  you 
may  take  as  many  sparks  from  it  as  you  please,  without 
making  any  change  in  the  charge  of  the  bottle  ;  for,  when 
you  stop  turning,  and  remove  the  communication  with 
the  conductor,  and  touch  the  outside  of  the  coating  with 
the  finger,  all  signs  of  the  vitreous  power  disappear ;  and 
when  the  circle  is  completed,  the  bottle  is  discharged  with 
as  loud  a  report  as  it  would  have  made  before  you  appli- 
ed the  conductor  to  the  coating  ;  for,  the  vitreous  power 
within  the  bottle  being  undisturbed,  kept  an  equal  quan- 
tity of  the  resinous  power  firmly  fixed  to  the  outside  of 
the  bottle. 

But  the  case  is  different  when  you  give  the  vitreous 
power  from  the  inside  an  opportunity  to  escape.  Thus, 
when  the  bottle  is  fully  charged  as  before,  remove  the 
wire  that  communicates  with  the  table,  and  bring  the  coat- 
ing in  connexion  with  the  conductor  ;  after  a  turn  or  two 
of  the  cylinder,  take  a  spark  from  the  ball  of  the  bottle, 
and  you  will  find  that  it  will  fly  to  a  considerable  dis- 
tance, often  double  the  distance  at  which  you  can  draw 
a  spark  from  the  conductor,  because  the  vitreous  power 
covering  the  resinous  power  on  the  coating,  lessens  the 
action  on  the  vitreous  power  within  the  bottle,  and  there- 
fore leaves  that  power  greater  freedom  to  fly  off;  but  as 
you  go  on  taking  sparks,  they  gradually  lessen,  because 
after  a  few,  the  vitreous  power  in  the  bottle  is  lessened, 
and  the  resinous  power  within  increased  by  the  quantity 
received  in  exchange  on  every  spark  ;  and  thus  by  a  few 
sparks  the  bottle  is  discharged  ;  but  if  you  go  on  to  take 
more  sparks,  the  bottle  will  be  re-charged  with  the  resi- 
nous power  withinside,  instead  of  the  vitreous,  with  which 
it  was  before  charged. 

Again,  suppose  fifty  turns  of  the  cylinder  will  charge 
your  bottle,  make  only  twenty-five,  and  then  remove  the 
communication  between  the  coating  and  table ;  and  as 
you  turn  on.  whether  you  continue  the  communication 
from  the  conductor  to  the  top  of  the  bottle,  or  shift  it 
to  the  coating,  you  will  find  the  bottle  electrified  on  both 
sides  with  the  vitreous  power ;  remove  the  bottle  from 


THEORY  OF  THE  LEYDEN  PHIAL.      301 

the  conductor,  and  then  discharge  it  with  an  insulated 
discharger,  and  you  will  find  the  bottle  still  electrified, 
both  within  and  without,  with  the  vitreous  power ;  but 
this  electricity  will  disappear,  by  touching  either  the  ball 
or  coating  writh  your  ringer. 

To  illustrate  further  the  reciprocal  exchange  of  the 
electric  powers,  here  is  an  insulated  bottle  with  a  wire 
proceeding  from  the  bottle,  at  right  angles  to  which  is  a 
wire  for  receiving  a  needle  with  reversed  points ;  make 
the  top  of  the  bottle  communicate  with  the  conductor, 
and  all  the  time  the  bottle  is  charging  the  needle  will 
turn ;  but  when  the  bottle  is  chatged,  the  needle  stops. 
Then  touch  the  top  of  the  bottle  with  your  finger,  or  any 
conductor,  and  the  needle  will  turn  till  the  bottle  is  dis- 
charged. Now,  while  the  bottle  is  charging,  if  you  touch 
the  needle  with  a  piece  of  bog-down,  or  a  cork  ball,  sus- 
pended by  silk,  you  will  find  it  electrified  by  the  vitreous 
power,  w:hich  flies  off  in  exchange  for  the  resinous  power 
drawn  in  from  the  air  to  the  outside  of  the  bottle ;  and 
while  the  bottle  is  discharging,  if  you  apply  the  dowrn  or 
ball  in  the  same  manner  to  the  needle,  you  will  find  them 
electrified  with  the  resinous  power,  which  flies  off  from 
the  outside  of  the  bottle  in  exchange  for  the  vitreous 
power  drawn  in  through  the  points  from  the  air ;  while 
the  vitreous  power  from  the  inside  of  the  bottle  makes 
the  same  exchange  for  the  resinous  power  through  your 
finger,  to  make  these  different  powers  equal  to  each 
other,  withinside  and  withoutside  the  bottle. 

Place  two  Levden  bottles  on  an  electric  stand,  with 
their  coatings  in  contact ;  and  while  you  charge  one  from 
the  conductor,  let  a  person  on  the  floor  touch  the  top  of 
the  other  bottle  with  his  finger  ;  you  wall  find  the  first  bot- 
tle charged  with  the  vitreous  power  inside,  and  the  second 
with  the  resinous  power  inside.  Now,  the  exchange  here 
is  evident ;  for,  while  the  resinous  power  from  the  inside 
of  the  first  bottle  changes  place  with  the  vitreous  thrown 
in  from  the  conductor,  the  vitreous  from  the  coating 
changes  place  for  so  much  of  the  resinous  from  the  coat- 
ing of  the  second  bottle ;  and  the  vitreous  in  that  bottle 
changes  place  for  so  much  of  the  resinous  power  drawn 
in  through  the  man  on  the  floor. 


302  EXPERIMENTS    ILLUSTRATING    THE 

I  charge  a  Leyden  phial,  and  set  it  aside  to  be  in  readi- 
ness to  ascertain  the  state  of  another.  I  now  take  the  bot- 
tle with  the  projecting  wires,  plate  1,  Jig.  10,  unscrewing 
the  ball  from  the  wire  at  the  coating,  and  suspending  a 
pair  of  pith  balls  therefrom  This  done,  I  bring  the  knob 
of  the  bottle  to  the  conductor  ;  I  work  the  machine,  and 
the  phial  will  charge  slowly,  and  the  balls  will  repel  each 
other  ;  while  I  am  turning  and  the  bottle  charging,  bring 
the  knob  of  the  first  bottle  towards  the  balls,  and  they 
will  be  repelled  thereby.  This  plainly  proves,  that  the 
outside  of  the  bottle  is  electrified  vitreously  while  it  is 
charging,  that  is,  with  the  same  electricity  as  the  inside. 

Let  us  discharge  the  bottle  with  the  projecting  wires, 
and  charge  it  again  as  before,  and  you  will  still  find,  that 
whilst  it  is  charging,  the  balls  will  fly  from  the  knob  of 
the  first  bottle  ;  I  cease  turning,  and  the  balls  cease  to  re- 
pel each  other ;  they  now  touch  each  other,  and  again 
recede,  but  with  a  contrary  electricity,  for  they  are  now 
attracted  by  the  knob  of  the  first  bottle.  This  shows  that 
the  difference  between  the  two  sides  cannot  appear,  while 
they  are  charging,  or  while  vitreous  electricity  is  forced 
through  the  jar. 

Let  us  now  discharge  both  bottles,  in  order  to  try  an- 
other experiment,  to  determine  the  state  of  the  outside  du- 
ring the  charge.  I  first  put  the  ball  on  the  end  of  the 
wire  of  the  bottle  with  the  projecting  wires,  bring  the 
knob  thereof  to  the  conductor,  holding  the  knob  of  the 
first  bottle  against  the  coating  of  that  with  the  projecting 
wires ;  by  working  the  machine,  both  will  be  charged. 
As  soon  as  they  are  pretty  well  charged,  and  while  the 
machine  is  working,  remove  the  first  bottle  from  the 
other  ;  after  this  is  removed,  cease  working  the  machine 
as  soon  as  possible.  I  now  connect  by  a  wire  the  two  out- 
side coatings,  and  bring  the  balls  to  each  other.  If,  while 
the  bottles  were  charging,  the  outside  of  that  with  pro- 
jecting wires  had  been  resinously  electrified,  the  inside  of 
the  second  would  have  been  so  also  ;  and  on  their  being 
thus  brought  together,  both  bottles  would  be  discharged  : 
but  this  is  not  the  case,  for  the  insides  of  both  are  charg- 
ed with  the  vitreous  electricity,  the  coating  having  ex- 
changed powers  with  the  bottle  charged  thereby.     This 


THEORY  OF  THE  LEYDEN  PHIAL.      303 

Experiment  shows,  that  to  consider  one  side  of  a  phial  to 
be  positive,  and  the  other  negative,  at  the  time  they  are 
charging,  is  erroneous. 

The  criterion  of  the  resinous  and  vitreous  electricity, 
as  determined  by  the  light  on  metallic  points,  gives  full 
evidence  in  favour  of  Mr.  Eeles's  theory,  while  it  is  di- 
rectly opposed  to  that  of  Dr.  Franklin.  For  you  will  here 
find,  that  during  the  time  that  the  bottle  is  charging,  the 
outside  exhibits  the  sign  of  vitreous  electricity.  To  prove 
this,  I  place  a  pointed  wire  at  the  end  of  the  conductor, 
and  place  this  apparatus  with  the  sliding  wire,  plate  1 , 
Jig.  11,  on  one  of  the  insulated  stands,  first  removing 
the  bottle  therefrom  ;  I  then  unscrew  the  balls  from  the 
projecting  wires  of  the  remaining  insulated  bottle,  and 
also  from  the  sliding  wire,  which  leaves  the  points  that 
were  under  the  bottle  exposed  and  ready  for  our  opera- 
tions. 

Things  being  thus  prepared,  I  place  the  insulated  bot- 
tle so  that  the  point,  from  the  inside,  may  be  about  half 
an  inch  distance  from  that  in  the  conductor,  and  let  one 
of  the  points  of  the  sliding  wire  be  at  the  same  distance 
from,  and  opposite  to,  the  point  projecting  from  the  out- 
side of  the  insulated  bottle.  I  now  turn  the  machine,  and 
as  soon  as  the  charge  begins,  the  signs  of  the  electrici- 
ties are  visible,  illuminating  the  points  of  the  interrupted 
circuit.  The  point  on  the  prime  conductor  gives  the 
brush  or  sign  of  vitreous  electricity;  the  sign  on  the  point 
opposed  to  it  on  the  knob  of  the  bottle  is  resinous.  The 
light  from  the  wire,  that  projects  from  the  coating  of  the 
bottle,  is  the  brush,  or  vitreous  ramified  light ;  but  that 
of  the  point  opposed  thereto  is  the  star,  or  sign  of  resi- 
nous electricity,  as  they  ought  to  be,  according  to  Mr. 
Eeles's  theory,  not  "  contrary  to  the  kind  or  source  of 
electricity  from  whence  they  proceed,"  which  is  the  case 
on  the  principles  of  the  Franklinian  theory.* 


*  Rcid's  Summary  View  of  Spontaneous  Electricity,  p.  81,  82 


[     304     ^ 


EXPERIMENTS   SHOWING  THAT  TN  THE  DISCHARGE 
THE   LEYDEN   JAR,    THE   TWO  ELECTRICITIES   RU! 
INTO  UNION  FROM  OPPOSITE  DIRECTIONS. 

The  three  first  experiments  I  shall  mention  to  you, 
were  made  by  Mr.  Atwood,  of  Cambridge,  and  were  de- 
scribed by  him  in  the  Analysis  of  a  Course  of  Lectures, 
which  he  read  at  Cambridge. 

He  slightly  charged  the  surfaces  of  an  electric  insulated 
plate,  and  discharged  it  through  an  interrupted  circuit, 
formed  of  needles  placed  in  a  groove  of  wax,  the  distance 
between  the  needles  being  very  small ;  the  two  powers 
were  visible,  on  the  discharge  illuminating  the  points  of 
the  interrupted  circuit,  each  power  extending  farther  from 
the  surface  contiguous  thereto,  in  proportion  to  the 
strength  of  the  charge ;  but  when  this  was  sufficiently 
strong  to  make  the  illuminations  proceeding  from  each 
side  meet,  there  was  an  explosion  of  the  whole  charge. 
The  length  of  the  interrupted  circuit  made  by  Mr.  Au 
wood  was  twelve  feet. 

Mr.  Atwood  charged  a  cylindrical  plate  of  air,  under 
the  receiver  of  an  air-pump,  and  found  that  the  more  the 
air  was  exhausted  from  between  the  surfaces,  the  more 
readily  and  easily  the  powers  united. 

He  made  an  exhausted  receiver  part  of  the  electric  cir- 
cuit, and  on  using  such  charges  as  were  not  sufficient  to 
form  an  explosion,  he  found  the  electric  light  proceeding 
in  opposite  directions  from  the  parts  communicating  with 
the  vitreous  and  resinous  surfaces. 

When  a  Leyden  jar  is  charged  but  slightly,  if  you  touch 
the  coating  with  a  finger  of  one  hand,  and  at  the  same 
time  bring  a  finger  of  the  other  to  the  knob  of  the  jar, 
you  will  receive  a  smart  blow  upon  the  tip  of  each  finger, 
but  the  sensation  reaches  no  higher.  Charge  the  jar  a 
degree  higher,  and  you  will  feel  a  stronger  blow,  reach- 
ing to  the  wrists,  but  no  further.  When  it  is  charged 
somewhat  higher,  a  severe  blow  will  be  received,  but 
which  will  not  reach  beyond  the  elbows.  Lastly,  when 
the  jar  is  strongly  charged5  the  shock  will  be  perceived 


FROM    OPPOSITE    DIRECTIONS.  305 

at  the  wrists  and  elbows,  but  the  principal  blow  is  felt  at 
the  breast,  as  if  a  blow  from  each  side  met  there.  This 
plain  and  simple  experiment  of  Mr.  Symmer  obviously 
suggests  the  existence  of  two  currents  proceeding  in 
contrary  directions,  accords  with  those  of  Atwood  and 
Volta*  and  is  in  direct  contradiction  to  that  assertion  of 
the  Franklinians,  "  that  the  same  quantity  of  electric 
matter,  which  is  thrown  upon  one  of  the  surfaces  of 
glass  in  charging,  is  driven  from  the  other,  and  that  in 
the  discharge  this  accumulated  quantity  is  restored  to 
the  deficient  surface." 

When  a  jar  is  charged  very  high,  the  electricities 
will  often,  in  their  endeavours  to  unite,  force  a  hole 
through  the  jar,  and  push  out  the  coating  on  both 
sides,  sometimes  melting  it ;  the  burr  of  tin-foil  pro- 
truded from  the  middle  of  the  glass  strongly  indicates, 
that  the  two  electricities  meet  at  the  middle  of  the 
glass  ;f  there  also  the  greatest  effort  is  exerted. 

Mr.  Read  says,  that  when  the  charge  for  melting  of 
fine  wire  is  of  a  proper  intensity  to  melt  it  into  fine 
globules,  he  has  observed  the  wire  to  be  of  a  paler  red 
heat  in  the  middle  than  at  the  extremities,  and  the 
melting  to  begin  at  the  middle,  leaving  a  portion  un- 
melted  at  each  end.  At  other  times,  though  less  fre- 
quent, the  wire  was  observed  to  be  of  a  more  glowing 
heat  in  two  parts,  and  these  were  generally  near  the 
middle.  These  effects  clearly  show,  that  the  vitreous 
and  resinous  electricities  of  the  charged  jar  meet  in 
great  force  near  the  middle  of  the  wire,  which  is  di- 
rectly contrary  to  the  leading  notions  of  Franklin's 
theory. 

The  remarkable  tendency  of  the  divided  fluids  to 
unite,  is  often  perceived  in  a  full-charged  Leyden  bot- 
tle, at  the  upper  edge  of  the  outside  coating,  and  at  the 
edge  of  the  cork  on  the  neck  of  the  bottle ;  rays  of 
light  darting  from  each,  and  soliciting  as  it  were  a 
union,  and  sometimes  forming  an  actual  circuit. 


*  See  my  Essay  on  Electricity, 
t  Read  on  Spontaneous  Electricity,  &c.  p.  44„ 
VOL,  IV,  3  R 


[     306     ] 


THE  SAME  PRINCIPLES  CONFIRMED  BY    THE    APPEAR- 
ANCES   OF    THE  ELECTRIC    SPARK. 

The  electric  spark  appears  of  different  colours  ac- 
cording to  its  density  ;  when  it  is  rare,  it  appears  of 
a  bluish  colour  ;  when  more  dense,  it  is  purple  ;  when 
highly  condensed,  it  is  clear  and  white,  like  the  light  of 
the  sun. 

The  middle  part  of  an  electric  spark  often  appears 
diluted,  and  of  a  red  or  violet  colour,  while  the  ends 
are  vivid  and  white  ;  this  appearance  cannot  be  account- 
ed for  by  the  theory  of  a  single  fluid  moving  in  one  di- 
rection, but  is  a  proof  of  two  currents  moving  in  op- 
posite directions ;  the  electric  signs  growing  weaker 
where  the  two  powers  unite.  Mr.  Read*  has  well, 
and  I  believe  first  observed,  that  the  place  of  re-union 
is  much  less  luminous,  and  in  some  cases  quite  dark ; 
and  that  this  is  the  natural  effect  of  the  union  of  the 
two  electricities ;  at  that  point  the  distinctions  of  the 
vitreous  and  resinous  cease,  and  there  the  electric  light 
vanishes.  These  appearances  are  best  observed,  by 
viewing  in  the  dark  a  strong  electric  spark  passing  be- 
tween two  bodies  electrified  with  contrary  electricities. 

Though  the  appearances  of  the  electric  light  on  a 
point  and  ball,  as  well  as  of  the  electric  spark,  are  sub- 
ject to  many  variations,  yet  there  are  certain  signs  ge- 
nerally peculiar  to  each  kind  of  electricity.  For  in- 
stance, if  the  resinous  part  of  a  spark  be  small,  or  what 
has  been  usually  termed  the  luminous  globule,  then  the 
middle  part  is  generally  of  a  purplish  colour.  When 
ramified  rays  issue  from  the  vitreous  part,  then  the 
resinous  is  more  extended,  stretching  out  towards 
the  vitreous.  When  the  vitreous  and  resinous  elec- 
tricities strike  into  each  other  in  dense  light,  in  va- 
rious parts  of  the  intermediate  space,  then  their  exact 
place  of  union  is  generally  observable  by  a  dark  spot. 
Mr.  Read  with  propriety  considers  the  loss  of  light  in 


*  Read's  Summary  View  of  Spontaneous  Electricity,  p.  47. 48,  and  4P» 


ELECTRIC    LIGHT  IN    VACUO.  307 

any  part  of  an  electric  spark,  whether  total  or  partial, 
as  the  immediate  effect  and  constant  sign  of  the  re- 
union of  the  two  electricities. 

Mr.  Read  observes,  that  whether  the  resinous  light 
•assumes  the  figure  of  an  oblong  flame,  or  of  a  luminous 
globule,  in  either  case  the  vitreous  light  is  seen  to  ap- 
proach, and  unite  with  it  in  all  possible  directions.  The 
effect  of  a  vitreous  surface  appears  to  extend  farther 
than  that  of  a  resinous  surface. 


THE  OPPOSITE  DIRECTIONS  OF  THE  TWO  ELECTRI- 
CITIES PROVED  BY  THE  APPEARANCES  OF  THE 
ELECTRIC    LIGHT    IN    VACUO. 

Though  I  have  already  pointed  out  to  you  some  ex- 
periments in  vacuo  that  illustrate  this  point,  yet  those 
of  Mr.  Read  are  so  decisive,  that  not  to  mention  them, 
would  be  to  deprive  you  of  essential  information  on  this 
subject. 

For  these  experiments,  Mr.  Read  used  a  glass  tube, 
three  feet  seven  inches  long,  furnished  at  each  end  with 
a  brass  cap,  one  of  the  caps  being  fitted  to  the  plate  of 
the  air-pump ;  from  each  cap  a  brass  wire,  on  which 
was  a  brass  ball,  projected  within  the  tube  ;  when  this 
tube  is  sufficiently  rarefied,  the  charge  of  a  Leyden 
phial  will  readily  pass  through  the  rarefied  air. 

In  making  these  experiments,  you  must  only  slightly 
charge  your  Leyden  jar  ;  for,  if  the  charge  be  strong 
enough  to  force  the  whole  contents  swiftly  through  the 
rarefied  air,  the  motion  of  the  fluid  is  too  rapid,  and 
the  light  to  resplendent  to  permit  an  exact  observation 
of  its  appearance. 

On  making  the  discharge  in  the  dark,  you  will  per- 
ceive, the  moment  the  circuit  is  formed  for  that  purpose, 
a  light  within  the  tube,  but  chiefly  at  each  end.  These 
lights  are  of  the  contrary  kinds  of  electricity,  and  ac- 
cord with  the  side  of  the  bottle  to  which  they  are  con- 
nected. You  may  sometimes  perceive  the  two  lights  to 
have  a  manifest  tendency  to  meet  near  the  middle  of  the 
resisting  medium.     Mr.  Read  has  observed  the  light: 


S08  OPPOSITE    DIRECTIONS   OF    THE 

within  the  tube  to  be  considerably  diminished  in  splen- 
dour where  the  two  powers  unite ;  and  so  it  ought  to 
be,  for  when  the  two  electricities  unite  and  regain  their 
natural  state,  they  loose  their  light,  for  it  is  only  in  a 
divided  state  that  the  electrical  matter  is  luminous  ;  the 
same  appearances  are  produced  in  the  tube  by  the  sim- 
ple spark,  that  is,  the  contrary  electricities  are  observed 
at  each  end*. 

But  this  is  still  further  confirmed  by  a  new  observa- 
tion and  decisive  experiment  of  Mr.  Read's.  He  sus- 
pended his  exhausted  tube  in  a  horizontal  direction, 
by  silk  lines  from  the  ceiling  ;f  one  end  was  placed  so 
as  to  receive  an  electric  spark  from  the  conductor  of 
his  machine  ;  at  half  an  inch  from  the  other  end,  there 
was  a  metallic  communication  with  the  earth. 

On  turning  the  machine,  the  tube  is  filled  with  elec- 
tric light,  and  continues  so  long  as  the  action  of  the 
machine  is  continued.  Mr.  Read  first  observed,  that 
the  instant  the  supply  ceases,  the  light  divides  near 
the  middle  of  the  tube,  and  flies  back  to  the  ends  ;  ful- 
ly evincing  the  truth  of  Mr.  Eeles's  theory,  by  showing 
that  the  light  within  the  tube  is  not  all  of  one  kind 
of  electricity  ;  the  tube  includes  both  electricities  in  one 
appearance  of  light.  The  moment  the  action  of  the 
machine  is  discontinued,  the  afflux  and  efflux  cease, 
and  each  electricity  returns  to  its  own  place,  where  the 
separation  first  commenced. 

To  ascertain  beyond  dispute,  that  the  light  within  this 
kind  of  exhausted  tube  consisted  of  vitreous  and  resi- 
nous light,  he  made  the  following  experiment.  The 
glass  tube  was  suspended  as  before,  and  two  Leyden 
phials  in  a  horizontal  position,  but  lying  on  glass  stands, 
were  placed  one  at  each  end  of  the  tube,  with  their  me- 
tallic knobs  nearly  in  contact  with  the  metallic  caps  of 
the  glass  tube.  In  this  disposition  of  the  apparatus,  the 
coating  of  one  bottle  is  to  receive  a  spark  from  the 


*  Read)  p.  51,  52,  53. 

t  It  is  more  convenient  to  insulate  the  glass  tube  or  luminous  conductor 
by  glass  pillars,  as  \>late  1,  Jig.  13. 


TWO    ELECTRICITIES    PROVED.  309 

prime  conductor,  and  the  coating  of  the  other  a  spark 
from  the  metallic  communication  with  the  earth. 

On  turning  the  cylinder,  sparks  were  perceived  to 
pass  in  the  four  intervals  of  air,  and  at  the  same  time 
a  luminous  appearance  within  the  glass  tube.  On  re- 
moving the  bottles,  and  examining  their  charges,  they 
were  found  to  correspond  with  the  lights  within  the  tube, 
to  which  they  were  opposed.  One  bottle  was  vitreous- 
ly,  the  other  resinously  electrified.* 

These  experiments  clearly  prove,  that  there  is  at  the 
same  time  one  power  acting  from  within,  towards  the 
outside  of  a  charged  Leyden  phial,  and  another  power 
acting  from  the  outside  towards  the  inside  of  the  phial ; 
and  thus  concur  with  others  in  showing,  that  electrici- 
ty consists  of  two  distinct  positive  powers  acting  in  con- 
trary directions,  and  towards  each  other. 

Here  is  a  glass  coated  flask  from  which  the  air  has 
been  exhausted,  that  you  will  find,  on  trial,  to  illustrate 
pleasingly  the  theory  of  electricity,  plate  l,Jig.  14. 

From  the  experiments  on  the  theory  of  the  Leyden 
bottle,  I  shall  now  proceed  to  some  entertaining  ones 
with  the  same  instrument.  No  electrical  experiments 
answer  so  well  the  joint  purposes  of  pleasure  and  sur- 
prize, as  those  that  are  made  with  the  Leyden  phial. 
And  philosophers  are  so  far  from  laughing  at  the  asto- 
nishment of  the  ignorant  at  these  experiments,  that  they 
cannot  help  viewing  them  with  equal,  if  not  greater 
astonishment  themselves.  There  are  indeed,  as  Dr. 
Priestley  has  observed,  many  electricians  still  living, 
who  can  well  remember  the  times  when,  with  respect 
to  these  things,  they  themselves  would  have  ranked 
among  the  same  ignorant  and  staring  vulgar. 

What  would  the  ancient  philosophers  have  said,  what 
would  Newton  himself  have  said,  to  see  the  present 
race  of  electricians  imitating,  in  minature,  all  the  known 
effects  of  lightning  ;  nay,  essaying  to  disarm  the  thun- 


Mr.  Relet  from  his  theory,  pointed  out,  in  1758,  the  mode  of  making 
mis  experiment,  ind  foretold  what  would  be  the  result.  This  is  only  one 
among  many  i:  s  ances,  where,  in  reasoning  a/morf,  he  has  pointed  out 
results,  that  the  Frankliniansof  the  day  denied. 


310  OPPOSITE    DIRECTIONS    OF    THE 

der  of  its  power  of  doing  mischief,  and  without  any  appre- 
hension of  danger  to  themselves,  drawing  lightning  from 
the  clouds  into  a  private  room,  and  amusing  themselves 
at  their  leisure,  by  performing  with  it  all  the  experiments 
that  are  exhibited  by  electrical  machines  ?  • 

One  cannot  indeed  consider  the  present  improved  state 
of  philosophy,  without  indulging,  with  the  Rev.  Mr.  Jones, 
a  wish  to  exhibit  to  the  wise  men  and  heroes  of  ancient 
times  some  of  those  wonderful  improvements  which 
are  now  so  familiar  to  us,  but  were  totally  unknown  to 

them.  . 

I  would  give,  says  he,  to  Aristotle  the  electrical  shock: 
I  would  carry  Alexander  to  see  the  experiments  upon 
"Woolwich  warren,  and  exhibit  to  him  all  the  evolutions 
and  firings  of  a  modern  battalion  :  I  would  show  to  Ju- 
lius Ctesar,  the  invader  of  Britain,  an  English  man  oi 
war;  to  Archimedes  a  steam  engine,  and  a  reflecting 

telescope.* 

Entertaining  electrical  experiments  are  not  withou! 
their  use,  for  they  give  even  to  philosophic  minds  an  op. 
portunity  of  examining  things  under  different  points  o: 
view,  and  often  arest  the  attention  to  objects  which  hac 
before  escaped  their  notice. 

To  strike  a  hole  through  a  card.  Having  charged  youi 
jar,  hold  a  card  with  one  hand  close  to  the  coating  of  th< 
jar'near  the  bottom,  then  apply  one  knob  of  the  discharg 
ing  rod  to  the  card,  and  the  other  to  the  ball  of  the  bot 
tie,  and  the  discharge  will  pass  through  the  card,  anc 
will  make  a  hole  through  it  with  a  burr  on  each  side,  o 
which  I  shall  take  more  notice  hereafter  ;  it  will  have  i 
strong  sulphureous  smell. 

If  die  experiment  be  made  with  two  cards  instead  o 
one,  the  cards  must  be  placed  but  at  a  very  small  distano 
from  each  other ;  each  of  the  cards,  after  the  explosion 
will  be  found  pierced  with  one  or  more  holes,  and  eacl 
hole  will  have  burrs  on  both  surfaces  of  the  card. 

To  stain  paper,  you  must  lay  a  chain  upon  a  sheet  o 
white  paper,  and  pass  a  shock  through  it ;  the  paper  Wil 


Jones's  Physiological  Disquisitions. 


TWO    ELECTRICITIES    PROVED.  311 

e  found  to  be  stained  with  a  blackish  tinge  at  every  junc- 
ure  of  the  links.  If  you  make  this  experiment  in  the 
.ark,  a  spark  with  a  kind  of  radiation  will  be  seen  at 
ach  juncture  ;  and  the  chain  will  appear  illuminated  like 
.  line  of  fire  ;  an  iron  chain  answers  best  the  purpose. 

You  may  also,  by  the  discharge,  stain  glass  with  go/d- 
eaf; for  this  end,  take  two  slips  of  common  window- 
dass,  each  about  an  inch  broad,  and  three  or  four  inches 
ong;  then  take  a  narrow  slip  of  gold  or  silver  leaf,  and 
;>ut  it  between  the  glasses  lengthwise,  letting  the  ends  of 
he  leaf  hang  half  an  inch  without  the  glasses  at  each  end ; 
)lace  the  glasses  in  the  small  wooden  press,  and  fix  them 
here  by  a  gentle  pressure,  and  then  lay  them  down  on 
he  table,  so  that  one  end  of  the  metal  leaf  may  be  in 
:ontact  with  the  coating  at  the  bottom  of  the  jar ;  and 
vhen  the  jar  is  charged,  put  one  end  of  the  discharging 
od  upon  that  part  of  the  leaf  that  lies  without  the  glass, 
vhich  is  farthest  from  the  jar,  and  apply  the  other  end 
)f  the  discharger  to  the  top  of  the  jar,  and  the  fluid  will 
>ass  through  the  metal  leaf;  and  when  the  glasses  are 
aken  asunder,  you  will  find,  that  the  leaf  has  been  actu- 
.lly  melted  by  the  electric  lightning,  and  driven  into  the 
ery  substance  of  the  glass.* 

A  pane  of  glass,  coated  on  each  side,  the  coating  being 
very  where  about  two  inches  from  the  edge,  with  a  pic- 
ure  pasted  on  the  upper  side,  and  put  into  a  frame,  is 
ailed  the  magic  picture  ;  one  line  of  tin-foil,  that  goes 
rom  the  coating  of  the  under  side,  is  made  to  communi- 
ate  with  the  bottom  of  the  frame ;  the  back  edge  of  the 
)ottom  rail  and  the  frame  is  covered  with  tin-foil.  Set 
he  face  of  the  picture  against  the  ball  of  the  conductor, 
md  turn  the  machine.  Then  take  it  away,  and  holding 
t  in  a  horizontal  position  by  the  top  of  the  frame,  drop 
t  small  piece  of  money  upon  the  head.  You  may  then 
lesire  any  person  to  take  hold  of  the  lower  rail  of  the 


*  The  wooden  press  is  generally  adapted  to  the  universal  discharger* 
date  1,  Jig.  15,  the  jointed  balls  and  wires  of  which  may  be  readily  and 
•onveniently  applied  to  the  extremities  of  the  gold-leaf  above-mentioned. 
V  chain  or  wire  from  the  outside  of  the  jar  is  to  be  connected  with  the  ring; 
>f  one  wire,  and  the  chain  of  the  discharging  red  with  the  other,  before  the 
Uncharge  is  made...„E,EDiT. 


312  OPPOSITE    DIRECTIONS    OF    THE 


frame  with  one  hand,  and  to  take  off  the  piece  of  money 
with  the  other ;  in  attempting  to  do  this,  he  will  fail  of 
his  design,  for  the  moment  he  touches  the  money  he  will 
receive  a  strong  shock.  You  must  continue  to  hold  the 
frame  all  the  while,  and  will  have  nothing  to  fear,  be- 
cause none  of  the  electric  virtue,  with  which  the  picture 
is  charged,  can  come  to  you,  as  you  are  not  in  the  cir- 
cuit. 

This  bottle  is  called  the  spotted  bottle,  plate  1,  Jig.  18, 
because  it  is  only  coated  with  small  pieces  of  tin-foil, 
placed  at  a  little  distance  from  each  other ;  charge  this 
bottle  in  the  usual  manner,  in  a  darkened  room,  and  you 
will  see  strong  sparks  of  electricity  fly  from  one  spot  of 
tin-foil  to  the  other,  making  the  passage  of  the  fluid  on 
the  outside  very  visible.  Discharge  this  bottle,  by  bring- 
ing a  pointed  wire  gradually  near  the  knob,  and  the  un- 
coated  part  of  the  glass  between  the  spots  will  be  pleas- 
ingly illuminated,  and  the  noise  will  resemble  that  of 
small  fired  crackers.  If  the  jar  be  discharged  suddenly, 
the  outside  surface  appears  illuminated.  To  produce 
these  appearances,  the  glass  must  be  very  dry. 

Hold  a  phial  in  the  hand  which  has  no  coating  on  tk 
outside,  and  present  its  knob  towards  an  electrified  con- 
ductor ;  the  fire,  while  it  is  charging,  will  pass  from  the 
outside  to  the  hand,  in  a  pleasing  manner ;  on  the  dis- 
charge, beautiful  ramifications  will  be  seen  upon  the  un- 
coated  part  of  the  jar. 

By  setting  fire  to  some  tow  in  a  tin  house,  you  have  a 
representation  of  that  awful  appearance,  a  house  inflames. 
To  make  this  experiment  succeed,  take  a  piece  of  soft 
tow,  dry  it  well,  and  then  rub,  or  fill  it  pretty  well  with 
rosin,  and  place  it  between  the  balls  in  the  inside  of  the 
house ;  the  balls  should  not  be  far  asunder,  nor  the 
charge  too  high  ;  connect  the  hook  at  the  bottom  of  tht 
house  with  the  bottom  of  the  jar  ;  let  the  top  of  the  jar  be 
connected  with  the  conductor,  and  when  it  is  charged5 
put  one  ball  of  the  jointed  discharger  on  the  conductor, 
and  bring  the  other  down  upon  the  ball  above  the  house ; 
the  explosion  will  set  the  tow  on  fire,  whose  flames  will 
pass  through  the  windows^  and  make  the  house  appear 
like  one  on  fire. 


TWO    ELECTRICITIES    PROVED.  313 

You  may  pleasingly  illustrate  the  nature  of  the  Leyden 
phial,  by  suspending  two  sets  of  bells  therefrom ;  one 
set  connected  with  the  inside,  the  other  with  the  outside, 
see  plate  1,  y%.  16.  Hook  up  the  chain  from  the  bells 
communicating  with  the  inside,  that  they  may  have  no 
connexion  with  the  table  ;  charge  \hk  bottle  in  the  usual 
manner ;  during  the  charge,  the  set  suspended  from  the 
outside  will  continue  to  ring.  After  the  bottle  is  charged, 
unhook  the  wire  of  the  bells  suspended  from  the  inside. 
Touch  now  the  wire  A,  and  the  bells  will  cease  ringing, 
but  the  other  set  will  begin  to  act ;  take  the  finger  from 
A,  and  apply  it  to  B,  and  the  bells  at  B  will  be  quiet, 
while  those  at  A  will  be  set  in  motion,  and  so  on  alter- 
nately, till  the  bottle  be  discharged. 


EXPERIMENTS    WITH    THE    ELECTRICAL    BATTERY. 

The  most  formidable  part  of  the  electrical  apparatus  is 
the  electrical  battery,  that  is,  a  number  of  jars  connected 
together  in  a  box ;  the  bottom  of  the  box  is  covered  with 
tin-foil ;  from  these  a  hook  projects  on  the  outside  of  the 
box,  by  which  any  substance  may  be  connected  with  the 
outside  of  the  jars ;  their  insides  are  all  connected  by 
wires. 

With  a  battery  you  may  perform  a  great  number  of 
very  surprising  and  interesting  experiments ;  and  though, 
if  very  large,  it  is  a  formidable  appendage  to  an  electri- 
cal machine,  and  ought  always  to  be  used  with  caution, 
yet  it  cannot  be  said,  that  the  apparatus  of  an  electrician 
is  complete  without  it ;  its  effects  in  rending  various  bo- 
dies, in  firing  gun-powder,  in  melting  wires,  and  in  imi- 
tating all  the  effects  of  lightning,  never  fail  to  be  viewed 
with  astonishment. 

There  is  some  caution  necessary  in  the  use  and  man- 
agement of  a  battery,  and  you  should  be  careful  never 
to  make  part  of  the  circuit,  and  to  prevent  those  that  are 
viewing  the  experiments  from  touching  the  battery,  or  ap- 
proaching too  near  any  part  of  the  apparatus ;  the  quad- 
rant electrometer,  plate  1,  Jig.  17,  should  be  always  used 

vol.  iv.  2  s 


5314  EXPERIMENTS    WITH    THE 

with  it ;  it  is  best  to  place  it  upon  the  ball,  which  unites 
the  internal  wires,  but  it  should  always  be  elevated  two 
or  three  feet  above  the  ball.  A  battery  cannot  be  charged 
so  high  in  proportion,  as  a  single  jar  ;  the  quadrant  elec- 
trometer, therefore,  never  rises  so  high  as  90  degrees, 
seldom  higher  than  to  60  or  70  degrees,  more  or  less. 
in  proportion  to  the  size  of  the  battery,  and  the  force  of 
the  machine.  I  must  observe  to  you  here,  that  if  one 
jar  in  your  battery  be  broken,  you  must  remove  the  bro- 
ken jar  before  the  rest  can  be  charged. 

Mr.  Atwood  made,  with  his  battery,  a  very  curious  ex- 
periment on  the  perforation  of  paper  by  the  electric  fluid; 
combined  with  those  that  I  shall  afterwards  relate  to  you, 
you  will  find  it  to  prove,  with  great  clearness,  the  existence 
and  action  of  the  two  electric  powers. 

He  suspended  a  quire  of  paper  by  a  line,  in  the  man- 
ner of  a  pendulum,  from  a  convenient  altitude,  while 
quiescent  in  a  horizontal  direction  perpendicular  to  the 
plane,  the  rods  of  communication  not  touching  the  pa- 
per ;  the  phenomena  were,  first,  the  aperture  mentioned 
in  the  leaves,  being  protruded  both  ways  from  the  mid- 
dle :*  second,  not  the  smallest  motion  was  communicated 
to  the  paper  from  the  force  of  the  discharge. 

A  quire  of  the  thickest  and  strongest  paper  was  made 
use  of  for  this  experiment,  the  height  from  which  it  was 
suspended  sixteen  feet.  It  is  an  extraordinary  appearance 
on  the  hypothesis  of  a  single  electric  fluid,  that  a  force  suf- 
ficient to  penetrate  a  solid  substance  of  great  tenacity  and 
cohesive  force,  should  not  communicate  the  smallest  mo- 
tion to  the  paper,  when  a  breath  of  air  would  cause  some 
sensible  vibration  in  it.  But  the  other  phenomenon,  /.  e> 
the  opposite  direction  in  which  the  leaves  are  protruded, 
tends  very  much  to  strengthen  the  opinion  of  two  oppo- 
site currents ;  indeed,  when  the  two  facts  are  taken  to- 
gether, it  is  scarcely  possible  to  reconcile  the  hypothesis 
of  a  single  power  with  matter  of  fact. 


*  The  burr  of  the  paper  pointed  one  way  on  one  side,  and  the  opposite 
way  on  the  other  side,  as  if  the  hole  had  been  made  in  the  quire,  by  draw- 
ing two  threads  through  it,  in  a  contrary  direction. 


ELECTRICAL    BATTERY.  315 

Mr.  Symmer  placed  in  the  middle  of  a  paper  book,  of 
the  thickness  of  a  quire,  a  slip  of  tin-foil ;  in  another  of 
the  same  thickness  he  put  two  slips  of  tin-foil,  including 
the  two  middle  leaves  between  them ;  upon  passing  the 
electric  stroke  through  them,  he  found  the  following  ef- 
fects. In  the  first,  the  leaves  on  the  side  of  the  foil  were 
pierced,  while  the  foil  itself  remained  unpierced  ;  but  at 
the  same  time  he  could  perceive,  that  an  impression  had 
been  made  on  each  of  its  surfaces,  at  a  small  distance 
from  each  other ;  such  impressions  were  still  more  visi- 
ble on  the  paper,  and  might  be  traced  as  pointing  differ- 
ent ways.  In  the  second,  all  the  leaves  of  the  book  were 
pierced,  excepting  the  two  holes  that  were  between  the 
slips  of  foil,  and  in  these  two,  instead  of  holes,  the  two 
impressions  in  contrary  directions  were  visible. 

When  a  quire  of  paper,  without  any  thing  between 
the  leaves,  is  pierced  by  the  electrical  stroke,  the  two  pow- 
ers keep  in  the  same  track,  and  make  but  one  hole  in 
their  passage  through  the  paper  ;  not  but  that  the  power 
from  above,  or  that  from  below,  sometimes  darts  into  the 
paper  at  two  or  more  different  points,  making  so  many- 
holes  ;  but  these  generally  unite  before  they  go  through 
the  paper.  They  seem  to  pass  each  other  about  the  mid- 
dle of  the  quire,  for  there  the  edges  are  most  visibly  bent 
different  ways ;  whereas,  on  the  leaves  near  the  outside, 
the  holes  very  often  carry  more  the  appearance  of  a  power 
issuing  out,  than  of  one  darting  into  the  paper. 

When  any  thin  metallic  substance,  such  as  gilt  leaf,  or 
tin-foil,  is  put  between  the  leaves  of  the  quire,  and  the 
whole  is  struck  ;  the  counteracting  powers  deviate  from 
the  direct  track,  and  make  their  way  in  different  lines  to 
the  metallic  body,  and  strike  it  in  two  different  points  dis- 
tant from  one  another,  about  one-fourth  of  an  inch,  more 
or  less ;  the  distance  appearing  to  be  generally  less  when 
the  power  is  greatest ;  and  whether  they  pierce  or  only 
make  impressions  upon  it,  they  leave  evident  marks  of 
motion  from  two  different  parts,  and  in  two  contrary  di- 
rections. 

When  two  slips  of  tin-foil  are  put  into  the  middle  of 
the  quire,  including  two  or  more  leaves  between  them,  if 
the  electricity  be  but  weak,  the  counteracting  powers  only 


316  EXPERIMENTS,    &C. 


strike  against  the  slips,  but  leave  an  impression ;  if  the 
shock  be  stronger,  one  of  the  slips  is  pierced,  but  seldom 
both  ;  and  it  appeared  in  general  to  Mr.  Symmer,  that  the 
power  which  issued  from  the  outside,  acts  with  greater 
force,  than  that  which  proceeded  from  within. 

To  break  thick  pieces  of  glass.  Place  a  thick  piece  of 
glass  on  the  ivory  plate  of  the  universal  discharger,  plate 
1,  Jig.  15,  and  a  thick  piece  of  ivory  on  the  glass,  on 
which  a  weight  from  one  to  seven  pounds  is  to  be  placed; 
take  off  the  balls  a,  b,  bring  the  points  of  the  wire  against 
the  edge  of  the  glass,  and  pass  the  discharge  through  the 
wires,  by  connecting  one  of  the  wires  with  the  hook  of 
the  battery,  and  forming  a  communication,  when  the 
battery  is  charged,  from  the  other  wire  to  the  ball.  By 
this  operation  the  glass  will  be  broken,  and  some  part  of 
it  shivered  to  an  impalpable  powder.  When  the  piece 
of  glass  is  strong  enough  to  resist  the  shock,  the  glass  is 
often  marked  by  the  explosion  with  the  most  lively  and 
beautiful  colours. 

Place  a  piece  of  very  dry  white  wood  between  the  balls 
of  the  universal  discharger,  the  fibres  of  the  wood  to  be  in 
the  same  direction  with  the  wire,  pass  the  shock  through 
them,  and  the  wood  will  be  torn  to  pieces ;  or  run  the 
points  into  the  wood,  and  then  pass  the  shock  through 
them. 

To  melt  wires  by  the  electrical fluid,  you  ought  to  have 
a  battery  containing  at  least  thirty  square  feet  of  coated 
surface  ;  you  may  then  connect  the  outside  coating  with 
a  wire  of  about  one-fiftieth  of  an  inch  in  diameter,  and 
from  twelve  to  twenty-four  inches  in  length  ;  fasten  the 
other  end  of  the  wire  to  one  of  the  balls  of  the  discharg- 
ing rod  ;  on  making  the  discharge  the  wire  will  become 
red-hot,  then  melt  and  fall  upon  the  floor  or  table  in  glow- 
ing globules.  Sometimes  the  sparks  are  thrown  to  a  con- 
siderable distance ;  if  the  force  of  the  battery  be  very  great, 
they  will  be  entirely  dispersed  by  the  explosion.* 


*  For  a  further  variety  of  experiments,  see  our  Author's  Essay  on  Elec- 
tricity. I  am  preparing  a  new  edition  oi  this  work  for  the  press,  which 
will  be  published  in  the  beginning  of  the  next  year,  17^9,  with  many  correc- 
tions and  augmentations.... E.  Edit. 


C     317     ] 


LECTURE  XLVIIL 


OF  LIGHTNING,  AND  THE  USEFULNESS  OF  METALLIC 
CONDUCTORS  TO  DEFEND  BUILDINGS  FROM  ITS  EF- 
FECTS, 


N  OTHING  can  be  more  natural  than  to  pass  from 
the  electrical  battery  to  lightning  itself,  for  the  former 
seems  to  be  more  than  an  imitation  ;  it  is  nature  invested 
in  her  own  attire.  The  light  and  sound  accompanying 
these  phenomena,  when  exhibited  on  the  great  scale  of 
nature,  are  indeed  so  awfully  sublime,  that  we  can  scarce 
with  propriety  reflect  on  the  weakness  of  those,  who,  in 
ages  less  informed,  supposed  it  to  be  the  immediate  minis- 
ter of  vengeance  from  an  angry  Deity.  They  are  now 
more  rationally  considered,  as  the  natural  means  of  re- 
storing a  necessary  equilibrium ;  the  rough  discords  of 
nature  productive  of  general  harmony. 

The  phenomena  of  lightning  are  always  surprizing, 
and  sometimes  terrible ;  there  is  no  appearance  in  which 
there  is  more  diversity,  no  two  flashes  being  observed  ex- 
actly similar  to  each  other. 

On  a  summer's  evening,  it  may  often  be  perceived  to 
play  among  the  clouds  ;  this  kind  is  quite  inoffensive,  and 
is  not  accompanied  with  thunder. 

When  the  lightning  is  accompanied  with  thunder,  it  is 
well-defined,  and  has  generally  a  zig-zag  form ;  some- 
times it  only  makes  one  angle  like  the  letter  V,  sometimes 
it  appears  like  the  arc  of  a  circle.  But  the  most  formida- 
ble and  destructive  form  which  lightning  is  ever  known 
to  assume,  is  that  of  balls  of  fire.  The  motion  of  these  is 
very  often  easily  perceptible  to  the  eye,  but  wherever 
they  fall,  much  mischief  is  the  result  of  their  explosion* 


318  OF    LIGHTNING* 

The  next  to  this,  in  its  destructive  effects,  is  the  zig- 
zag kind;  for  that  species,  whose  flashes  are  indistinct, 
and  whose  form  cannot  be  easily  observed,  is  seldom 
known  to  do  much  hurt.  You  may  consider  the  co- 
lour of  lightning  as  an  indication  of  its  power  to  do  mis- 
chief, the  palest  and  brightest  flashes  being  the  most 
destructive. 

There  seems  to  be  a  kind  of  omnipresent  property 
in  the  zig-zag  kind  of  lightning  when  near.  If  two 
persons  be  standing  in  a  room,  looking  different  ways, 
when  a  loud  clap  of  thunder  happens,  accompanied 
with  the  zig-zag  lightning,  they  will  both  distinctly  see 
the  flash,  not  only  by  that  indistinct  kind  of  illumina- 
tion of  the  atmosphere,  which  is  occasioned  by  fire  of 
any  kind,  but  the  very  form  of  the  lightning  itself,  and 
every  angle  it  makes  in  its  course  will  be  as  distinctly 
perceptible,  as  though  they  had  looked  directly  at  the 
cloud  from  whence  it  proceeded.  If  a  person  were  at 
that  time  io  be  looking  on  a  book,  or  other  object  which 
he  held  in  his  hand,  he  would  distinctly  see  the  form  of 
the  lightning  between  him  and  the  object.  This  pro- 
perty seems  peculiar  to  lightning. 

The  effects  of  lightning  are  generally  confined  with- 
in a  small  space  :  and  are  seldom  similar  to  those  which 
accompany  explosions  of  gun-powder,  or  of  inflam- 
mable air  in  mines.  Instances  of  this  kind,  however, 
have  occured ;  the  following  is  one  of  the  most  remark- 
able of  which  we  have  any  distinct  account :  "  Au- 
gust 2,  1 763,  about  six  in  the  evening,  there  arose  at 
Anderlight,  about  a-league  from  Brussels,  a  conflict  of 
several  winds  borne  upon  a  thick  fog.  This  conflict 
lasted  four  or  five  minutes,  and  was  attended  with  a 
frightful  hissing  noise,  which  could  be  compared  to  no- 
thing but  the  yellings  of  an  infinite  number  of  wild 
beasts.  The  cloud  then  opening,  discovered  a  kind  of 
very  bright  lightning,  and  in  an  instant  the  roofs  of 
one  side  of  the  houses  were  carried  off  and  dispersed  at 
a  distance;  above  1000  large  trees  were  broken  off, 
some  near  the  ground,  others  near  the  top,  some  torn 
up  by  the  roots  ;  and  many  both  of  the  branches  and 
tops  carried  to  the  distance  of  60,  100,  or  120  paces  ; 


OF   LIGHTNING.  319 

whole  coppices  were  laid  on  one  side,  as  corn  is  by  or- 
dinary winds.  The  glass  of  the  windows,  which  were 
most  exposed,  was  shivered  to  pieces.  A  tent  in  a  gen- 
tleman's garden  was  carried  to  the  distance  of  4000 
paces ;  and  a  branch  torn  from  a  large  tree,  struck  a 
o-irl  in  the  forehead  as  she  was  coming  into  town,  at  the 
distance  of  40  paces  from  the  trunk  of  the  tree,  and 
killed  her  on  the  spot." 

Thunder-storms  will  sometimes  produce  most  violent 
whirlwinds,  such  as  are  by  some  philosophers  attributed 
to  electricity  ;  nay,  even  occasion  an  agitation  of  the 
waters  of  the  ocean  itself;  and  all  this  too  after  the 
thunder  and  lightning  has  ceased.  Of  this  we  have 
the  following  instances : 

"  Great  Malvern,  October  16,  1761.  On  Wednes- 
day last  we  had  the  most  violent  thunder  ever  known 
in  the  memory  of  man.  At  a  quarter  past  four  in  the 
afternoon,  we  were  surprised  with  a  most  shocking  and 
dismal  noise;  100  forges  all  at  work  at  once,  could  scarce 
equal  it.  Upon  the  side  of  the  hill,  about  400  yards 
to  the  south-west,  there  appeared  a  prodigious  smoke, 
attended  with  the  same  violent  noise,  as  if  a  volcano 
had  burst  out  of  the  hill ;  it  soon  descended,  and  passed 
on  within  about  one  hundred  yards  of  the  south  end  of 
the  house  ;  it  seemed  to  rise  again  in  the  meadow  just 
below  it,  and  continued  its  progress  to  the  east,  rising 
in  the  same  manner  for  four  different  times,  attended 
with  the  same  dismal  noise  as  at  first ;  the  air  being 
filled  with  a  nauseous  and  sulphureous  smell ;  it  gradu- 
ally decreased  till  it  was  quite  extinguished  in  a  turnip 
field,  about  a  quarter  of  a  mile  below  the  house  ;  the 
turnip  leaves,  with  leaves  of  trees,  dirt,  sticks,  &c.  fill- 
ed the  air,  and  flew  higher  than  any  of  these  hills. 
The  thunder  ceased  before  this  happened,  and  the  air 
soon  after  became  calm  and  serene." 

Lightning  is  in  the  hands  of  nature,  what  electricity 
is  in  ours  ;  the  wonders  we  now  exhibit  at  pleasure  are 
little  imitations  of  those  great  effects  which  frighten  and 
alarm  us,  they  seem  to  depend  on  the  same  mechanism ; 
the  same  properties,  the  zig-zag  sparks,  their  similar 
action  on  conducting  substances,  the  power  of  rending,, 


320  OF    LIGHTNING. 

inflaming,  and  dispersing  in  every  direction  the  sub. 
stances  on  which  it  acts  with  power,  the  giving  polari- 
ty to  feruginous  matter,  &c-  all  concur  to  show  their 
identity.  But  independent  of  these  similarities,  the 
thing  is  proved  by  the  plainest  and  clearest  evidence ; 
when  the  atmosphere  is  charged  with  thunder  clouds, 
we  can  by  an  electrical  kite  draw  from  it  the  matter  of 
lightning,  and  with  this  matter  perform  every  known 
electrical  experiment. 

You  have  seen,  that  the  electric  powers  never  become 
sensible  to  us,  except  when  they  are  separated,  and  then 
chiefly  in  their  passage  from  one  body  to'  another  in 
opposite  directions  ;  and  that  an  equal  quantity  of  a  dif- 
ferent power  must  be  conducted  from  the  earth  to  the 
cloud  to  produce  lightning.  There  must  be  the  same 
reciprocal  exchange  of  powers  to  occasion  lightning 
from  one  cloud  to  another. 

When  two  clouds,  which  are  highly  electrified  with 
the  different  powers,  come  near  together,  they  approach 
with  an  increasing  force  till  they  flash  in  exchanging 
powers.  But  as  clouds  are  formed  of  distinct  particles, 
and  every  particle  has  its  share  of  both  electric  powers, 
according  to  the  equality  or  inequality  of  quantity  of 
each  power  in  each  particle,  it  is  more  or  less  electrified; 
and  on  the  various  combinations  of  these  powers,  will 
arise  the  mode  in  which  the  clouds  approach  each  other, 
and  in  which  they  exchange  their  different  powers. 

When  the  electrified  particles  are  made  so  to  approach 
each  other,  that  their  atmospheres  are  pressed  off  toge- 
ther to  a  great  distance  from  the  cloud,  they  then  act 
nearly  the  same  as  if  the  cloud  was  one  continuous  bo- 
dy ;  but  after  the  flash,  those  particles  which  have 
exchanged  powers,  and  in  which  the  two  electricities 
are  united,  being  no  longer  buoyed  up  by  these  agents, 
fall  dow  in  rain,  hail,  &c. 

That  these  atmospheres  are  extended  to  a  great  dis- 
tance from  the  cloud,  appears  from  all  experiments 
made  both  here  and  abroad  ;  for  in  them  it  is  plain, 
that  an  atmosphere  goes  up  from  the  earth,  of  the  pow- 
er which  is  contrary  to  that  of  the  cloud,  which  would 


OF   LIGHTNING.  321 

not  take  place  if  the  atmosphere  of  the  cloud  did  not 
reach  the  earth. 

When  one  of  these  highly  electrified  clouds  ap- 
proaches so  near  to  the  earth  as  to  exchange  powers 
with  it,  then  is  the  damage  done  to  those  things  through 
which  the  exchange  is  made,  which  are  generally  those 
bodies  that  rise  nearest  the  cloud. 

Many  are  the  observations  which  show,  that  the  at- 
mosphere of  the  clouds  are  condensed  at  the  time  of 
their  junction  by  a  flash,  and  that  the  contrary  electrici- 
ty is  then,  as  it  were,  drawn  up  from  the  earth.  Thus, 
in  Mr.  Ludolfs  account,  Phil.  Trans,  vol.  xlvii.  at  eve- 
ry clap  of  thunder  the  electricity  seemed  extinct,  and 
did  not  return  till  after  the  space  of  about  30  seconds ; 
the  threads  which  by  their  divergence  indicated  the  elec- 
tricity, approached  each  other  suddenly,  as  if  they  had 
been  pushed  together  with  force.  The  Abbe  NoIIet9 
and  many  others,  have  observed  similar  appearances. 
In  an  observation  of  Abbe  Nollet,  the  clap  of  thunder 
put  a  stop  for  some  time  to  the  force  of  the  electrici- 
ty ;  all  this  may  be  easily  illustrated  by  our  electrical 
apparatus.  Bring  two  cork  balls  suspended  by  linen 
threads  from  the  end  of  a  wire,  within  the  atmosphere 
of  an  electrified  conductor,  and  they  will  be  electrified 
with  a  power  contrary  to  that  which  electrifies  the  con- 
ductor, receding  from  each  other,  but  flying  towards 
the  conductor  ;  take  a  spark  from  the  conductor,  and 
they  immediately  collapse,  the  electricity  drawn  into 
them  from  your  body  returning  thereto. 

It  often  happens,  as  before  observed,  that  clouds 
electrified  with  the  contrary  powers  are  driven  together, 
and  the  particles  coming  into  contact,  the  powers  is  ex- 
changed without  that  violent  flash  which  usually  accom- 
panies a  thunder  storm.  In  this  case,  the  particles  ge- 
nerally descend  in  heavy  showers  of  rain ;  but  the  ex- 
change of  powers  is  most  complete  in  the  middle  of  the 
united  clouds,  and  the  heaviest  part  of  the  shower  is 
generally  from  the  middle  of  the  cloud. 

In  confirmation  of  this,  I  shall  only  mention  one  ob- 
servation, though  many  might  be  produced  j  it  was 

VOL,  IV.  3  T 


322  OF   LIGHTNING. 

made  by  Mr.  Eeles'in  October  1760  ;  the  clouds  were 
very  distinct,  and  the  showers  heavy.  In  three  differ- 
ent clouds  he  found  the  showers  from  the  beginning 
electrified  with  the  vitreous  power ;  the  showers  from 
the  middle  of  each  cloud  showed  no  sign  of  electrici- 
ty, and  the  end  of  each  cloud  was  resinously  electrified, 
the  wind  N.  W.  There  was  no  appearance  of  electri- 
city in  the  middle  of  the  showers,  because  the  electric 
powers  were  there  united  to  each  other  in  every  drop ; 
their  atmospheres  and  actions  were  therefore  insensible. 

Rain,  hail,  and  snow,  often  exhibit  signs  of  being 
electrified,  for  the  clouds  are  seldom  so  equally  electri- 
fied with  the  different  powers  of  electricity,  as  upon 
meeting  to  render  them  equal  in  each  descending  drop. 
In  large  flakes  of  snow,  the  electricity  is  often  very 
evident  ;  for  when  they  come  near  a  non-electric  body, 
they  are  driven  towards,  and  cling  about  it  like  an  elec- 
trified feather. 

It  is  not  easy  to  form  any  idea  of  what  some  writers 
mean  by  a  negative  cloud  or  negative  stroke.  Is  it  a 
mere  inanity  which  knocks  down  steeples,  rends  trees, 
tears  up  the  earth,  and  kills  men  and  cattle,  &c.  Can 
that,  which  is  not,  act  ? 

You  saw,  by  an  experiment  I  lately  exhibited  to  you, 
that  if  two  electric  plates,  or  two  jars,  be  charged,  and 
a  communication  be  made  from  the  vitreous  side  of  one, 
to  the  resinous  side  of  the  other,  no  discharge  will  fol- 
low, unless  a  communication  be  formed  between  the 
other  two  surfaces  at  the  same  time. 

The  natural  electricity  in  the  atmosphere  is  frequent- 
ly discharged  in  this  manner :  two  clouds  being  elec- 
trified with  opposite  powers,  the  surfaces  of  the  earth 
immediately  under  them  are  likewise  electrified  with 
powers  contrary  to  those  in  the  clouds  above  them  ; 
and  the  moisture  of  the  earth  forming  a  communication 
between  the  two  contiguous  charged  surfaces,  when- 
ever the  two  clouds  meet,  there  will  fellow  a  discharge 
both  of  the  clouds  and  surfaces  on  the  earth  opposite  to 
them.  If  the  earth  should  be  dry,  and  consequently 
afford  a  resistance  to  the  union  of  the  two  electricities 
accumulated  on  or  under  its  surface,  there  will  follow 


OF    LIGHTNING,  323 

an  explosion  in  the  earth  as  well  as  in  the  atmosphere, 
which  will  produce  concussions  and  other  phenomena 
which  have  Frequently  been  observed  to  happen  in  dry 
seasons,  particularly  in  those  climates  which  are  the 
most  liable  to  storms  of  thunder  and  lightning. 

The  various  cases  of  lightning  are  too  numerous  to 
be  here  considered,  and  too  imperfectly  known  to  be  ac- 
curately explained.  What  I  have  said  will,  I  hope, 
give  some  general  notions  of  the  method  in  which  it 
operates,  and  lead  you  to  a  further  investigation  of  the 
subject.  You  may  from  thence  also  readily  account  for 
its  seemingly  capricious  nature ;  sometimes  it  will  strike 
trees,  high  nouses,  &c.  without  touching  cottages,  men, 
or  animals  in  the  neighbourhood  ;  while  in  other  in- 
stances, low  houses  and  cattle  have  been  struck,  while , 
high  tre&s,  steeples,  &c.  near  them  have  escaped. 

All  this  is  very  easily  accounted  for,  upon  Mr.  Eeles's 
theory  of  a  double  current,  and  the  efforts  in  nature  to 
restore  the  electrical  fluid  to  a  latent  state,  whenever  by 
any  means  the  powers  thereof  have  been  separated. 
Thus,  in  great  thunder  storms,  there  is  a  portion  of 
the  earth  under  the  cloud  which  is  electrified  thereby 
with  the  contrary  electricity  ;  those  objects  therefore, 
which  form  the  most  perfect  conductors  between  the 
clouds  and  that  portion  of  the  earth,  will  most  probably 
be  struck,  as  being  the  readiest  way  by  which  the  two 
opposite  powers  can  unite,  and  restore  the  electrical 
equilibrium  both  in  the  cloud  and  the  earth,  one  part 
of  the  flashes  ascending  from  the  earth,  the  other  de- 
scending from  the  cloud. 

Let  us  suppose  a  cloud,  vitreously  electrified,  to  be 
formed  over  a  certain  part  of  the  earth's  surface  ;  the 
electric  power  of  the  cloud  first  separates  that  of  the  at- 
mosphere, and  while  it  is  thus  operating,  the  atmosphere 
is  resinously  electrified  ;  in  a  little  time  the  air  becomes 
vitreously  electrified,  and  then  both  it  and  the  cloud 
act  as  one  body.  The  surface  of  the  earth  then  begins 
to  be  electrified,  and  the  powers  therein  to  be  separated, 
and  a  continual  effort  is  made  by  the  contrary  electri- 
cities to  unite  between  the  earth  and  the  cloud.  If  those 
causes  which  first  produced  the  electricity  still  act,  the 


324  OF    LIGHTNING. 


c     in 


power  becomes  inconceivably  great,  and  the  flashes  in 
uniting  will  tear  every  thing  to  pieces  that  resists  their 
passage. 

Mr.  Read  justly  observes,  that  a  portion  of  the  earth 
may  be  highly  electrified,  and  yet  we  may  be  insensible 
thereof,  because  we  are  involved  therein  ;  for  where  all 
things  are  equally  involved  in  an  electrical  atmosphere, 
there  can  be  no  visible  signs  of  the  presence  of  the  elec- 
tric matter.  Thus,  if  two  or  more  persons  be  electri- 
fied, while  standing  on  the  same  insulation,  they  show 
no  signs  to  each  other  of  being  electrified.*  Whatever 
be  a  person's  situation,  whether  in  the  house  or  open  field, 
he  is  liable  to  be  involved  in  an  electric  charge,  whether 
it  be  stationary,  or  moving  with  the  clouds.  Mr.  Read 
found  himself  so  involved  once  in  Hyde  Park  ;  the  at- 
mosphere had  a  menacing  appearance  with  a  heavy  black 
cloud  at  no  great  distance  ;  on  taking  his  pocket  elec- 
trometer out  of  its  case,  and  holding  it  in  his  hand, 
it  instantly  diverged  near  one  inch.  It  is  not  probable, 
that  the  restoration  of  the  equilibrium,  or  returning 
stroke,  as  it  is  often  called,  will  hurt  any  one,  unless 
he  be  in  the  direct  path  of  the  flash. 

I  have  already  observed,  that  it  is  probable  that  the 
operations  of  the  electrical  matter  are  most  universal 
and  important  in  its  latent  and  united  state ;  and  that, 
whenever  by  separation  it  becomes  visible,  there  is  then 
a  general  stress  throughout  the  greater  part  of  our  sys- 
tem to  restore  the  equilibrium  ;  and  that  this  stress  is 
greater  in  proportion  to  the  quantity  separated  ;  that 
this  separation  in  many  instances  is  spontaneous  ; ,  and 
that  as  this  fluid  is  universally  disseminated,  there  is  no 
occasion  to  consider  the  appearances  of  electricity  in  va- 
pour, &c.  as  the  means  whereby  this  fluid  is  conveyed 
to  the  clouds. 

From  M.  de  Luc's  observations,  it  would  hence  ap- 
pear, that  lightning  often  arises  from  the  sudden  pro- 
duction of  a  great  quantity  of  the  electrical  fluid,  that 
which  is  then  manifested,  not  being  apparent  as  elec- 


*  Ready  p.  61  and  68. 


OF    LIGHTKING.  325 

tricity,  but  just  before  we  perceive  its  effects.  This  is 
further  confirmed  by  his  observations  when  on  moun- 
tains, where  he  had  often  opportunities  of  viewing  these 
phenomena.  Thus,  in  a  storm  on  the  Buet,  one  of 
the  Alps,  while  the  air  was  perfectly  transparent  and 
dry  (the  last  circumstance  being  determined  by  the 
hygrometer),  clouds  began  to  form  in  different  parts ; 
these,  when  thickened  and  united,  embraced  the  sum- 
mit of  the  Buet,  and  supported  themselves  against 
Mount  Blanc,  and  the  summits  of  the  neighbouring 
mountains.  M.  de  Luc  and  his  companions  were  over- 
whelmed  with  rain  ;  there  was  also  a  vast  deal  of  light- 
ning, which  was  often  violent,  and  lasted  for  a  conside- 
rable time.  M.  de  Saussure  has  also  given  instances 
where  the  clouds  formed  a  conducting  communication 
with  the  ground,  and  yet  the  lightning  continued  with- 
out interruption. 

From  these  phenomena,  air  perfectly  transparent 
and  dry,  containing  neither  the  vapours  of  which  the 
cloud  is  formed,  nor  the  electric  fluid,  but  only  the  in- 
gredients proper  to  give  them  birth,  he  infers,  that  by 
some  unknown  cause,  clouds  of  a  certain  kind  are 
formed  spontaneously,  and  during  the  progress  of  their 
formation,  the  electricity  is  produced  in  great  abun- 
dance exploding  every  time  it  is  thus  formed ;  and 
that  before  this,  the  electric  fluid  no  more  existed  in 
that  state,  than  the  aerial  fluids  which  are  disengaged 
from  gunpowder,  existed  as  such  before  the  gunpowder 
was  exploded.  I  need  scarce  observe  to  you,  how  much 
Mr.  Eeles's  theory  is  confirmed  by  this  account  of  M.  de 
Luc. 

You  may  gain  some  idea  of  the  prodigious  quantity 
of  the  electric  fluid,  that  is  sometimes  manifested,  and 
passing  between  the  clouds  and  the  earth,  by  an  instance 
or  two  with  which  we  are  furnished  by  M.  de  Luc. 
Thus,  a  cloud  was  observed  at  the  top  of  the  mountains 
of  Turin :  it  was  formed  of  a  mass,  whose  obscurity 
rendered  it  terrific,  producing  in  those  places,  over 
which  it  was  situate,  night  at  noon  day  ;  this  mass  was 
ploughed  as  it  were  by  lightning,  which  was  soon  after 
followed  by  a  grumbling  kind  of  thunder ;  there  fell 


326        POINTED    AND    KNOBBED    CONDUCTORS. 


so  prodigious  a  quantity  of  water  and  ice  from  the 
cloud,  that  the  country  was  ravaged  by  the  torrents, 
the  hedges  were  bearen  down,  and  the  ditches  half  filled 
with  hail.  Erfurt,  a  small  city  in  Germany,  was  struck 
in  one  night  in  forty-two  different  places  ;  seven  per- 
sons were  killed,  three  houses  were  set  on  fire,  but 
quenched  by  the  rain,  which  came  down  in  torrents. 
Now  where  shall  we  find,  on  the  vapour  theory,  known 
humidity  in  any  strata  of  transparent  air,  sufficient  to 
explain  the  formation  of  such  clouds,  and  the  torrents 
of  rain  which  were  discharged  from  them  ? 


OF    CONDUCTING    RODS. 

We  are  now  prepared  to  consider  the  advantages  of 
conducting  rods.  You  know  that  the  electrical  fluid 
is  always  impelled  to  those  places  where  an  exchange 
of  powers  can  be  most  easily  made,  or  where  the  union 
of  the  two  powers  is  least  resisted.  If  then  there  should 
happen,  in  any  of  the  preceding  instances,  to  be  a  house 
furnished  with  a  conducting  rod,  directly  between  that 
part  of  the  cloud  and  that  part  of  the  earth,  where  there  is 
the  greatest  effort  for  restoring  the  equilibrium,  the  con- 
ductor will  be  struck,  and  will  probably  prevent  the 
building  from  receiving  any  injury.  If  there  be  no  con- 
ductor, the  lightning  will  for  the  foregoing  reasons  pass 
at  the  same  place,  but  the  building  will  probably  be 
damaged,  because  the  materials  resist  the  passage  of  the 
electrical  powers. 

OF    POINTED    AND    KNOBBED    CONDUCTORS. 

A  great  dispute  has  been  carried  on  among  electri- 
cians concerning  the  termination  of  conducting  rods, 
for  preserving  buildings  from  lightning  ;  some  warmly 
contending,  that  they  should  be  terminated  by  knobs 
or  balls ;  others  as  strenuously  contending,  that  they 
should  be  pointed. 

Ever  since  the  identity  of  electricity  and  lightning 
has  been  proved,  conductors  of  some  kind  have  beer 


POINTED   AND    KNOBBED    CONDUCTORS.        327 

generally  allowed  to  be  necessary  for  the  safety  of  build- 
ings in  thunder  storms,  as  they  afford  a  ready  passage 
for  the  union  of  the  contrary  electricities.  Electricians 
seems  to  have  forgotten,  that  neither  lightning  nor  elec- 
tricity ever  strikes  a  body,  merely  for  the  sake  of  the 
body,  but  because  that  body  is  a  means  of  restoring  the 
disturbed  equilibrium. 

When  a  quantity  of  electricity  is  excited  by  means  of 
an  electric  machine,,  a  body  communicating  with  the 
earth,  will  receive  a  strong  spark  from  the  prime  con- 
ductor 5  it  receives  this  spark,  not  because  it  is  capable 
of  containing  all  the  electricity  of  the  cylinder  and  con- 
ductor, but  because,  the  natural  situation  of  the  fluid  be- 
ing disturbed  by  the  motion  of  the  machine,  the  natural 
powers  make  an  effort  to  restore  the  equilibrium.    No 
sooner,  then,  is  a  conducting  body,  communicating  with 
the  earth,  presented  to  the  prime  conductor,  than  the 
whole  effort  of  the  electricity  is  directed  against  that 
body ;  not  merely  because  it  is  a  conductor,  but  because 
it  affords  a  place,  by  which  the  natural  powers  can  more 
readily  unite,  and  which  they  would  do  by  other  means, 
though  that  body  were  not  to  be  presented.     That  this 
is  the  case,  we  may  easily  see,  by  presenting  the  same 
conducting  substance  in  an  insulated  state  to  the  prime 
conductor  of  the  machine,  when  we  shall  find  only  a 
small  spark  will  be  produced.     In  like  manner,  when 
lightning  strikes  a  tree,  a  house,  or  a  conducting  rod,  it 
is  not  because  these  objects  are  high,  but  because  they 
are  situate  in  that  place,  where,  from   a  variety  of 
causes,  the  impetus  of  the  two  powers  can  be  lessened 
by  uniting  with  each  other. 

From  hence  you  will  perceive  the  fallacity  of  that  ! 
kind  of  reasoning,  which  is  generally  employed  con- 
cerning the  use  of  thunder  rods. 

Because  a  point  presented  to  an  electrified  body,  in 
our  experiments,  draws  off  the  electricity  in  a  silent 
manner,  Dr.  Franklin  and  his  followers  have  conclud- 
ed, that  a  pointed  conductor  will  do  the  same  thing  to 
a  thunder  cloud,  and  thus  prevent  any  kind  of  danger 
from  a  stroke  of  lightning* 


328        POINTED    AND    KNOBBED    CONDUCTORS. 


, 


But,  for  this  very  reason,  Mr.  Wilson  and  his  part 
have  determined,  that  the  use  of  pointed  conductors  is 
utterly  unsafe  ;  they  justly  consider  the  Franklinian  idea 
of  exhausting  the  clouds  of  their  electricity,  to  be  not 
less  absurd,  than  it  would  be  to  clear  away  an  inundation 
with  a  shovel,  or  exhaust  the  atmosphere  with  an  air- 
pump.  They  bring  many  instances,  where  a  point  will 
receive  a  full  stroke,  and  assert  that  it  solicits  a  discharge ; 
and  that,  being  often  unable  to  conduct  the  whole  elec- 
tricity of  the  atmosphere,  it  is  impossible  for  us  to  know 
whether  the  discharge  it  solicits  may  not  be  too  great 
for  the  conductor  to  bear;  and,  consequently,  all  the  mis- 
chiefs arising  from  thunder  storms  may  be  expected, 
with  this  mortifying  circumstance,  that  this  very  con- 
ductor hath  probably  solicited  the  fatal  stroke. 

I  must  also  further  observe  to  you,  that  the  Franklini- 
ans,  granting  them  all  they  ask,  still  make  their  pointed 
conductors  of  too  much  consequence  ;  for  it  is  now  well 
known,  that  points  have  no  influence  at  all,  unless  they 
be  immerged  in  the  electrified  atmosphere.  If  a  point- 
ed body  do  not  communicate  with  the  earth,  but  the 
communication  be  interrupted  by  a  short  interval,  it  will 
receive  a  full  spark.  It  will  also  receive  a  full  spark,  if 
it  be  suddenly  brought  sufficiently  near  a  strongly  electri- 
fied body  :  this  case  applies  strongly  against  pointed  con- 
ducting rods  for  shipping.  It  will  also  receive  a  full 
spark  at  a  considerable  distance,  if  surrounded  with  non- 
conducting substances.  The  circumstances  on  which 
an  explosion  depends  are  too  many  to  be  here  enumerat- 
ed ;  in  general  it  may  be  said  that,  with  respect  to  a 
point,  it  will  depend  on  the  suddenness  of  the  discharge, 
on  the  proximity  of  the  cloud,  on  the  velocity  in  its  mo- 
tion, on  the  quantity  of  electricity  contained  in  it,  and 
on  the  contrary  electricity  opposed  to  it.  If  a  small 
cloud  hang  suspended  under  a  large  cloud  loaded  with 
electric  matter,  pointed  conductors  on  a  building  under- 
neath will  receive  the  discharge  by  explosion,  in  prefer- 
ence to  those  terminated  by  balls ;  the  small  cloud  will 
form  an  interruption,  which  allows  only  an  instant  of 
time  for  the  discharge.  If  a  single  electric  cK  u  J  be  dri- 
ven with  considerable  velocity  near  to  a  pointed  con- 


rOINTED    AND    &NOBBED    CONDUCTORS.         329 

ductor,  the  charge  may  be  caused  to  explode  upon  it  by 
the  motion  of  the  charged  body. 

A  pointed  conductor  has  not  even  the  power  of  at- 
tracting  the  lightning  a  few  feet  out  of  the  direction  it 
would  choose  itself:  of  this  we  have  a  most  decisive 
instance  in  what  happened  to  the  magazine  at  Purfleet, 
in  Essex.  That  house  was  furnished  with  a  conductor, 
raised  above  the  highest  part  of  the  building ;  never- 
theless, a  flash  of  lightning  struck  an  iron  cramp  in  the 
corner  of  the  wall  of  the  building,  considerably  lower 
than  the  top  of  the  conductor,  and  only  forty-six  feet  in 
a  sloping  line  distant  from  the  point. 

The  conductor,  with  all  its  power  of  drawing  off  the 
electric  matter,  was  neither  able  to  prevent  the  flash,  nor 
to  turn  it  forty-six  feet  out  of  its  way.  The  matter  of 
fact  is,  the  lightning  was  determined  to  enter  the  earth 
at  the  place  where  the  board-house  stands,  or  near  it ; 
the  conductor  fixed  on  the  house  offered  the  easiest  com- 
munication, but  forty-six  feet  of  air  intervening  between 
the  point  of  the  conductor  and  the  place  of  the  explosion, 
the  resistance  was  less  through  the  blunt  cramp  of  iron 
and  a  few  bricks  moistened  with  the  rain  to  the  side  of 
the  metalline  conductor,  than  through  the  forty-six  feet 
of  air  to  its  point ;  for  the  former  was  the  way  in  which 
the  lightning  actually  passed. 

An  objection  to  the  use  of  conductors  of  either 
kind  may  be  also  drawn  from  the  accident  which  hap- 
pened to  the  poor-house  at  Heckingham,  Norfolk,  which 
was  struck  by  lightning,  though  furnished  with  eight 
pointed  conductors,  and  which,  I  am  well  assured  from 
good  authority,  were  uninterrupted,  continuous,  and  at 
the  time  of  the  stroke  perfectly  connected  with  the  com- 
mon stock.  Hence  it  is  evident,  that  the  effect  of  con- 
ductors, in  general  is  too  inconsiderable  either  to  lessen 
fear  or  animate  hope. 

The  thunder  house ,  plate  2,  fig.  3,  as  it  is  usually  call- 
ed, is  the  apparatus  principally  used  to  illustrate  the 
Franklinian  method  of  preserving  houses  from  damage 
by  lightning.  It  consists  of  a  mahogany  board,  shaped 
like  the  gable  end  of  a  house.  It  is  fixed  upright  on  a 
VOL.  iv.  u 


330         POINTED    AND    KNOBBED    CONDUCTORS. 

horizontal  board  as  a  stand ;  a  square  hole  is  made  in 
the  gable  board,  into  which  is  fitted,  so  as  to  go  in  and 
out  easily,  a  square  piece  of  wood  ;  a  wire  is  fixed  in  the 
one  diagonal  of  this  board,  and  wires  are  also  fixed  in 
the  gable  board,  one  from  the  upper  part,  the  lower 
end  of  which  comes  to  one  corner  of  the  square  hole; 
the  upper  end  of  the  other  wire  coincides  with  the  oppo- 
site corner,  and  goes  down  to  the  bottom  of  the  gable 
board.  The  upper  wire  has  a  brass  ball  on  the  top  ;  this 
may  be  occasionally  taken  off,  which  leaves  a  point  ex- 
posed ;  at  the  bottom  of  the  lower  wire  there  is  a  hook : 
connect  the  hook  at  the  bottom  with  the  outer  coat- 
ing of  a  jar,  place  the  square  piece  in  the  hole,  so 
that  the  metallic  wire  shall  not  coincide  with  the  other 
two  ;  when  the  jar  is  charged,  bring  the  discharging  rod 
from  the  knob  thereof  to  the  ball  of  the  house  ;  an  ex- 
plosion will  ensue,  and  the  square  piece  be  driven  out  to 
a  good  distance  from  the  gable  board. 

Put  the  square  piece  into  the  hole  in  such  a  manner, 
that  the  ends  of  the  diagonal  may  not  coincide  with  the 
ends  of  the  wire  of  the  gable  board,  then  make  the  dis- 
charge as  before,  and  the  metallic  circuit  being  now  com- 
plete, the  square  board  will  remain  jn  its  place.* 

Take  off  che  ball,  and  the  point  will  prevent  an  explo- 
sion, and  its  accumulating  therein  in  any  sufficient  quan- 
tity to  do  any  damage. 

The  prime  conductor  is  supposed  to  represent  a  thun- 
der cloud  discharging  its  contents  on  some  metal  pro- 
jection on  the  top  of  a  building ;  and  -this  is  considered 
as  receiving  no  damage  when  the  conductor  is  perfect; 
but  when  the  connexion  is  imperfect,  the  fluid  in  pass- 
ing from  one  part  to  the  other,  damages  the  building. 


*  If  two  square  pieces  be  made  and  placed  in  die  two  different  direc- 
tions, the  property  is  then  shown  atone  discharge  of  the  jar. 

In  respect  to  the  best  form  for  conductors;  from  a  great  variety  of  ex 
periments  that  1  have  made,  and  also  been  a  witness  to,  pointed  conduc- 
tors have  all  the  properties  of  blunt  ones  under  all  circumstances,  and, 
moreover,  some  advantageous  properties  peculiar  to  their  pointed  termina- 
tions. Upon  the  whole,  I  recommend,  and  give  the  preference  to  con 
tors  with  very  fine  and  pure  metallic  points....E.  Edit, 


I     331      ] 


LECTURE  XLIX. 


ON    THE    NATURE    OF    ELECTRICITY;     OF    ANIMAL 
ELECTRICITY,   &C. 


AFTER  pointing  out  to  you  the  principal  phenome- 
na of  electricity,  and  exhibitijig  to  you  many  of  the  most 
interesting  and  entertaining  experiments  in  this  branch  of 
natural  philosophy ;  I  shall  now  endeavour  to  trace  out 
its  connexion  with  the  great  agents  in  the  operations  of 
nature,  and  thus  lead  you  to  form  some  idea  of  what 
electricity  is,  and  of  its  use  in  the  great  system  of  things. 
Whatever  it  may  be,  it  is  certain,  and  that  without  any 
exaggeration,  that  whether  you  look  to  the  heaven's  above, 
or  the  earth  beneath,  you  can  scarce  perceive  any  thing 
that  is  not  acted  upon,  and  in  a  manner  perfectly  subject- 
ed to  the  operations  of  this  wonderful  fluid. 

That  electricity  is  real  matter,  and  not  a  mere  proper- 
ty, is  evident  from  a  variety  of  circumstances.  When  it 
passes  between  bodies,  it  divides  the  air,  and  puts  it  into 
those  undulations  which  give  us  the  idea  of  sound.  It 
emits  the  rays  of  light  in  every  direction,  and  those  rays 
are  variously  refrangible  and  colorific,  as  other  light  is: 
and,  if  light  is  acknowledged  to  be  matter,  it  is  contra- 
ry to  reason  and  experience  to  suppose  that  the  thing 
which  emits  it  should  not  likewise  be  material  ;  neither 
are  the  other  senses  unaffected  at  its  presence :  its  smell 
is  strongly  posphoreal  or  sulphureous.  The  sense  of 
feeling  is  a  witness  of  its  presence,  not  only  from  the 
sparks  which,  when  received  from  the  conductor  of  a 
powerful  machine,  are  pungent,  and  will  pass  through 
two  or  three  persons  standing  on  the  ground,  but  also 


332  ON    THE    NATURE    OF    ELECTRICITY. 

from  the  shock.     A  stream  of  electric  matter  has  also 
evidently  a  subacid  taste. 

In  contemplating  the  system  of  nature,  you  perceive 
three  kinds  of  fluids  of  extreme  subtilty,  and  very  much 
resembling  one  another :  these  three  are,  fire,  light,  and 
electricity.  Their  resemblance  is  so  great,  that  it  is  not 
surprizing  to  find  it  the  general  conception  of  all  unin- 
formed minds,  that  they  are  ultimately  the  same  ;  on  ex- 
amining the  evidence  of  their  identity,  you  will  find  it 
exceedingly  strong. 

If  it  is  true,  that  natural  effects  are  not  to  be  ascribed 
to  many  different  means  or  agents,  where  one  will  suf- 
fice, these  three  should  be  considered  as  different  modi- 
fications or  states  of  the  same  fluid.  Light  or  solar  fire 
will  burn  in  fuel,  and  act  in  solid  matter  with  greater 
effect  than  the  most  violent  fire  of  a  furnace.  Common 
fire,  like  that  of  the  sun,  will  promote  vegetation  and 
ripen  fruits.  The  electric  fire  will  light  a  candle  and  fire 
gunpowder,  like  the  common  fire -,  will  afford  a  spectrum 
of  the  seven  primordial  colours,  in  common  with  light  j 
and  will  throw  metals  into  fusion  with  a  violent  scorching 
heat.*  Let  us  leave  generals,  and  descend  more  into 
particulars. 

These  three  fluids  all  agree  in  one  property,  that  of 
exciting  heat  in  certain  circumstances,  and  not  doing  so 
in  others. 

Fire,  in  the  common  acceptation  of  the  word,  always 
excites  heat ;  but  in  its  latent  state,  it  lays  aside  this  pro- 
perty, and  in  vapour,  for  instance,  is  cold  to  the  touch. 

Light,  when  collected  into  a  focus  by  a  burning  glass, 
/.  e.  when  its  rays  converge  to  a  centre,  and  diverge,  or 
attempt  to  diverge  from  one,  produces  heat. 

Electricity,  when  its  force  is  concentrated  and  converg- 
ed, produces  heat,  as  I  shall  soon  show  you  by  its  effect 
on  a  thermometer.  This  does  away  the  objection  for- 
merly made  to  those  who  asserted,  that  electricity  was 
that  elementary  fire  which  pervaded  all  substances :  the 
objection  was,  that  though  the  electric  matter  emitted 


*  J^ieSs  Ph}siol;gic  al  Disquisitions. 


ON    THE    NATURE    OF    ELECTRICITY.  333 

light,  and  had  the  appearance  of  fire,  it  wanted  ics  essen- 
tial characteristic  of  burning ;  and  where  great  quanti- 
ties of  the  fluid  were  forced  through  substances,  they  in- 
sinuated, that  it  might  be  occasioned  by  the  internal  com- 
motion excited  among  their  small  particles. 

There  is  no  occasion  to  dwell  upon  the  weakness  and 
fallacy  of  the  objection,  as  it  is  completely  removed  by 
many  facts.  1 .  By  the  effect  of  electricity  upon  the  ther- 
mometer. 2.  By  the  experiment  that  was  made  at  the 
Pantheon  by  Mr.  Wilson,  with  the  immense  apparatus 
that  was  constructed  for  making  experiments  on  the 
preferable  utility  of  pointed  or  knobbed  conductors,  for 
preserving  buildings  from  lightning.  The  electric  aura 
from  this  machine  fired  gunpowder  in  the  most  unfa- 
vourable circumstances  that  can  be  imagined,  namely, 
when  it  was  drawn  off  by  a  sharp  point,  in  which  case  it 
has  generally  the  least  force.  Upon  a  staff  of  baked  wood 
a  stem  of  brass  was  fixed,  which  terminated  at  the  top  in 
a  wooden  point;  this  point  was  put  into  the  end  of  a  small 
tube  of  Indian  paper,  made  somewhat  in  form  of  a  car- 
tridge, about  an  inch  and  a  quarter  long,  and  T0ths  of  an 
inch  in  diameter.  When  the  cartridge  was  filled  with 
common  gunpowder  unbruised,  a  wire  communicating 
with  the  earth  was  fastened  to  the  bottom  of  the  brass 
stem.  The  charge  in  the  large  conductor  being  kept  up 
by  the  motion  of  the  cylinder,  the  top  of  the  cartridge  was 
brought  near  to  the  conductor,  so  as  even  frequently  to 
touch  the  tin-foil  with  which  it  was  covered.  In  this 
situation,  a  small  faint  luminous  stream  was  frequently 
observed  between  the  top  of  the  cartridge  and  the  metal. 
Sometimes  this  stream  would  set  fire  to  the  gunpowder 
the  moment  it  was  applied ;  at  others,  it  would  require 
half  a  minute  or  more,  before  it  would  take  effect.  The 
difference  in  time  was  supposed  to  arise  from  some  mois- 
ture in  the  powder.  Tinder  was  fired  much  more  rea- 
dily. 

It  now  appears  clearly,  that  the  electric  fluid  moving 
through  bodies,  either  in  small  quantities,  or  with  rapidi- 
ty, or  in  very  great  quantities,  will  produce  heat,  and  set 
them  on  fire  :  it  seems  therefore  scarce  disputable,  that 
this  fluid  is  the  same  with  the  element  of  fire. 


334  ON    THE    NATURE    OF    ELECTRICITY. 

These  are  far  from  being  the  only  instances  of  their 
identity  ;  for  fire  is  brought  into  action  by  friction,  as 
well  as  electricity.  Fire  dilates  all  bodies :  the  electric 
fluid  has  also  a  dilating  power,  which  is  evident  from  its 
action  on  a  thermometer,  though,  in  general,  the  force 
with  which  bodies  cohere  together  is  greater  than  the 
dilating  power  of  electricity. 

Fire  promotes  and  accelerates  vegetation  as  well  as 
germination.    Electricity  does  the  same. 

Electricity,  as  well  as  fire,  accelerates  evaporation. 

The  experiments  made  by  Mr.  Achard  on  the  eggs  of 
a  hen,  and  by  others  on  the  eggs  of  moths,  prove  that 
electricity,  as  well  as  heat,  favours  the  developement  of 
those  animals.  The  electric  fluid,  in  common  with  fire, 
will  throw  metals  into  fusion. 

If  substances  with  equal  degrees  of  heat  touch  each 
other,  the  heat  is  diffused  uniformly  between  them.  In 
the  same  manner,  if  two  bodies  with  unequal  degrees,  or 
different  kinds  of  electricity,  touch  each  other,- an  equili- 
brium will  be  established. 

If  bodies  of  different  kinds,  and  of  equal  degrees  of 
heat,  are  placed  in  a  medium  of  a  different  temperature, 
they  will  all  acquire,  at  the  end  of  a  certain  time,  the 
same  degree  of  heat.  There  is  a  considerable  difference, 
however,  in  the  space  of  time  in  which  they  acquire  the 
temperature  of  the  medium  :  ex.  gr.  metals  take  less  time 
than  glass  to  acquire  or  lose  an  equal  degree  of  heat. 

On  an  attentive  examination  of  the  bodies  which  re- 
ceive and  lose  their  heat  soonest,  when  they  are  placed 
in  mediums  of  different  temperatures,  they  will  be  found 
to  be  the  same  which  receive  and  lose  the  electric  signs 
soonest.  Metals,  which  become  warm  or  grow  cool  the 
quickest,  are  the  substances  in  which  the  electric  powers 
unite  most  readily.  Wood,  which  requires  more  time  to 
be  heated  or  cooled,  receives  and  loses  electricity  slower 
than  metals.  Lastly,  glass  and  resinous  substances,  which 
receive  and  lose  slowly  the  electric  fluid,  acquire  with 
difficulty  the  temperature  of  the  medium  which  surrounds 
them. 

If  one  extremity  of  an  iron  rod  be  heated  red-hot,  the 
other  extremity,  though  the  bar  be  several  feet  long,  will 


ON    THE    NATURE    OF    ELECTRICITY.  535 

become  so  warm  in  a  little  time,  that  the  hand  cannot 
hold  it,  because  the  iron  conducts  fire  readily  ;  but  a  tube 
of  glass,  only  a  few  inches  long,  may  be  held  in  the  hand, 
even  while  the  other  end  is  melting.  The  electric  fluid, 
in  the  same  manner,  passes  with  great  velocity  from  one 
end  of  a  rod  of  iron  to  the  other ;  but  it  is  a  considera- 
ble time  before  a  tube  of  glass,  at  one  end  of  which  an 
excited  electric  is  held,  will  give  electric  signs  at  the 
other. 

These  observations  prove,  that  several  bodies  that  re- 
ceive and  lose  with  difficulty  their  actual  degree  of  heaty 
receive  and  lose  also  with  difficulty  their  electricity. 

The  electric  powers  may  be  put  in  action  by  heat  and 
cold.  Mr.  Canton  procured  some  thin  glass  balls,  of  about 
an  inch  and  a  half  in  diameter,  with  stems  or  tubes  about 
eight  or  nine  inches  in  length,  and  electrified  them,  some 
vitreously  on  the  inside,  others  resinously,  and  then  seal- 
ed them  hermetically ;  soon  after  he  applied  the  naked 
balls  to  his  electrometer,  and  could  not  observe  the  least 
sign  of  their  being  electrical ;  but  holding  them  at  the 
fire,  at  the  distance  of  five  or  six  inches,  they  became 
strongly  electrical  in  a  short  time,  and  more  so  when 
they  were  cooling.  These  balls  would,  every  time  they 
were  heated,  give  the  electric  power  to,  or  take  it  from 
other  bodies,  according  to  the  vitreous  or  resinous  state 
of  it  within  them.  Heating  them  frequently  diminished 
their  power,  but  keeping  one  of  them  under  water  a  week 
did  not  in  the  least  impair  it.  The  balls  retained  their 
virtue  above  six  years. 

The  tourmalin,  and  many  other  precious  stones,  are 
also  known  to  acquire  electricity  by  heat.  The  tourmalin 
has  always  at  the  same  time  a  vitreous  and  resinous  elec- 
tricity ;  one  side  of  it  being  in  one  state,  the  other  in  the 
opposite.  Sometimes  one  side  will  at  the  same  time  pos- 
sess both  electricities.  These  powers  may  be  excited  by 
friction  and  by  heat ;  nay,  even  by  plunging  it  in  boiling 
water. 

Many  instances  prove,  that  electricity  is  produced  by 
liquifaction.  Thus,  where  chocolate  is  manufactured  in 
large  quantities,  a  vivid  light  is  frequently  seen  flashing 
upon  its  surface  after  melting,  and  it  will  also  attract  light 


336  ACTION    OF    ELECTRICITY. 

substances,  separate  pith  balls,  &rc.  When  it  had  lost 
this  property,  Mr.  Henly  found  it  might  be  restored  by 
melting  it  together  with  a  small  quantity  of  olive-oil.  If 
sulphur  be  melted  in  a  glass  vessel,  and  taken  out  when 
cool,  both  it  and  the  glass  will  be  found  strongly  electri- 
fied. 

I  have  already  shown  you,  that  electricity  is  produced 
by  the  evaporation  of  water ;  I  shall  now  relate  Mr. 
Read's*  mode  of  performing  this  experiment.  He  insu- 
lates  a  large  hollow  tin  cone,  containing  about  four  sheets 
of  tin  plates,  with  many  yards  of  small  wires  coiled  up 
within  it ;  one  end  of  the  wire  is  extended  from  the  cone 
to  a  very  sensible  electrometer.  The  cone  and  wire  col- 
lect and  condense  the  ascending  electrified  vapour,  as  it 
quits  the  insulated  vessel  containing  the  fluid.  The  elec- 
trometer connected  with  the  cone  is  vitreously  electrified ; 
that  connected  with  the  vessel  from  whence  the  vapour 
arose,  is  in  a  resinous  state. 

Mr.  Read  has  also,  by  burning  different  substances  in 
insulated  vessels  under  his  tin  cone,  shown  that  bodies, 
in  passing  from  a  solid  to  a  fluid  state,  produce  the  two 
electricities ;  the  quantity  observed  is  in  general  very 
small,  on  account  of  the  intimate  affinity  between  flame 
and  electricity. 

ACTION    OF    ELECTRICITY    ON    A    THERMOMETER.' 

Insulate  a  sensible  mercurial  thermometer,  and  place 
the  bulb  between  two  balls  of  wood,  one  affixed  to  the 
conductor,  the  other  communicating  with  the  ground ; 
and  the  electric  fluid,  in  passing  between  the  two  balls, 
will  raise  the  mercury  in  the  thermometer  considerably. 
With  a  cylinder  of  about  seven  inches  and  a  half  in  dia- 
merer,  the  fluid  passing  from  a  ball  of  lignum  vitse  to  a 
ball  of  beech,  and  thence  to  the  ground,  elevated  the 
quicksilver  in  the  thermometer  from  68°  to  1 10°,  repeat- 
edly to  105°.  The  thermometer  was  raised  from  68°  to 
85°,  by  the  fluid  passing  from  a  point  of  box  to  a  point  of 


Read's  Summary  Mew  of  Spontanecus  Electricity,  &c. 


ON    A    THERMOMETER.  §37 

lignum  vitae;  from  67°  to  100°  from  a  point  of  box  to 
a  ball  of  box ;  from  66°  to  100°  from  a  ball  of  box  to  a 
brass  point;  from  69°  to  100°  from  ball  to  ball;  the 
bulb  of  the  thermometer  being  covered  with  flannel. 

<c  If  then  these  fluids,  fire,  light,  and  electricity,  which 
thus  mutually  and  in  all  respects  assume  each  others  pro- 
perties, be  not  the  same ;  experiment  is  a  thing  not  to 
be  depended  upon,  and  the  most  obvious  rules  of  philo- 
sophizing, adopted  and  approved  by  all  parties,  are  no 
better  than  specious  deceptions. " 

More,  indeed,  need  not  be  said  to  any  observer  of  na- 
ture ;  but  it  is  necessary  to  accumulate  proof,  in  order  to 
lessen  the  prejudices  of  modern  philosophers,  who  have 
altogether  neglected  to  study  and  trace  the  great  agents 
of  nature.  For  these,  it  may  be  necessary  to  point  out 
other  links,  in  which  they  may  see  the  connexion  between 
fire,  light,  and  electricity. 

Thus,  as  heat  is  diminished,  or  bodies  are  cooled,  elec- 
tricity succeeds  in  its  place.  All  electric  bodies  by  heat 
are  rendered  conductors,  and  can  no  longer  be  excited  ; 
but,  as  soon  as  the  heat  is  removed,  their  electric  pro- 
perty returns. 

Water  is  a  conducting  substance ;  by  being  frozen, 
its  conducting  powers  are  lessened ;  when  cooled  down 
to  twenty  degrees  below  0  of  Fahrenheit's  scale,  it  be- 
comes an  electric,  and  will  emit  sparks  by  friction,  like 
glass.  The  atmosphere  is  a  natural  electric  ;  but,  by  a 
certain  degree  of  heat,  it  loses  in  a  degree  this  property, 
and  becomes  a  conductor,  nor  is  there  any  doubt  that  its 
electric  properties  are  increased,  in  proportion  to  the  de- 
gree of  cold  imparted  to  it. 

Mr.  JEpinus  mentions  some  facts  in  a  letter  to  Dr. 
Guthrie^  which  will  illustrate  this  subject ;  they  relate  to 
phenomena  that  are  known  to  take  place  in  Russia,  when 
a  great  cold  has  continued  for  several  weeks.  Mr.  JEpi- 
nus was  sent  for,,  to  see  an  uncommon  phenomenon.  On 
going  into  the  apartment  of  Prince  Or/off,  he  found  him 
at  his  toilet,  and  that  at  every  time  his  valet  drew  the 
comb  through  his  hair,  a  pretty  strong  crackling  noise 
was  heard  ;  and  on  darkening  the  room,  the  sparks  were 
seen  following  the  comb  in  great  abundance,  while  the 

VOL.  IV.  2  X 


338  ACTION    OF    ELECTRICITY 

Prince  was  so  completely  electrified,  that  strong  sparks 
could  be  drawn  from  his  hands  and  face ;  nay,  he  was 
even  electrified,  when  he  was  only  powdered  with  a  puff. 

A  few  days  after,  Mr.  JEpinus  was  witness  to  a  more 
striking  effect  of  the  electric  state  of  a  Russian  atmos- 
phere. The  Great  Duke  of  Russia  sent  for  him  one 
evening  in  the  twilight,  and  told  him,  that  having  briskly 
drawn  a  flannel  cover  off  a  green  damask  chair  in  his 
bed-chamber,  he  was  astonished  at  the  appearance  of  a 
strong  bright  flame  that  followed  ;  but,  considering  it  as 
an  electrical  appearance,  he  had  tried  to  produce  a  simi- 
lar illumination  on  different  pieces  of  furniture,  and  could 
then  show  him  a  beautiful  and  surprizing  experiment. 
His  Highness  threw  himself  on  his  bed,  which  was  co- 
vered with  a  damask  quilt,  laced  with  gold,  and  rubbing 
it  with  his  hands  in  all  directions,  the  young  Prince,  who 
had  then  reached  his  twelfth  year,  appeared  to  be  swim- 
ming in  fire,  as  at  every  stroke  flames  arose  all  around 
him,  darted  to  the  gold-lace  border,  ran  along  it,  and  up 
to  that  of  the  bed,  and  even  to  the  very  top. 

While  his  Highness  was  showing  this  experiment, 
Prince  Orloff  came  into  the  room  with  a  sable  muff  in 
his  hand,  and  showed  us,  that  by  only  whirling  it  five  or 
six  times  round  his  head  in  the  air,  he  could  electrify 
himself  so  strongly,  as  to  send  out  sparks  from  all  the 
uncovered  parts  of  his  body.  The  inlaid  floors  had  be- 
come so  dry,  as  to  form  a  complete  insulation. 

In  the  winter  time,  therefore,  we  must  consider  the 
frozen  surface  of  the  earth,  the  water,  and  atmosphere, 
as  forming  one  electrical  machine  of  enormous  magni- 
tude ;  for,  the  natural  cold  of  those  countries  is  often 
so  great  as  to  cool  water  to  more  than  20  degrees  be- 
low 0,  and  thus  render  it  an  electric.  That  something  of 
this  kind  is  real,  appears  from  the  excessive  bright  au- 
rora borealis,  and  other  electric  appearances,  far  exceed- 
ing any  thing  in  this  country.  In  the  summer  time  these 
appearances  are  not  remarkable,  but  an  excessive  heat 
prevails  from  the  long  continuance  of  the  sun  above  the 
horizon.  The  quantity  of  heat  in  summer  being  succeed- 
ed by  a  proportionable  quantity  of  electricity  in  winter, 
one  can  scarce  avoid  concluding,  that  the  heat  in  sum- 


ON    A    THERMOMETER.  339 

mer,  or  disengaged  fire,  becomes  electric  fluid  in  winter, 
wMch  going  off  through  the  celestial  expanse,  returns 
a  i  ro  the  grand  source  of  light  and  heat ;  thus  mak- 
iu  room  for  the  succeeding  quantities  which  are  to  en- 
li     :  the  earth  during  the  following  season. 

If  the  identity  of  light,  fire,  and  electricity,  be  admit- 
ted, the  source  from  whence  the  electric  fluid  is  derived 
into  the  earth  and  atmosphere  is  very  evident ;  it  can  be 
no  other  than  the  sun  or  source  of  light.  The  vast 
quantity  of  light  continually  proceeding  from  the  sun 
to  the  earth,  must  in  a  great  measure  be  absorded  there- 
by ;  but,  from  the  other  operations  in  nature,  it  is  pre- 
vented from  remaining  there:  it  is  therefore  in  continu- 
al circulation,  to  make  room  for  new  quantities  continu- 
ally coming  from  the  sun.  It  must  however  be  observed, 
that  as  this  fluid  is  variously  combined,  it  cannot  appear 
in  its  natural  form  of  fire  or  light,  till  it  be  disengaged, 
and  capable  of  receiving  a  motion  similar  to  what  it  had 
when  proceeding  from  the  sun. 

This  change  of  matter  into  a  different  form,  with  the 
subsequent  regeneration  of  it  into  its  primative  form,  is, 
says  Mr.  Jones,  one  of  the  greatest  secrets  of  nature, 
whereby  the  world  is  kept  from  decaying,  either  with 
respect  to  its  matter  or  its  motion.  By  means  of  a  cir- 
culation in  matter,  the  lasting  motions  of  nature  are 
maintained,  and  its  stores  unexhausted.* 

The  experiments  that  I  shall  now  lay  before  you, 
do  in  the  strongest  manner  prove  the  identity  of  the 
electric  fluid  and  light,  and  that  both  are  transmitted 
through  electric  as  well  as  other  substances  ;  and  that 
it  is  on  the  motion  of  this  fluid  that  transparency  de- 
pends ;  that,  when  this  medium  is  at  rest,  the  body  is 
opake  j  when  set  in  motion,  it  becomes  transparent. 

LUMINOUS    EXPERIMENTS. 

To  render  an  ivory  ball  luminious.  Take  a  strong 
spark  through  the  centre  of  the  ball,  and  it  will  be  il- 
luminated throughout. 


*  See  Jonse's  Physiological  Disquisitions,  p,  51. 


J40  LUMINOUS    EXPERIMENTS. 


™k 
a 

i 


To  obtain  a  crimson  coloured  spark.     Take  a  spark 
through  a  ball  of  box  wood,  and  it  will  appear  of  a 
beautiful  crimson,  or  rather  a  fine  scarlet  colour ;  or 
the  shock  may  be  passed  through  pieces  of  wood  o 
different  thicknesses  and  density ;  which  will  afford 
very  ample  field  for  observation  and  experiment. 

To  make  a  bottle  of  water  luminous*  Connect  one  end 
of  a  chain  with  the  outside  of  a  charged  jar,  let  the  other 
end  lie  on  the  table,  place  the  end  of  another  piece  o 
chain  at  about  one  quarter  of  an  inch  distance  from  th 
former  ;  then  set  a  decanter  of  water  on  these  separat 
ed  ends,  and,  on  making  the  discharge  through  th 
chain,  the  water  will  appear  perfectly  and  beautifull 
luminous. 

There  is  scarce  any  substance,  fluid  or  solid,  bu 
what  may  be  rendered  luminous,  by  passing  the  elec 
trie  fluid  through  it,  and  thereby  separating  the  elec 
trie  powers  inherent  in  the  body.  In  water,  spirit,  oil 
animal  fluids  of  all  kinds,  the  discharge  of  a  Leyde 
phial  of  almost  any  size  will  appear  very  splendid 
provided  you  take  care  to  place  them  in  the  circuit, 
that  the  fluid  may  not  pass  through  too  great  a  quant 
ty  of  them. 

To  perform  this,  place  the  fluid,  on  which  the  exp 
riment  is  to  be  made,  in  a  tube  three  quarters  of 
inch  in  diameter  and  four  inches  long ;  stop  up  th 
orifices  of  the  tube  with   two  corks,  through  whic 
push  two  pointed  wires,  so  that  the  points  may  approac 
within  one-eighth  of  an  inch  of  each  other;  the  fluid, 
in  passing  through  the  interval  which  separates  the  wires 
is  always  luminous,  if  a  force  be  used  sufficiently  strong  ; 
the  glass  tube,  if  not  very  thick,  always  breaks  when 
this  experiment  succeeds.* 

To  make  the  passage  of  the  fluid  luminous  in  the 
acids,  they  must  be  placed  in  capillary  tubes,  and  two 
wires  introduced,  as  in  the  preceding  experiment,  whose 
points  shall  be  very  near  each  other.  It  is  a  well-known 
fact,  that  the  discharge  of  a  small  Leyden  phial,  in  pass- 


See  Mr.  Morgan's  paper,  Phil.  Trans. 


LUMINOUS    EXPERIMENTS.  S41 

ig  over  a  strip  of  gold,  silver,  or  Dutch  metal  leaf, 
ill  appear  very  luminous.  By  conveying  the  contents 
f  a  jar,  measuring  two  gallons,  over  a  strip  of  gold- 
:af  one-eighth  of  an  inch  in  diameter  and  a  yard  long, 

will  frequently  give  the  whole  a  dazzling  brightness, 
ou  may  give  this  experiment  a  curious  diversity,  by 
tying  the  gold  or  silver  leaf  on  a  piece  of  glass,  and 
len placing  the  glass  in  water;  for,  the  whole  gold  leaf 
ill  appear  most  brilliantly  luminous  in  the  water,  by 
xposing  it  thus  circumstanced  to  the  explosion  of  a 
attery. 

The  difficulty  of  making  any  quantity  of  the  electri- 
al  fluid  luminous  in  any  body,  increases  as  the  con- 
ucting  power  of  that  body  increases  ;  because  the  two 
owers  unite  sooner  in  proportion  to  the  conducting 
ower,  and  consequenrly  all  electric  signs  vanish. 

In  order  to  make  the  contents  of  a  jar  luminous  in 
•oiling  water,  a  much  higher  charge  is  necessary,  than 
rould  be  sufficient  to  make  it  luminous  in  cold  water, 
?hich  is  universally  allowed  to  be  the  worse  conductor. 

There  are  various  reasons  for  believing  the  acids  to 
>e  very  good  conductors  ;  if,  therefore,  into  a  tube 
illed  with  water  and  circumstanced  as  has  been  already 
lescribed,  a  few  drops  of  either  of  the  mineral  acids 
)e  poured,  it  will  be  almost  impossible  to  make  the 
luid  luminous  in  its  passage  through  the  tube,  as  the 
wo  powers  unite  immediately. 

The  ease  with  which  the  electrical  fluid  is  rendered 
uminous  in  any  particular  body,  is  increased  by  increas- 
ng  the  rarity  of  the  body.  The  appearance  of  a  spark, 
)r  of  the  discharge  of  a  Leyden  phial,  in  rarefied  air 
s  well  known.  But  we  need  not  rest  the  truth  of  the 
^receding  observation  on  the  several  varieties  of  this 
act ;  similar  phenomena  attend  the  rarefaction  of  ether, 
rf  spirits  of  wine,  and  of  water. 

Spark  in  rarefied  water,  spirit  of  wine,  ether,  and 
uids.  Into  the  orifice  of  a  tube  forty-eight  inches  long 
md  two-thirds  of  an  inch  in  diameter,  Mr.  Morgan 
zemented  an  iron  ball,  so  as  to  bear  the  weight  which 
presses  upon  it  when  the  tube  is  filled  with  quicksilver, 
leaving  only  an  interval  at  the  open  end,  which  con- 


342  LUMINOUS    EXPERIMENTS, 

tains  a  few  drops  of  water.  Having  inverted  the  tube, 
and  plunged  the  open  end  of  it  into  a  bason  of  mercu- 
ry, the  mercury  in  the  tube  stood  nearly  half  an  inch 
lower  than  it  did  in  a  barometer  at  the  same  instant,  ow- 
ing to  the  vapour  which  was  formed  by  the  water.  But 
through  this  rarefied  water,  the  electrical  spark  passed 
as  luminously  as  it.  does  through  air  equally  rarefied. 

If,  instead  of  water,  a  few  drops  of  spirits  of  wine 
be  placed  on  the  surface  of  the  mercury,  phenomena, 
similar  to  those  of  the  preceding  experiment,  will  be  dis- 
covered, with  this  difference  only,  that  as  the  vapour  in 
this  case  is  more  dense,  the  electrical  spark,  in  its  pas- 
sage through  it,  is  not  quite  so  luminous  as  it  is  in  the 
vapour  of  water. 

Good  ether  substituted  in  the  room  of  the  spirits  of 
wine,  will  press  the  mercury  down  so  low  as  the  height  of 
16  or  17  inches.  The  electrical  fluid,  in  passing  through 
this  vapour,  unless  the  force  be  very  great  indeed,  is 
scarcely  luminous ;  but  if  the  pressure  on  the  surface 
of  the  mercury  in  the  bason,  be  gradually  lessened  by 
the  aid  of  an  air-pump,  the  vapour  will  become  more 
and  more  rare,  and  the  electric  spark,  in  passing  through 
it,  more  and  more  luminous. 

The  brilliancy  and  splendour  of  the  electric  spark  is 
always  increased  when  it  is  compressed  into  a  smaller 
compass.  That  is,  a  spark,  or  the  discharge  of  a  bat- 
tery, which  we  might  suppose  equal  to  a  sphere  one 
quarter  of  an  inch  in  diameter,  will  appear  much  more 
brilliant,  if  the  same  quantity  of  fluid  be  compressed  into  a 
sphere  one-eighth  of  an  inch  in  diameter.  This  obser- 
vation is  the  obvious  consequence  of  many  known  facts; 
if  the  machine  be  large  enough  to  afford  a  spark,  whose 
length  is  nine  or  ten  inches,  this  spark  may  be  seen  some- 
times forming  itself  into  a  brush,  in  which  state  it  oc- 
cupies more  room,  but  appears  very  faintly  luminous ;  at 
other  times,  the  same  spark  is  seen  dividing  itself  into  a 
variety  of  ramifications  which  shoot  into  the  surrounding 
air.  In  this  case,  likewise,  the  fluid  is  diffused  over  a 
large  surface,  and  in  proportion  to  the  diffusion,  so  is  the 
faintness  of  the  appearance.  A  spark,  which  in  the  open 
air  cannot  exceed  one-quarter  of  an  inch  in  diameter, 


LUMINOUS    EXPERIMENTS.  343 

will  appear  to  fill  the  whole  of  an  exhausted  receiver, 
four  inches  wide  and  eight  inches  long  :  but  in  the  for- 
mer case  it  is  brilliant,  and  in  the  latter  it  grows  fainter 
and  fainter,  as  the  size  of  the  receiver  increases.  This 
observation  is  further  illustrated  by  the  following  ex- 
periments. 

Introduce  two  pointed  wires  into  the  vacuum,  so  that 
the  fluid  may  easily  pass  from  the  point  of  the  one  to  the 
point  of  the  other  ;  when  the  distance  between  them  is 
not  more  than  the  one-tenth  of  an  inch,  in  this  case  we 
shall  find  a  brilliancy  as  great  as  in  the  open  air. 

Into  a  Torricellian  vacuum,  thirty-six  inches  long,  con- 
vey as  much  air  as  will  fill  two  inches  only  of  the  ex- 
hausted tube  if  it  were  inverted  in  water ;  this  quanti- 
ty of  air  will  afford  resistance  enough  to  condense  the 
fluid,  as  it  passes  through  the  tube  in  a  spark  thirty- 
eight  inches  in  length.  The  brilliancy  of  the  spark  in 
condensed  air,  in  water,  and  in  all  substances  through 
which  it  passes  with  difficulty,  depends  on  principles  si- 
milar to  those  which  account  for  the  preceding  facts. 

In  the  appearances  of  electricity,  as  well  as  in  those  of 
burning  bodies,  there  are  cases  in  which  all  the  rays  of 
light  do  not  escape;  and  the  most  refrangible  rays  are 
those  which  escape  first  or  most  easily.  The  electrical 
brush  is  always  of  a  purple  or  bluish  hue.  If  you  con- 
vey a  spark  through  a  Torricellian  vacuum,  made  with- 
out boiling  the  mercury  in  the  tube,  the  brush  will  dis- 
play the  indigo  rays. 

To  an  insulated  metallic  ball,  four  inches  in  diameter, 
fix  a  wire  a  foot  and  a  half  long ;  this  wire  should  ter- 
minate in  four  ramifications,  each  of  which  must  be  fix- 
sd  to  a  metallic  ball  half  an  inch  in  diameter,  and  placed 
at  an  equal  distance  from  a  metallic  plate,  which  must 
communicate  by  metallic  conductors  with  the  ground. 
A  powerful  spark,  after  falling  on  the  large  ball  at 
:>ne  extremity  of  the  wire,  will  be  divided  in  its  passage 
from  the  four  small  balls  to  the  metallic  plate.  When 
you  examine  the  division  of  the  fluid  in  a  darkened 
room,  you  will  discover  some  little  ramifications,  which 
will  yield  the  indigo  rays  only :  indeed,  at  the  edges 
>f  all  weak  sparks,  the  same  purple  appearance  may  be 


844  LUMINOUS    EXPERIMENTS. 

discovered.  You  may  likewise  observe,  that  the  near 
6r  you  approach  the  centre  of  the  spark,  the  greater  h 
the  brilliancy  of  its  colour. 

The  influence  of  different  media  on  electrical  light  i 
analogous  to  their  influence  on  solar  light,  and  will  heir 
us  to  account  for  some  very  singular  appearances. 

Let  a  pointed  wire,  having  a  metallic  ball  fixed  tc 
one  of  its  extremities,  be  forced  obliquely  into  ; 
piece  of  wood,  so  as  to  make  a  small  angle  with  tfec 
surface  of  the  wood,  and  to  make  the  point  lie  abou 
one-eighth  of  an  inch  below  the  surface.  Let  anothei 
pointed  wire,  which  communicates  with  the  ground,  bi 
forced  in  the  same  manner  into  the  same  wood,  so  tha 
its  point  likewise  may  lie  about  one-eighth  of  an  inct 
below  the  surface,  and  about  two  inches  distant  fron 
the  point  of  the  first  wire.  Let  the  wood  be  insulated 
and  a  strong  spark,  which  strikes  on  the  metallic  ball 
will  force  its  passage  through  the  interval  of  wood  whicl 
lies  between  the  points,  and  appear  as  red  as  blood.  Tc 
prove  that  this  appearance  depends  on  the  wood's  absorp 
tion  of  ail  the  rays  but  the  red  ;  when  these  points  wen 
the  deepest  below  the  surface,  the  red  only  came  tc 
the  eye  through  a  prism  ;  when  they  were  raised  a  little 
nearer  the  surface,  the  red  and  orange  appeared  ;  whei 
nearer  still,  the  yellow  ;  and  so  on,  till,  by  making  th< 
spark  pass  through  the  wood  very  near  its  surface,  ai" 
the  rays  were  at  length  able  to  reach  the  eye. 

"  Previous  to  the  discoveries  that  have  been  made  ii 
modern  times,  relative  to  the  chemical  effects  of  light 
somcmathematical  philosophers  disputed  its  existeno 
as  a  particular  fluid,  and  even  that  of  fire  itself;  the; 
crudely  imagined,  that  the  phenomena  of  light  aru 
heat  were  only  particular  modifications  of  the  sub 
stances,  in  which  they  appeared  ;  a  kind  of  vibratioi 
of  their  particles,  transmitted  by  means  of  a  medium 
as  in  the  case  of  sounds." 

,  "  They  applied  the  mathematics  to  this  hypothesis 
in  order  to  explain  some  particular  phenomena  ;  and  a 
every  thing  that  appears  to  be  deduced  from  mathema 
tical  theorems  easily  seduces  those  who  do  not  apph 
themselves  to  examine  data,  this  theory,  which  effect u 


LUMINOUS    EXPERIMENTS.  345 

ally  barred  the  road  to  the  most  important  physical  re- 
searches, met  with  many  partizans  :  but  chemistry  and 
meteorology  have  now  come  in  to  terminate  the  contro- 
versy ;  and  there  are  at  present  very  few  philosophers 
who  do  not  agree,  that  lucidity  and  heat  are  the  effects 
of  two  fluids,  namely,  light  and  fire,  which  produce  those 
particular  phenomena  whenever  they  are  at  liberty  ;  but 
which,  at  the  same  time,  may  be  so  combined  with  other 
substances  as  to  lie  hidden  in  them  without  vproducing 
these  effects,  till  again  set  at  liberty.  By  an  attention  to 
these  great  agents,  the  stud  of  nature  has  proceeded 
with  rapidity,  and  the  present  asra  will  probably  on  this 
account  be  as  much  celebrated  in  the  history  of  science, 
as  those  in  which  Pascal  demonstrated  the  pressure  of 
the  air  on  bodies,  and  in  which  Newton  discovered  the 
principle  of  gravity. 

"  Our  progress  in  the  knowledge  of  the  origin  of 
bodies,  has  been  much  advanced  in  this  age,  since  che- 
mists and  philosophers  have  begun  to  examine  their  vo- 
latile products,  in  other  words,  the  elastic  fluids  ;  but 
this  would  have  been  doing  but  little,  had  not  the  ad- 
vances in  other  branches  of  natural  knowledge  led  them 
to  discover,  that  the  phenomenon  of  heat  proceeded  from 
a  particular  substance  susceptible  of  chemical  affinities, 
namely,  fire,  the  immediate  cause  of  heat.  Here  is  then 
a  substance  of  the  highest  importance  in  the  composi- 
tion of  bodies,  which  nevertheless  escaped  the  attention 
of  philosophers,  while  they  only  estimated  and  express- 
ed the  amount  of  their  products  by  their  weights.  Is 
it  possible  for  any  one  to  suppose,  that  we  have  hereby 
discovered  all  the  imponderable  substances  that  enter 
into  the  composition  of  natural  bodies  ?" 

Ought  we  to  neglect  the  phenomena  of  lucidity,  while 
every  thing  announces  to  us,  that  light  is  a  chemical 
substance  ?  This  neglect  is  scarcely  now  to  be  appre- 
hended, as  philosophers  are  aware,  that  great  chemical 
effects  may  be  produced  by  imponderable  substances. 
The  phosphoric  phenomena  of  certain  mineral  substan- 
ces indicate  clearly,  that  light  enters  as  an  ingredient  into 
their  composition.     Wilson  and  Beccaria  have  shown, 

vol.  iv,  2  Y 


MS  LUMINOUS    EXPERIMENTS. 

that  every  substance  in  nature  is  more  or  less  phospho- 
rical ;  and  you  have  just  seen,  that  there  is  scarce  any 
substance  but  what  you  may  render  luminous  by  sepa- 
rating its  electric  powers. 

The  relation*  of  these  two  imponderable  substances, 
whose  existence  is  now  established  beyond  a  doubt,  is 
such  as  in  many  other  instances  is  found  to  subsist  be- 
tween such  substances  as  enter  into  the  composition  the 
one  of  the  other.  Light  frequently  does  not  sensibly  act 
otherwise  than  as  the  cause  of  lucidity,  or  of  luminous 
phenomena ;  and  fire  in  the  same  manner,  only  as  the 
ciuse  of  heat :  but  at  other  times  fire,  in  producing  heat, 
produces  also  in  the  end  its  luminous  effects ;  and  in  some 
circumstances  light,  in  making  visible  the  objects,  by  its 
reflexion  contributes  to  produce  heat.  These  phenomena 
clearly  indicate,  that  one  of  these  substances  contains  the 
other,  but  that  under  certain  circumstances  it  may  be  so 
decomposed,  as  to  permit  either  of  them  to  exercise  its 
own  peculiar  properties. 

The'most  excellent  Boerhaave,  in  bis  analysis  of  fire, 
has  so  clearly  established  the  universality  and  importance 
of  this  element,  and  so  stripped  it  of  the  mystic  dress,  in 
which  it  was  enveloped  before  his  time,  that  one  would 
imagine  it  scarce  possible  for  philosophers  to  have  resolv- 
ed so  many  of  its  subtile  effects  into  occult  or  fanciful 
properties ;  yet,  that  such  has  been  the  case  is  evident 
from  the  slightest  inspection  of  modern  theories.  Again, 
though  the  most  obvious  phenomena  in  nature,  and  nu- 
merous experiments,  tend  to  ascertain  beyond  all  doubt, 
that  the  matter  of  common  light  or  fire  pervades  all  na- 
ture, and  fills  all  things ;  yet,  as  I  have  before  observed, 
the  whole  has  been  overlooked  as  an  accidental  filtration 
that  implied  no  consequences,  nor  interfered  with  the 
various  unintelligible  properties  of  bodies,  notwithstand- 
ing its  access  to  their  innermost  penetralia. 

It  is  evident,  that  the  natural  omnipotence  of  light  de- 
pends on  the  sun ;  by  him  in  a  natural  sense  the  matter 
of  fire,  as  his  issue,  is  omnipresent  and  all-sufficient.  If 
the  life  of  all  things  depends  on  the  activity  he  commu- 
nicates to  them,  is  it  not  probable,  that  it  is  the  influence 
of  the  solar  fluid  that  generates  and  maintains  that  life  in 


LUMINOUS    EXPERIMENTS.  347 

all  its  specific  characters,  in  every  being  according  to  its 
kind  ?  And  that  life,  whether  it  be  vegetable  or  animal, 
is  such  as  it  is  according  to  the  state  of  the  fire  in  it ;  and 
that  every  dead  £ hing  is  only  so,  because  its  fire  is  quench- 
ed ?  The  ancient  philosophers  affirmed,  that  the  light  of 
the  sun,  which  gave  life  and  motion  to  all  things,  must 
be  in  all  things ;  they  therefore  conceived  all  things  to  be 
replete  with  this  fluid. 

Is  it  not  highly  probable  then,*  that  this  terraqueous 
globe  is  only  an  accumulation  of  materials  introduced  in 
the  boundless  ocean  of  the  solar  fluid  as  a  theatre,  on  which, 
under  the  direction  and  guidance  of  the  Almighty,  it  may 
display  its  inexhaustible  energy  and  powers  ;  the  terres- 
trial mass  being  so  disposed  and  arranged  by  its  Divine 
Author,  as  to  become  a  seminal  bed  of  materials,  where 
light  and  fire  may  pierce,  animate,  and  display  an  endless 
variety  and  succession  of  beings  ?  This  fluid  extricates 
all  the  forms,  and  generates  all  the  powers  of  nature,  out 
of  the* materials  provided  for  it  to  possess. 

It  is  impossible  to  form  any  clear  or  distinct  idea  of  the 
agency  of  the  solar  fluid  in  the  air,  in  animals,  in  vege- 
tables, &c.  without  first  considering  it  more  in  general ; 
nor  can  you  properly  have  a  view  of  the  universal  agency 
of  the  element  productive  of  fire,  light,  and  electricity, 
and  its  importance  to  the  animal  frame,  unless  you  take 
an  enlarged  prospect  of  its  action.  Besides,  knowledge 
often  makes  more  rapid  advances  by  reasoning  upon 
known  facts  than  by  discovering  new  ones,  which,  by 
their  novelty,  too  often  lead  to  hasty  undigested  theories. 
In  the  disquisition  upon  these  fluids,  I  have  always  an 
eye  upon  the  doctrine  of  electricity;  and  the  preceding 
as  well  as  following  experiments  all  concur  in  showing 
the  analogy  that  runs  through  nature  ;  and  you  will  find 
that  electricity,  though  not  in  name,  has  been  the  doc- 
trine of  all  ages.  I  shall  therefore  continue  to  treat  of 
these  wonderful  fluids.  Of  all  that  are  known  in  the  uni- 
verse; the  mobility  of  the  matter  of  light  is  the  greatest. 
There  is  not  the  smallest  speck  of  colour  in  the  beams  of 


See  vol.  ii.  Lecture  xxi. 


348  LUMINOUS    EXPERIMENTS. 

the  sun,  that  does  not  obediently  receive  perpetual  im- 
pressions from  him  in  all  lineal  directions,  by  night  as 
well  as  by  day.  The  sun,  as  the  fountain  of  motion,  is 
also  continually  agitating  this  fluid  either  radially  or  ob- 
liquely, by  the  lateral  shocks  and  friction  of  the  radii  upon 
those  parts  of  the  fluid  that  lie  out  of  the  line  of  the  sun's 
irradiation  ;  these,  together  with  the  constant  vicissitudes 
of  day  and  night,  preserve  a  constant  motion  in  ail  its 
internal  parts. 

But  even  this  is  inadequate  to  convey  to  you  a  just 
idea  of  the  constant,  positive,  intense  energy,  from  the 
activity  of  the  matter  of  light.  Of  this  you  will  form  a 
better  idea,  by  examining  the  mode  of  its  action  in  the 
interior  parts  of  the  most  rigid  and  solid  bodies.  For  in 
the  most  secret  recesses  of  the  most  solid  and  passive 
substances,  the  matter  of  light  is  so  far  from  existing  in 
an  indolent  quiescent  state,  that  it  is  impossible  to  form 
an  adequate  idea  of  its  incessant  and  active  energy  under 
these  circumstances.  Yet  this  state  of  bodies  is  but  little 
thought  of  by  philosophers  in  their  researches  into  its 
properties,  either  common  or  special ;  which  I  shall  illus- 
trate by  considering  the  cases  of  sonorous  bodies,  and 
the  phenomenon  of  hammering  cold  iron  red-hot. 

If  this  fluid  resided  within  bodies  in  an  indolent  and 
passive  state,  it  could  exert  no  reluctation  on  any  mecha- 
nical force,  disturbing  its  passive  occupation  within  bo- 
dies ;  whereas,  in  fact,  its  natural  state  is  never  disturb- 
ed without  an  active  irritation  being  excited  in  the  fluid, 
to  recover  and  repossess  its  organical  and  interstitial  in- 
herency, greater  than  that  by  which  it  was  expelled ;  it 
returns  with  a  force  not  barely  sufficient  to  recover  the 
dimensions  it  occupied  within  bodies,  but  with  a  violence 
capable  of  expanding  them  as  much  beyond  their  natu- 
ral size  as  the  external  blow  or  concussion  tended  to 
compress  them  within  it :  hence  a  vibratory  colluctation 
takes  place  between  that  action  which  preserves  bodies  in 
their  natural  crasis,  and  the' 'rapid  returns  of  the  fluid  to 
its  natural  state;  these  vibrations  continue  for  a  time,  and 
die  away  imperceptibly. 

This  intense  agitation,  excited  by  the  collision  of  bo- 
dies, is  not  confined  to  their  points  of  contact,  but  per- 


LUMINOUS    EXPERIMENTS.  349 

vades  their  whole  substance,  and  oscillates  in  every  part. 
This  is  demonstrated  to  the  eye  and  ear,  when  a  musical 
chord  is  struck.  You  have  specimens  also  of  it  in  all  elastic 
sonorous  bodies.  When  a  bell  is  struck,  the  sound  conti- 
nues labouring  in  the  ear  for  a  considerable  time  after- 
wards ;  nor  is  the  tumult  subsided  when  our  sense  of  it 
fails  ;  it  passes  through  a  gradual  decay  below  the  stand- 
ard of  sense. 

Suspend  an  iron  poker  from  the  head,  by  the  teeth, 
and  the  iron  discovers  no  great  degree  of  any  sonorous 
quality  ;  yet  if  it  be  struck,  you  will  have  a  very  striking 
sensation  of  the  vibratory  motion  its  whole  substance  re- 
ceives from  the  stroke,  by  the  teeth's  transmisson  of  their 
feeling  to  the  ear. 

Physicians  talk  of  the  irritability  of  our  nervous  system, 
as  a  very  mysterious  and  wonderful  phenomenon ;  but 
there  are  more  striking  examples  of  this  irritability  in  the 
most  rigid  dead  substances.  Substances,  such  as  glass, 
bell-metal,  &c.  which  are  so  rigid  that  few  instruments 
will  make  an  impression  on  them,  yet  are  capable  of  be- 
ing agitated  through  every  atom  of  their  substance  ;  nay, 
in  some  cases,  to  be  burst  in  pieces  by  the  impression  of 
certain  sounds.  A  wine-glass  will  burst  in  pieces  by  the 
action  excited  through  its  substance  by  certain  tones  of 
voice  ;  columns  of  marble  or  porphyry  are  tremulous  to 
thunder  explosions,  and  to  certain  tones  of  an  organ. 

This  excessive  mobility  of  parts  throughout  the  whole 
substance  of  the  most  rigid  bodies,  clearly  implies  a  great 
turgency  of  their  substance  with  some  very  active  fluid, 
so  that  a  small  increase  of  its  action  is  ready  to  burst  them 
in  pieces.  A  slight  resistance  to  the  internal  agitation  of 
a  bell  will  cause  it  to  crack. 

Now  it  is  impossible  to  conceive,  that  such  a  tremulous 
motion  should  be  produced  through  the  whole  continuity 
of  such  hard  bodies,  unless  they  contained  in  themselves 
some  inconceivably  active  element,  exerting  a  constant 
nisus  to  force  their  parts  to  as  great  a  distance  from  each 
other  as  possible,  and  barely  counteracted  by  the  power 
that  maintains  their  cohesion. 

■  The  symptoms  of  this  restless  activity  within  solid  bo- 
dies are  not  confined  to  such  as  are  commonly  called  elas- 


350  LUMINOUS    EXPERIMENTS. 

tic.  Thus  iron  yields  more  striking  proofs  of  this  latent 
active  principle,  than  any  substance  of  greater  elasticity 
than  itself,  and  thus  discloses  to  our  sensible  conviction 
precisely  what  that  principle  or  restless  element  is,  that 
exerts  its  energy  so  powerfully  within  all  terrestrial  bo- 
dies. 

For  the  power  within  bodies,  that  sustains  and  preserves 
their  form,  is  not  a  passive  power.  It  is  positive  re-action 
to  the  approach  of  the  parts  of  the  body.  The  law  of  re- 
action, being  equal  to  action,  resides  ultimately  in  the  con- 
stitution of  this  powerful  fluid  medium.  Whenever  the 
spaces  it  occupies  within  the  surfaces  of  bodies  are  press- 
ed nearer  one  another  by  any  sudden  shock  or  collision, 
and  consequently  this  medium,  for  an  instant,  driven  out, 
the  next  instant  it  returns  with  violence,  not  enough  to  re- 
gain its  place  in  the  body,  but  equal  to  that  with  which  it 
was  ejected  ;  and  therefore,  in  returning,  it  dilates  its  spa- 
ces as  much  beyond  their  sizes,  as  they  were  compressed 
below  their  natural  standard  by  their  collision  ;  by  which 
means,  a  temporary  oscillation  is  excited  between  the  ef- 
forts of  that  power,  which  circumscribes  bodies,  and  binds 
them  to  their  natural  sizes,  and  the  internal  medium,  which 
was  irritated  by  the  stroke,  to  act  with  a  force  equal  thereto. 

If  the  strokes,  which  dispossess  this  fluid  of  the  spaces 
it  naturally  obtains  within  bodies,  be  quickly  and  succes- 
sively renewed,  before  the  coliuctations  raised  by  former 
ones  have  subsided,  the  internal  agitation  may  thereby 
soon  be  raised  to  such  a  height,  as  to  break  forth  and 
manifest  itself  in  the  form  of  actual  fire. 

Every  material  being  through  all  the  forms  of  nature, 
is  a  composition  of  this  celestial  fluid  and  terrestrial  mat- 
ter ;  you  will  find  the  distribution  of  material  substances 
into  these  two  classes  to  be  the  real  key  to  all  natural 
knowledge  ;  it  not  only  distinguishes  this  globe  from  the 
celestial  fluid  in  which  it  swims,  but  it  is  to  be  applied  to 
every  individual  terrestrial  substance;  which  must  be  con- 
sidered, if  you  would  comprehend  the  phenomena  of  na- 
ture, as  an  intimate  composition  of  these  two  elements ; 
the  latter  being  the  organ  or  case  to  the  energy  of  the 
former,  and  the  modifier  to  its  incessant  activities,  while 
the  former  is  the  medium  used  by  mind  to  impress  those 


LUMINOUS    EXPERIMENTS.  351 

characters  on  the  latter,  which  are  known  as  the  distin- 
guishing properties  of  different  bodies. 

This  fluid,  according  to  the  variety  of  the  phenomena 
by  which  its  energy  has  been  discovered  to  us,  has  been 
called  under  different  circumstances,  light,  fire,  electri- 
city, materia  subtilis,  materia  media,  &c.  At  other  times 
it  has  been  divested  of  its  materiality,  and  has  been  con- 
sidered merely  as  a  principle  annexed  to  or  inherent  in 
matter,  under  the  terms  of  occult  quality,  nisus,  attrac- 
tion, electric  attraction,  elasticity,  irritability,  stimulus,  sym- 
pathy, vital  principle,  life,  &c.  &c. 

This  invisible  fire  is  ever  ready  to  exert  and  show  it- 
self in  its  effects,  cherishing,  heating,  fermenting,  dis- 
solving, shining,  and  operating  in  the  various  manners, 
according  to  the  subjects  which  employ  and  determine 
its  force.  It  is  present  in  all  parts  of  the  earth  and  fir- 
mament, though  in  most  cases  latent  and  unobserved, 
till  some  occasion  produces  it  in  act,  and  renders  its  ef- 
fects visible  ;  it  exists  in  our  constitution,  and  indeed  in 
every  form  in  nature  in  two  modes,  interstitially  and  or- 
ganically. If  the  pores  of  gold,  which  is  one  of  the  dens- 
est known  substances,  exceed  its  solid  or  earthly  parts, 
how  much  greater  must  the  proportion  of  solar  fluid  be 
in  our  frame  than  in  that  of  gold  1  To  illustrate  this  I 
shall  refer  to  the  element  of  water. 

Now  water,  by  its  transparency,  certifies  to  your  senses, 
that  light  has  free  access  into  and  through  its  substance ; 
and  that  it  probably  fills  up  its  interstices,  as  water  does 
a  spunge  when  soaked  in  it.  But  we  know  further,  by 
the  fluidity  and  the  volatilization  of  water,  that  the  matter 
or  light  of  fire  has  not  only  access  to  its  interstices,  but 
penetrates  and  occupies  its  similar  elementary  particles ; 
for  these  particles  could  not  be  rendered  volatile,  but  by 
internal  dilation,  nor  could  they  be  dilated,  but  by  some- 
thing that  reached  their  internal  parts. 

These  particles  then  are  the  organical  parts  of  water, 
which  have  their  individuality  as  separable  elementary 
parts,  as  well  as  their  similarity  of  character,  preserved 
by  that  etherial  principle  that  possesses  them. 

^  These  points  being  cleared,  you  will  now  have  an  ob- 
vious solution  of  the  difficulties  which  have  attended  the 


352  LUMINOUS    EXPERIMENTS. 

question,  What  is  the  principle  of  natural  life  ?  Modern 
physiology  has  indeed  bewildered  the  conception  of  its 
pupils,  by  not  distinguishing  between  the  term  life,  used 
metaphysically  for  our  system  of  consciousness,  or  as  the 
result  of  our  whole  composition  explicable  only  by  the 
Creator,  and  the  same  term  life,  used  physically  to  denote 
the  natural  power  that  presides  in  reciprocally  regulating, 
and  being  regulated  by  the  mechanism  and  disposition  of 
the  whole,  and  every  part  and  particle  of  our  corporeal 
frame. 

It  is  by  the  unremitting  reciprocal  corruscations  of  this 
vital  principle  in  the  fluids  and  solids,  according  to  the 
different  qualities  and  consistencies  they  assume  in  dif- 
ferent parts  of  our  constitution,  that  the  whole  system 
of  life  is  displayed  and  maintained  in  every  individual. 
Light  is  not  more  instantaneously  dispatched  by  reflexion 
from  a  mirror,  or  by  that  power  which  every  point  of 
the  air  has  of  reflecting  lightning,  than  that  with  which 
the  same  fluid,  under  the  character  and  modification  of 
the  vital  principle,  acts  from  place  to  place  in  the  human 
frame. 

For  the  moment  of  willing,  and  moving  any  member, 
is  undistinguishably  the  same ;  so  likewise  the  moment 
of  being  touched,  and  the  touch  being  felt.  But  these 
instantaneous  transmissions  in  our  frame  are  not  confined 
to  such  as  we  have  a  conscious  perception  of;  they  are 
incessantly  transacting  ;  the  remotest  vibrating  artery  cor- 
responding with  the  heart,  does  not  more  immediately 
and  constantly  feel  its  power,  than  the  material  princi- 
ple of  vitality  through  its  whole  form  in  our  structure 
feels  the  permanent  influence  of  its  own  power  concen- 
tered in  and  irradiating  from  the  brain,  the  nerves  being 
the  directors  of  the  various  intended  energy  of  the  pow- 
ers of  natural  life.  This  vivifying  plenum,  occupying  and 
organizing  every  particle  and  interstice  in  our  composi- 
tion, can  discharge  its  whole  nisus  according  to  the  inti- 
mation and  direction  of  any  nerve  or  nerves,  as  instantly 
as  electricity  does  through  the  substance  of  the  body  that 
receives  the  shock. 

When  you  consider  the  rarefying  and  expansive  force 
of  this  element,  which  is  capable  in  an  instant  of  time  to 


LUMINOUS    EXPERIMENTS.  353 

produce  the  greatest  and  most  stupendous  effects,  you 
have  a  full  proof  not  only  of  the  power  of  fire,  but  al- 
so of  the  wisdom  with  which  it  is  managed,  and  with- 
held from  bursting  forth  to  the  utter  ravage  and  de- 
struction of  all  things  ;  and  it  is  very  remarkable,  that 
this  same  element,  so  fierce  and  destructive,  should  yet 
be  so  variously  tempered,  and  applied  by  Divine  Provi- 
dence, as  to  be  the  genial  and  cherishing  flame  of  all 
natural  life. 

So  bright  and  lively  are  the  signatures  of  a  Divine 
Mind  operating  and  displaying  itself  in  fire  and  light 
throughout  the  world,  that,  as  Aristotle  observes,  "  all 
things  seem  full  of  divinities,  whose  apparitions  on  all 
sides  strike  and  dazzle  our  eyes."  And  indeed  the 
wisest  men  of  antiquity,  how  much  soever  they  attri- 
buted to  second  causes,  and  the  force  of  fire,  yet  sup- 
pose it  always  to  be  governed  by  a  mind  or  intellect  ac- 
tive and  provident,  restraining  its  force,  and  directing 
its  operations. 

The  order  and  course  of  things,  together  with  what 
we  daily  experience,  fully  proves  that  there  is  a  Mind 
that  governs  and  actuates  this  mundane  system,  as  the 
proper  real  agent  and  cause,  and  that  the  inferior  instru- 
mental cause  is  pure  ether,  fire,  or  the  substance  of 
light,  which  is  applied  and  determined  by  an  Infinite 
Mind  in  the  macrocosm  or  universe  with  unlimited 
power,  and  according  to  stated  rules,  as  it  is  in  the  mi- 
crocosm, with  limited  power  and  skill  by  the  human 
mind.  There  is  no  proof  from  reason,  or  experiment, 
of  any  other  agent  or  efficient  cause  than  mind  or  spirit. 
When  I  speak  therefore  of  corporeal  agents,  or  corpo- 
real causes,  you  understand  them  as  used  in  a  different, 
subordinate,  and  improper  sense. 

The  principles  whereof  a  thing  is  compounded,  the 

instrument  used  in  its  production,  and  the  end  for  which 

it  was  designed,  are  all  in  vulgar  use  termed  causes, 

though  none  of  them  be,  strictly  speaking,  agent  or 

efficient.     Therefore  when  I  speak  of  the  element  of 

fire  as  acting,  it  is  to  be  understood  only  as  a  mean  or 

instrument,  which  is  indeed  the  case  of  all  mechanical 
VOL.  iv.  2  z 


354  OF    ANIMAL    ELECTRICITY. 

causes  whatsoever.  They  are  nevertheless  sometimes 
termed  agents,  or  causes,  although  by  no  means  active 
in  a  strict  and  proper  signification :  when  therefore 
force,  power,  virtue,  or  action,  are  mentioned  as  sub- 
sisting in  an  extended,  corporeal,  or  mechanical  being, 
these  terms  are  not  to  be  taken  in  a  true,  genuine,  real, 
but  only  in  a  gross  and  popular  sense,  which  sticks  in 
appearances,  and  does  not  analyze  things  to  their  first 
principles.  In  compliance  with  established  language, 
and  the  use  of  the  world,  we  must  employ  the  current 
phrases ;  but  for  the  sake  of  truth,  we  should  distin- 
guish their  meaning.* 

What  I  have  here,  as  well  as  in  my  former  lectures, 
laid  before  you,  concur  in  proving  (nay,  all  nature 
gives  testimony  thereto),  "  that  the  fluid  etherial  mat- 
ter of  the  heavens  acts  by  impulse  on  the  solid  matter 
of  the  earth ;  is  instrumental  in  every  one  of  its  pro- 
ductions, and  necessary  to  all  the  stated  phenomena  of 
nature.  The  elements  may  then  be  divided  into  active 
and  passive  ;  not  that  they  are  such  by  any  inherent  or 
essential  difference,  but  that  according  to  the  order  es- 
tablished by  the  Divine  Architect,  they  are  observed  to 
subsist  under  such  relations."! 


OF    ANIMAL    ELECTRICITY. 

I  shall  here  introduce  you  to  the  reasons  and  expe- 
riments, which  induced  Dr.  Shebbeare  to  adopt  electri- 
city, as  the  principle  of  vital  heat  and  motion,  in  1755; 
and  then  show  how  far  his  opinion  has  been  confirmed 
by  subsequent  information. 

A  muscle  put  in  motion  by  the  will,  may  yet  be  more 
actuated  by  a  farther  extension  of  volition,  as  from 
walking  to  running  ;  by  this  operation  of  the  mind, 
there  is  more  of  the  vital  fire  determined  to  the  muscles 
employed  in  those  actions ;  muscles  are  also  brought 
into  action  by  the  fire  from  the  electric  machine,  and 


*  Siris,  No.  154,  155. 
t  Jones's  Essay  on  the  the  First  Principles  of  Philosophy,  p.  8. 


OF    ANIMAL    ELECTRICITY.  S55 

palsied  limbs  have  been  rendered  plump  by  the  same 
machine,  and  a  power  of  motion  and  action  restored 
to  those  whose  palsies  have  not  been  of  long  standing, 
and  which  do  not  take  their  source  from  the  spinal  mar- 
row. This  offers  a  convincing  proof,  that  vital  fire  is 
the  cause  of  muscular  motion,  and  that  the  vital  fire  is 
of  the  same  kind  with  that  produced  by  our  electical 
machines. 

After  so  many  experiments  on  the  electrical  fluid, 
and  after  the  discovery  of  so  many  phenomena,  which 
are  no  ways  to  be  distinguished  from  those  of  fire,  it  will 
scarce  be  any  longer  disputed,  that  they  are  the  same 
in  their  own  nature.  Nor  will  any  one,  I  presume,  af- 
ter the  fire  put  in  action  in  electrical  experiments  has 
been  perceived  by  all  our  senses,  suppose  that  there  can 
be  no  less  reality  in  it,  than  in  earth,  air,  water,  or  fire, 
whose  reality  with  respect  to  mankind  depends  on  the 
evidence  of  those  very  senses.  Electricity  communi- 
cates ideas  to  every  sense ;  it  is  light  to  the  eye,  odour 
to  the  nose,  stroke  to  the  touch,  subacid  to  the  taste. 

If  you  apply  heat,  either  by  means  of  water,  or  any 
other  method,  to  the  heart  of  a  viper  or  of  an  eel  taken 
from  the  body  of  those  animals,  it  will  again  begin  to 
vibrate.  Now  heat  is  fire  in  action,  and  thus  you  see 
the  same  effect  is  produced  as  was  effected  in  the  pal- 
sied limb. 

The  reason  why  the  hearts  of  vipers  and  eels,  and 
such  like  animals,  are  put  into  motion  by  a  power  of 
the  same  nature,  though  in  a  less  degree  than  that  which 
moves  the  heart  of  larger  animals,  is,  because  they  are 
extremely  cold  by  nature,  and  therefore  a  less  degree 
of  fire  actuates  on  their  heart  than  on  those  of  larger 
animals.  It  is  not  improbable  that  the  same  degree  of 
heat,  which  is  necessary  to  keep  a  fowl  alive,  would  de- 
stroy a  frog  or  viper,  and  burst  the  cells  of  the  tunica 
cellularis.  After  the  heart  of  a  viper  has  discontinued 
to  beat  with  the  application  of  any  certain  degree  of 
heat,  it  will  vibrate  again  on  the  application  of  a  supe- 
rior degree. 

The  heart,  which  in  the  open  air  had  ceased  to  move 
with  a  certain  degree  of  heat,  will  vibrate  again  in 
vacuo  with  the  same  degree ;  for  the  pressure  of  the 


256  OF    ANIMAL    ELECTRICITY. 

atmosphere  being  removed,  a  less  power  is  required  to 
distend  the  fibres. 

Dr.  Shebbeare  took  the  heart  of  an  eel,  which  had 
been  some  time  dead,  and  placing  it  on  a  card,  put  it 
on  the  conductor ;  the  first  motion  that  was  communi- 
cated to  it  was  its  swelling,  or  the  diastole  of  the  ven- 
tricles, which  not  being  immediately  followed  by  the 
contraction  or  systole,  he  took  the  electrical  spark 
therefrom,  on  which  it  contracted  ;  it  then  dilated  again, 
and  upon  the  application  of  his  finger  again  contracted; 
and  thus  having  repeated  it  several  times,  the  heat  con- 
tinued to  perform  its  dyastole  and  systole,  without  be- 
ing touched  ;  and  when  it  was  removed  it  ceased,  but 
began  again  upon  being  placed  on  the  bar. 

Lord  Bacon  has  given  us  a  very  remarkable  instance 
of  the  effect  of  fire  upon  the  human  heart.  He  says, 
"  that  upon  the  embowelling  of  a  criminal,  he  had  seen 
the  heart  of  a  man,  after  it  was  thrown  into  the  fire, 
leap  up  for  several  times  together,  at  first  to  the  height 
of  a  foot  and  a  half,  and  then  gradually  lower,  to  die 
of  his  memory,  for  the  space  of  seven  or  eight  mi- 
nutes. 

Trace  vital  heat  and  motion  from  their  source,  and 
you  will  find  these  phenomena  still  more  clearly  illus- 
trated. An  egg,  though  it  includes  all  the  parts  neces- 
sary for  the  formation  of  an  animal,  will  never  produce 
a  chicken,  unles  it  be  kept  in  a  certain  degree  of  heat 
for  a  certain  time  ;  which  heat,  regularly  conducted,  is 
all  that  is  necessary  to  the  production  of  an  animal 
similar  to  the  parent. 

That  there  is  nothing  more  necessary  to  the  produ- 
cing this  animal  from  an  egg,  than  common  fire,  has  been 
long  known  and  practised  in  Egypt,  and  demonstrated 
by  Mr.  Reaumur.  There  is  no  other  vital  principle 
transfused  from  the  hen  to  the  embryo,  than  from  a 
common  fire.  Thus  is  fire  plainly  proved  to  be  the 
first  mover  in  the  animal  machine,  and  is  the  only 
active,  material,  or  natural  principle  during  its  exist- 
ence ;  and  it  is  a  principle  absolutely  necessary  for  the 
preservation  of  health,  and  generating  wholesome  fluids. 
Shall  fire  be  allowed  to  have  the  power  of  beginning 


OF    ANIMAL    ELECTRICITY.  357 

the  vital  motion  in  the  womb,  or  egg,  and  shall  it  be 
refused  the  power  of  continuing  it  after  the  birth  ? 

Now,  for  many  reasons,  which  will  be  seen  as  we  pro- 
ceed, it  appears  that  the  fluid  of  fire  passes  by  the 
nerves  to  the  brain  and  spinal  marrow,  and  from  thence 
to  the  heart  for  supplying  trie  cause  of  involuntary 
motion,  and  that  a  sufficient  quantity  is  always  detained 
there  to  go  to  the  muscles  at  particular  times  for  the 
performing  voluntary  motion. 

This  fire  (the  reality  of  whose  existence  is  proved 
by  all  the  demonstrations  which  can  attend  the  proof  of 
any  existence,  and  whose  general  properties  are  now 
well  known)  is  lodged  in  the  brain,  medulla  spinalis, 
ganglions,  and  nerves,  and  thence  operates  on  aU-the 
different  parts  of  the  body.  The  diminution  and  waste 
of  this  fire  is  continually  supplied  from  the  earth. 

The  nerves,  which  are  destined  to  the  sense  of  feel- 
ing, are  the  conductors  of  this  fire  to  the  brain  ;  while 
those  which  are  destined  to  motion,  are  the  conductors 
by  which  it  is  conveyed  to  the  muscles.  For  a  parti- 
cular explanation  of  the  manner  in  which  it  acts,  I  must 
refer  you  to  Dr.  Shebbeare's  masterly  performance. 

It  is  not  the  fluid  of  fire  alone  that  constitutes  and 
preserves  the  vital  heat  and  vital  motion  ;  but  it  must 
for  this  purpose  be  brought  into  a  certain  state  or  de- 
gree of  action,  which,  in  a  healthy  man,  amounts  to  98° 
of  Fahrenheit* s  thermometer ;  and  according  to  the  de- 
grees of  heat  originally  destined  to  each  animal,  and  the 
excess  or  decrease  of  it,  will  be  the  state  of  its  activity 
and  health. 

Nor  is  it  confined  to  animals  ;  something  of  the 
same  kind  seems  to  take  place  in  vegetables.  The  heat 
which  produces  an  apple  to  perfection  would  never 
bring  forth  a  pine-apple ;  and  the  firs,  which  thrive 
and  look  green  on  the  bleak  and  snowy  hills  of  Norway, 
would  perish  in  the  burning  sands  of  Barca  ;  whilst 
the  spicy  vegetables  of  the  east,  which  breathe  incessant 
sweets  amid  the  glowing  soil  of  Arabia,  would  languish 
md  expire  in  that  cold  clime  which  breeds  the  lofty 
i>ak. 


35S  OF    ANIMAL    ELECTRICITY. 

The  heat  which  hatches  the  chicken  from  an  egg 
would  destroy  the  whole  race  of  fishes,  if  it  affected 
their  spawn  ;  and  thus  the  very  same  element,  which 
makes  an  animal  complete  in  one  degree,  and  in  one 
species  destroys  its  existence  in  another  species  with  the 
same  degree. 

The  degree  of  heat,  which  would  injure  the  life  of  a 
frog,  would  not  be  sufficient  to  keep  the  heart  of  a 
sheep  in  action.  Health  depends  on  a  degree  of  heat 
which  is  natural  to  each  animal,  and  which  was  first 
imparted  to  it  by  that  Divine  Intelligence,  who  is  alone 
able  to  actuate  and  inform,  and  who  has  furnished  us 
with  powers  to  keep  up  this  degree,  and  counteraf. 
and  throw  off  a  greater. 

In  this  account  of  vital  heat  and  motion,  there  is  no- 
thing new  supposed  ;  no  new  property  assigned  either 
to  fire  or  electricity ;  no  new  formation  given  to  any 
part  of  the  human  body. 

We  require  no  more  of  the  nerve  than  that  it  exists, 
and  that  it  be  a  conductor  of  the  electric  fluid  ;  which 
experiment  proves,  vital  heat  and  vital  motion  are  here 
as  they  are  in  nature,  beginning  together,  and  conti- 
nuing so  through  life.  Solar  fire  and  the  electric 
fluid  are  one  and  the  same  vivifying  principle,  actuating 
all  the  different  orders  of  material  beings  :  they  are  so 
radically  the  same,  that  in  various  instances  you  find 
that  what  was  one  becomes  the  other  ;  and  thus  facts 
and  philosophy  are  united  ;  and  the  cause  of  natural 
life  and  motion  is  discovered  by  reason  and  experience 
to  be  the  same  with  what  our  senses  inform  us  to  be  in- 
tuitively the  true  one.  And  permit  me  to  tell  you,  that 
in  general,  whenever  the  account  given  to  explain  the 
cause  of  any  phenomena  in  nature,  is  contradictory  to 
the  obvious  apprehension  of  the  senses  of  a  plain  un- 
derstanding, there  is  reason  to  suspect  its  truth.  That 
to  the  agency  of  fire  all  animal  motion  and  animal  heat 
are  owing  is  obvious  to  the  meanest  capacity ;  and  if 
this  element  cease  to  act,  or  if  it  be  disunited  from  the 
body,  death  is  the  certain  consequence.  Every  part  of 
nature  affords  facts  to  support  this  opinion.  Contem- 
plate the  great  luminary  which  enlightens  the  universe, 


OF    ANIMAL    ELECTRICITY.  359 

and  you  will  find  every  ray  to  be  fraught  with  fire,  which 
it  is  ready  to  manifest  on  meeting  a  proper  recipient. 
Without  the  genial  warmth  they  communicate,  both 
animal  and  vegetable  life  must  cease,  and  all  nature  be- 
come one  lifeless,  torpid,  dismal  ruin. 

All  nature  bears  testimony  to  the  existence  of  this  ethe- 
rial  fluid,  and  to  its  incessant  active  energy.  To  us,  indeed, 
it  often  remains  latent ;  and  peculiar  circumstances  are  ne- 
cessary to  excite  those  signs  which  render  its  effects  most 
visible  to  our  senses.  The  ancients,  viewing  nature  as  she 
is,  often  attained  more  accurate  notions  of  her  operations 
than  modern  philosophers.  These,  by  multiplying  expe- 
riments, without  first  attaining  a  correct  idea  of  the  facts 
continually  presented  for  observation  in  the  great  labora- 
tory of  nature,  have  often  wasted  their  time  and  talents; 
and,  in  the  end,  have  bewildered  themselves  in  an  inex- 
plicable labyrinth,  or  at  best,  have  only  placed  one  spe- 
cies of  ignorance  in  the  deserted  room  of  another. 

The  Platonists  and  Pythagoreans  maintained,  that  fire 
was  the  great  instrumental  cause  in  the  universe,  subor- 
dinate to  the  Infinite  Creative  Mind  ;  and  that  it  actuated 
the  macrocosm  and  animated  the  microcosm. 

The  old  naturalists  have  universally  maintained,  that 
fire  was  in  all  bodies ;  and,  however  indistinctly  they 
were  able  to  write  of  it,  what  they  wrote  was  true.  The- 
opbrastus  has  spoken  of  fire  in  terms  that  bespeak  a  con- 
siderable knowledge  thereof.  Far  from  supposing  mo- 
tion to  be  the  cause,  much  further  from  supposing  it  to 
be  the  essence  of  fire,  he  asserts,  that  fire  is  a  very  dis- 
tinct thing  from  the  matter  in  which  we  see  it  lodged, 
and  from  the  motions  which  we  see  excite  it ;  and  that  it 
is,  in  its  pure  natural  state,  fine,  etherial,  imperceptible, 
and  at  perfect  rest.  He  hints,  that  this  fire  was  the  breath 
which  the  Creator  diffused  in  all  matter,  which,  passing 
over  the  waters,  made  out  of  them  metals,  stones,  and 
earth ;  and  asserts,  that  it  is  the  instrument  which  he 
employs  to  give  all  things  life  and  motion. 

They  in  general  considered  earth  and  water,  air  and  fire, 
as  the  component  elements  of  all  visible  and  known  cor- 
poreal beings,  and  that  life  was  conveyed  to  them  through 
the  elements  of  air  and  fire ;  that  this  fire  was  continu- 


360  OF    ANIMAL    ELECTRICITY. 

ally  operating  to  apply  and  adjoin  to  these  bodies  the  newly 
arrived  matter,  converting  this  matter  into  a  substance  of 
the  same  nature  or  form  with  that  part  to  which  ii  was 
applied,  and  thus  fitting  it  for  the  growth  or  increase,  as 
well  as  the  aliment  of  the  part.  But  then  they  also  con- 
sidered natural  life  as  only  possessed  of  these  powers,  be- 
cause  it  was  the  immediate  agent  of  mind :  for  mind  is 
evidently  the  cause  of  form  to  all  things  formed  by  man  ; 
and  the  cause  of  union  or  conjunction  to  all  things  united 
or  conjoined  by  art. 

It  is  hardly  possible  not  to  agree  in  many  respects  with 
these  ancient  sages:  for,  when  you  look  round  with  a  phi- 
losophic eye,  and  contemplate  the  universe  with  sedulous 
attention,  you  will  find  that  there  is  no  effect  either  beau- 
tiful,  great,  marvellous,  or  terrible,  but  what  proceeds 
from  fire. 

It  can,  therefore,  be  no  matter  of  surprize,  that  after 
the  discovery  of  electricity,  it  was  considered  as  the  phy- 
sical cause  of  motion,  irritability,  &c.  but  it  is  surely  a 
subject  of  regret,  that  medical  men  have  shown  such  re-  ; 
luctance  to  the  investigation  of  this  subject,  and  that  too 
many  have  in  every  possible  way  endeavoured  to  discoun- 
tenance its  application  in  medicine ;  though  the  agency 
of  this  fluid,  and  its  existence  in  animated  nature,  has 
been  so  fully  proved  by  a  variety  of  experiments,  that 
there  can  be  very  little  doubt  that  it  is  essentially  con- 
nected with,  and  continually  exerting  its  influence  on  the 
human  frame.  I  shall  here  lay  before  you  some  further 
instances  to  corroborate  what  has  been  already  advanced. 
By  means  of  a  small  condensing  plate,  Mr.  Cavallo  ob- 
tained very  sensible  signs  of  electricity  from  various  parts 
of  his  own  body,  and  the  head  of  almost  any  other  per- 
son. The  strong  electricity  obtained  in  frosty  weather 
from  silk  stockings,  &c.  on  being  pulled  off,  as  well  as 
that  obtained  by  combing  the  hair,  have  been  long  known. 
Among  others,  Mr.  Brydone  mentions  a  lady,  who,  on 
combing  her  hair  in  frosty  weather  in  the  dark,  had  ob* 
served  sparks  of  fire  to  issue  therefrom.  This  made  him 
think  of  trying  to  collect  the  electrical  fire  from  human 
hair  alone.  To  this  end,  he  desired  a  young  lady  to  stand 
on  wax,  and  comb  her  sister's  hair,  who  was  sitting  in  a 


OF    ANIMAL    ELECTRICITY.  361 

chair  before  her ;  soon  after  she^had  begun  to  comb,  the 
young  lady  on  the  wax  was  surprized  to  find  her  whole 
body  electrified,  and  darting  out  sparks  of  fire  against 
every  object  that  approached  her.  Her  hair  was  strongly 
electrical,  and  affected  an  electrometer  at  a  considerable 
distance.  He  charged  a  metallic  conductor  from  it,  and 
in  the  space  of  a  few  minutes  collected  a  sufficient  quan- 
tity of  fire  to  kindle  common  spirits ;  and,  by  means  of 
a  small  jar,  gave  many  smart  strokes  to  all  the  com- 
pany. 

When  the  discoveries  in  this  science,  says  Mr.  Brydone, 
are  further  advanced,  we  may  find,  that  what  we  call  sen- 
sibility of  nerves,  and  many  other  diseases,  which  are 
known  only  by  name,  are  owing  to  the  bodies  being  pos- 
sessed of  too  large  or  too  small  a  quantity  of  this  subtile 
fluid,  which  is,  perhaps,  the  vehicle  of  all  our  feelings.  It 
is  known,  that  in  damp  and  hazy  weather,  when  this  fire 
is  blunted  and  absorbed  by  the  humidity,  its  activity  is 
lessened,  and  what  is  collected  is  soon  dissipated  ;  then 
our  spirits  are  more  languid,  and  our  sensibility  is  less 
acute.  And,  in  the  fierce  wind  at  Naples,  when  the  air 
seems  totally  deprived  of  it,  the  whole  system  is  unstrung, 
and  the  nerves  seem  to  lose  both  their  tension  and  elasti- 
city, till  the  north-west  wind  awakens  the  activity  of  the 
animating  power,  which  soon  restores  the  tone,  and  enli- 
vens all  nature,  which  seemed  to  droop  and  languish  in 
its  absence.  Nor  can  this  appear  surprizing,  if  it  be  from 
the  different  state  of  this  fire  in  the  human  body  that  the 
strictum  and  laxum  proceed,  and  not  from  any  alteration 
in  the  fibres  themselves,  or  their  being  more  or  less  braced 
up  (among  which  bracers  cold  has  been  reckoned  one,) 
though  the  muscular  parts  of  an  animal  are  more  braced 
when  they  are  hot,  and  relaxed  when  they  are  cold. 

From  the  perpetual  electricity  of  the  atmosphere,  which 
is  no  longer  a  problem,  as  its  existence  and  agency  in  that 
mass  of  air  which  surrounds  our  globe,  has  been  ascer- 
tained by  numerous,  clear,  and  decisive  experiments,  it 
seems  but  just  to  infer,  that  it  must  exert  a  certain  influ- 
ence on  all  the  beings  contained  therein,  and  principally 
on  organized  bodies,  among  which  the  human  frame 
claims  the  pre-eminence. 

VOL.  IV.  3  A 


362  OF    THE    LATER    EXPERIMENTS 

But  there  is  no  necessity  for  deductions  from  a  general 
view  of  nature,  for  we  are  nowi:.  possession  of  facts,  which 
prove  that  it  is  a  principal  agent  in  promoting  the  func- 
tions of  animated  beings  ;  as  in  the  gymnotus  electricus 
torpedo,  and  silurus  electricus.  For  the  similitude  esta- 
blished between  the  electrical  fluid  of  these  animals,  and 
that  of  nature  at  large,  is  such  that  in  a  physical  sense 
it  may  be  considered  the  same. 


OF    THE    LATER    EXPERIMENTS    ON    ANIMAL    ELEC- 
TRICITY. 

When  Mr.  Walsh  first  attributed  the  sensations  pro- 
duced by  the  torpedo,  &c.  to  electricity,  his  opinions,  and 
the  inferences  deduced  from  his  experiments,  were  vehe- 
mently opposed  by  most  of  the  best  electricians  of  the 
day  :  the  conceptions  of  these  men  being  limited  to  the 
minutiae  of  experiments,  they  were  incapable  of  grasping 
a  more  extensive  subject,  or  one  that  was  not  in  all  re- 
spects conformable  to  the  appearances  they  were  used  to. 
Whereas  a  just  view  of  things  should  have  prepared  them 
to  expect  various  anomalies,  while  they  were  investigating 
the  nature  of  an  invisible  and  subtile  agent,  subject  to  a 
variety  of  modifications  from  the  substance  through  which 
it  passes,  or  with  which  it  may  be  combined.  Hence,  in 
the  pursuit  of  animal  electricity,  you  must  not  expect  to 
meet  with  every  electric  sign  ;  as,  from  the  very  nature  of 
its  connexion  with  animated  beings,  it  will  certainly  ac- 
quire properties  that  are  not  to  be  found  when  it  is  disen- 
gaged therefrom. 

Before  I  relate  any  of  the  experiments  of  Val/i,  &c.  I 
shall  lay  before  you  those  principles,  which  I  conceive  will 
throw  great  light  on  the  subject  of  animal  electricity,  and 
by  which  they  may  be  reconciled  to  the  general  agency 
of  nature.  You  have  seen,  by  a  great  variety  of  experi- 
ments, that  electricity  is  first  rendered  sensible  by  a  solu- 
tion of  continuity  ;  you  have  also  every  reason  to  suppose, 
that  the  electric  matter  is  carrying  on  its  most  important 
functions  when  we  are  unable  to  perceive  any  signs  of  elec- 
tricity ;  you  have  seen  that  the  electric  matter,  and  what 


ON    ANIMAL    ELECTRICITY.  363 

we  term  electricity,  are  not  inseparable  beings,  that  the 
one  may  subsist  when  the  other  ceases  to  appear.  As 
the  air  may  occupy  a  space  without  producing  sound, 
so  the  electric  matter  may  reside  in  a  body  without  ex- 
hibiting any  electric  signs.  We  know  also  by  universal 
observation,  as  well  as  partial  experiments,  that  there 
is  a  principle  in  all  bodies  which  is  continually  endea- 
vouring to  extend  their  form,  but  whose  energies  are 
continually  counteracted  by  an  exterior  force.  Now  it 
must  be  evident,  that  every  solution  of  continuity  will 
give  an  opportunity  for  this  expansive  dilating  substance 
to  escape,  when  it  puts  on  new  and  unexpected  appear- 
ances. Hence,  as  we  know  this  expanding  substance  to  be 
fire,  and  have  a  proof  that  on  its  escape  it  exhibits  elec- 
tric signs,  we  have  a  further  confirmation  of  the  iden- 
tity of  these  elements. 

I  think  this  view  of  the  subject  is  in  itself  a  sufficient 
refutation  of  Dr.  Munro's  attempt  to  prove  that  the  ner- 
vous fluid  or  energy  is  not  the  same  with  the  electrical;* 
though  many  other  arguments  may  be  adduced  to  an- 
swer the  same  purpose. 

His  difficulty  in  conceiving  how  the  electrical  fluid 
can  be  accumulated  within  our  nervous  system,  is  not 
greater  than  that  of  conceiving  how  it  is  accumulated 
amidst  a  conducting  fluid  in  the  torpedo,  &c.  nor  indeed 
than  of  its  being  accumulated  in  the  Leyden  phial,  as 
glass  is  now  known  to  be  permeable  thereto.  But  the 
difficulty  with  respect  to  animals  vanishes,  when  we  con- 
sider that  electrical  appearances  are  occasioned  by  a  state 
of  the  fluid  altogether  different  from  that  under  which 
it  exists  in  the  animal  frame  ;  when  it  is  in  the  latter,  its 
powers  are  united,  and  its  operations  imperceptible ; 
when  it  appears  as  electricity,  its  powers  are  divided 
and  some  of  their  effects  rendered  sensible. 

So  far  as  mechanical  stimuli  have  any  relation  to  fire, 
so  far  they  will  be  in  some  degree  similar  to  the  electri- 
cal fluid,  and  act  in  the  same  manner ;  for,  stimulants 
act  only  as  they  are  the  vehicles  of  fire.     The  second 


*  Mimro's  Experiments  on  the  Nervous  System, 


364  OF    THE    LATER    EXPERIMENTS 

objection,  therefore,  of  the  professor  falls  to  the  ground 
The  same  reasoning  applies  to  his  sixth  objection. 

His  fourth  reason,  so  far  from  proving  tl^at  the  ner- 
vous and  electrical  fluids  are  not  the  same,  may  be  con- 
sidered as  a  clear  proof  of  their  identity,  for  the  two  elec- 
trical powers  always  act  in  opposite  directions. 

On  the  same  principle,  the  nervous  energy  (the  elec- 
trical fluid  in  its  united  state)  cannot  pass  readily  up  or 
down  a  nerve  that  has  been  tied  or  cut,  for  the  tying  or 
cutting  of  the  nerve  changes  the  state  of  the  fluid. 

Before  I  proceed  to  give  you  an  account  of  the  ex- 
periments relating  to  animal  electricity,  I  shall  lay  be- 
fore you  some  remarks  of  the  Rev.  Mr.  William  Jones,* 
from  whom  we  have  already  profited  so  much  in  the 
course  of  these  lectures,  and  which  are  intimately  con- 
nected with  our  subject.  "  As  the  force  of  the  electri- 
cal fluid,  says  he,  is  principally  exerted  on  the  nerves 
and  tendons  of  the  body,  there  is  reason  to  believe  that 
this  fluid  is  the  same  with  that  something,  which  many 
physicians  have  discoursed  upon  under  the  name  of  ani- 
mal spirits.  The  nerves  do  not  appear  as  if  they  were 
designed  to  admit  any  animal  fluid  or  liquor,  unless  it 
be  an  indolent  lymph  necessary  to  keep  them  moist : 
but  their  pellucidity  indicates  that  they  are  properly 
adapted  to  give  a  direct  passage  to  the  fluid  light ;  for 
they  are  transparent,  and  that  not  transversely,  but 
longitudinally,  or  in  the  direction  of  their  fibres.  This 
Mr.  Jones  observed  accidentally,  as  some  eyes  of  sheep 
and  oxen,  which  he  had  procured  for  dissection,  lay  on 
the  table  ;  one  of  these  eyes  shone  in  the  day  time  much 
in  the  same  manner  as  the  eyes  of  some  animals  do  in 
the  dark;  on  examining  into  this  circumstance,  he  found, 
that  if  his  hand  were  interposed  between  the  nearest  win- 
dow and  the  extremity  of  the  optic  nerve,  a  part  of  which 
nearly  an  inch  in  length  remained  with  the  eye,  and  was 
accidentally  pointed  towards  the  window,  the  light  im- 
mediately disappeared. " 


*  Jnnen\s  Essay  on  the  First  Principles  of  Natural  Philosophy,  p 


ON    ANIMAL    ELECTRICITY.  365 

From  this  he  was  led  to  consider,  whether  the  light 
that  appears  in  the  eyes  of  some  animals  in  the  night 
time,  is  really  a  reflection  of  light  from  the  eye,  as  is 
commonly  supposed ;  or  whether  it  does  not  rather 
pass  into  the  eye,  through  the  optic  nerve,  from  the 
body  of  the  animal  ?  It  is  not  easy  to  conceive  how  this 
shining  can  be  occasioned  by  a  reflexion  of  light  from 
the  choroides  in  the  bottom  of  the  eye,  when  the  light 
to  be  reflected  (as  in  a  dark  night)  is  not  visible  before 
its  entrance  into  the  eye.  If  a  candle  be  held  before  the 
eyes  of  a  dog,  and  you  place  yourself  in  the  line  of  re- 
flection, the  light  will  be  visibly  reflected  from  his  eyes, 
because  the  illumination  is  sufficiently  strong;  but 
when  there  is  no  visible  illumination  at  all,  how  should 
it  account  for  the  like  effect  ?  Whence  it  is  more  reason- 
able, that  this  appearance  should  be  owing  to  a  light 
from  within  the  body  of  the  animal,  which  being  weaker 
than  the  light  of  the  day,  but  stronger  than  the  light  of 
the  night,  is  visible  in  the  night  but  not  in  the  day.  The 
light  of  other  bodies  which  shine  in  the  dark  is  inhe 
rent  in  those  bodies,  as  in  putrifying  veal,  fish,  rotten 
wood,  phosphorus,  the  glow-worm,  &c.  concerning  the 
last  of  these,  the  eminent  anatomist  and  philosopher,  T. 
Bartholine,  has  the  following  observation.  If  a  glow-worm 
be  examined,  it  will  appear  to  have  a  lucid  liquor  in  the 
hinder  part  of  its  body,  where  the  heart  is  placed,  by 
which  the  heart  is  moved  and  illuminated  ;  and  this 
fluid  retains  its  light  so  long  as  the  heart  of  the  insect  re- 
tains its  life  and  motion. 

Dr.  Priestley ,  in  his  Heads  of  Lectures  on  a  Course  of 
Experimental  Philosophy,  has  given  so  excellent  and 
compendious  a  view  of  the  principal  experiments  that 
have  been  made  by  Valli  and  others,  to  determine  the 
electricity  of  animals,  that  I  cannot  do  better  than  lay  it 
before  you  ;  which  I  the  more  readily  do,  as  it  will  save 
us  from  the  disgusting  detail  of  a  variety  of  cruel  expe- 
riments ;  experiments  that  I  hope  you  will  never  be  in- 
duced to  repeat.  One  alone  will  suffice  to  give  you  an 
idea  of  the  nature  of  these  operations. 

Mr.  Valli  opened  the  abdomen  of  a  frog,  in  order  to 
lay  bare  the  spine  of  the  back,  and  discover  the  crural 


366  OF    THE    LATER    EXPERIMENTS 


. 


nerves  which  issue  from  it ;  a  few  lines  above  this  poi 
he  cut  the  animal  in  two,  and  passing  his  scissars  imme- 
diately under  the  origin  of  these  nerves,  removed  the 
remaining  portion  of  the  vertebral  column,  so  as  only 
to  leave  the  vertebral  which  united  the  bundle  of  nerves ; 
this  portion  of  the  vertebras  was  enveloped  with  a  piece 
of  sheet  lead  ;  the  coated  part  was  touched  with  one 
end  of  a  metallic  conductor,  and  with  the  other,  the  sur- 
face of  the  thighs  which  were  previously  stripped  of 
their  skins.  The  movements  produced  thereby  were 
violent  and  continued  for  a  long  time. 

Having  thus  explained  to  you  the  manner  in  which 
the  animal  is  prepared  for  these  experiments,  I  shall 
proceed  to  point  out  the  principal  results,"  as  furnished 
by  Dr.  Priestley. 

The  nerve  of  the  limb  of  an  animal  being  laid  bare, 
and  surrounded  with  a  piece  of  sheet-lead,  or  tin-foil,  if 
a  communication  be  formed  between  the  nerve  thus 
armed,  and  any  of  the  neighbouring  muscles  by  means 
of  a  piece  of  zinc,  strong  contractions  will  be  produced 
in  the  limb. 

If  a  portion  of  the  nerve  which  has  been  laid  bare 
be  armed  as  above,  contractions  will  be  produced  as 
powerfully,  by  forming  the  communication  between  the 
armed  and  bare  part  of  the  nerve,  as  between  the  armed 
part  and  muscle. 

A  similar  effect  is  produced  by  arming  a  nerve,  and 
simply  touching  the  armed  part  of  the  nerve  with  the 
metallic  conductor. 

Contractions  will  take  place  if  a  muscle  be  armed,  and 
a  communication  be  formed  by  means  of  the  conductor 
between  it  and  a  neighbouring  nerve ;  the  same  effect 
will  be  produced,  if  the  communication  be  formed  be- 
tween the  armed  muscle  and  another  muscle  which  is 


contiguous  to  it 


Contractions  may  be  produced  in  the  limb  of  an  ani- 
mal, by  bringing  the  pieces  of  metal  into  contact  with 
each  other  at  some  distance  from  the  limb,  provided  the 
latter  make  part  of  a  line  of  communication  between 
the  two  metallic  conductors. 


ON    ANIMAL    ELECTRICITY.  367 

The  experiment  which  proves  this  is  made  in  the  fol- 
lowing manner.  The  amputated  limb  of  an  animal  be- 
ing placed  upon  a  table,  let  the  operator  hold  with  one 
hand  the  principal  nerve,  previously  laid  bare,  and  in 
the  other  let  him  hold  a  piece  of  zinc  ;  let  a  small  plate 
of  lead  or  silver  be  then  laid  upon  the  table  at  some 
distance  from  the  limb,  and  a  communication  be  form- 
ed by  means  of  water  between  the  limb  and  the  part  of 
the  table  where  the  metal  is  lying.  If  the  operator 
touch  the  piece  of  silver  with  the  zinc,  contractions 
will  be  produced  in  the  limb  the  moment  that  the  me- 
tals come  into  contact  with  each  other.  The  same  ef- 
fect will  be  produced,  if  the  two  pieces  of  metals  be 
previously  placed  in  contact,  and  the  operator  touch 
one  of  them  with  his  finger.  This  fact  was  discovered 
by  Mr.  William  Cruikshank. 

Contractions  can  be  produced  in  the  amputated  leg 
of  a  frog,  by  putting  it  into  water,  and  bringing  the 
two  metals  into  contact  with  each  other,  at  a  small  dis- 
tance from  the  limb. 

The  influence  which  has  passed  through,  and  excited 
contractions  in  one  limb,  may  be  made  to  pass  through, 
and  excite  contractions  in  another  limb.  In  perform- 
ing this  experiment,  it  is  necessary  to  attend  to  the 
following  circumstances ;  let  two  amputated  limbs  of 
a  frog  be  taken,  let  one  of  them  be  laid  upon  a  table,  and 
its  foot  be  folded  in  a  piece  of  silver  ;  let  a  person  lift 
up  the  nerve  of  this  limb  with  a  silver  probe,  and  an- 
other person  hold  in  his  hand  a  piece  of  zinc,  with 
which  he  is  to  touch  the  silver  including  the  foot ;  let 
the  person  holding  the  zinc  in  one  hand,  catch  with 
the  other  the  nerve  of  the  second  limb,  and  he  who 
touches  the  nerve  of  the  first  limb  is  to  hold  in  his  other 
hand  the  foot  of  the  second  ;  let  the  zinc  now  be  ap- 
plied to  the  silver  including  the  foot  of  the  first  limb, 
and  contractions  will  immediately  be  excited  in  both 
limbs. 

The  heart  is  the  only  involuntary  muscle,  in  which 
contractions  can  be  excited  by  these  experiments ;  con- 
tractions are  produced  more  strongly,  the  farther  the 
coating  is  placed  from  the  origin  of  the  nerve. 


368  OF    THE    LATER    EXPERIMENTS 

Animals  which  were  almost  dead,  have  been  found 
to  be  considerably  revived  by  exciting  this  influence. 
When  these  experiments  are  repeated  upon  an  animal 
that  has  been  killed  by  opium,  or  by  the  electric  shock, 
very  slight  contractions  are  produced  ;  and  no  contrac- 
tions whatever  will  take  place  in  an  animal  that  has 
been  killed  by  corrosive  sublimate,  or  that  has  been  starv- 
ed to  death.  Zinc  appears  to  be  the  best  exciter  when 
applied  to  gold,  silver,  molybdena,  steel,  or  copper ;  the 
latter  metals,  however,  excite  but  feeble  contractions 
when  applied  to  each  other  ;  next  to  zinc,  in  contact 
with  these  metals,  tin  and  lead  appear  most  powerful 
exciters. 

It  has  been  found,  that  if  a  plate  of  zinc  be  applied  to 
the  upper  part  of  the  point  of  the  tongue,  and  a  plate 
of  silver  to  its  under  part,  on  bringing  the  two  metals 
into  contact  with  each  other,  a  pungent  disagreeable  feel- 
ing, which  it  is  difficult  to  describe,  is  produced  in  the 
point  of  the  tongue.  And  if  a  plate  of  zinc  be  placed 
between  the  upper  lip  and  the  gums,  and  a  plate  of 
gold  applied  to  the  upper  or  under  part  of  the  tongue, 
on  bringing  these  two  metals  into  contact  with  each 
other,  the  person  imagines  that  he  sees  a  flash  of  light- 
ning, which  however  a  by-stander  in  a  darkened  room 
does  not  perceive ;  and  the  person  performing  the  ex- 
periment perceives  the  flash  though  he  be  hood-winked.* 

After  performing  this  experiment  repeatedly,  Dr. 
Munro  constantly  felt  a  pain  in  his  upper  jaw,  at  the 
place  to  which  the  zinc  had  keen  applied,  which  conti- 
nued for  an  hour  or  more  ;  and  in  one  experiment,  af- 
ter he  had  applied  a  blunt  probe  of  zinc  to  the  septum 
narium,  and  repeatedly  touched  with  a  crown  piece  of 
silver  applied  to  the  tongue,  and  thereby  produced  the 
appearance  of  a  flash,  several  drops  of  blood  fell  from 
that  nostril ;  and  Dr.  Fowler ,  after  making  such  an  ex- 
periment on  his  ears,  observed  a  similar  effect. 


Munro's  Experiments  on  the  Nervous  System,  p.  25. 


[     369     ] 


RESEMBLANCE  OF  THE  FLUID  PUT  IN  MOTION  BY 
THE  FOREGOING  EXPERIMENT,  TO  THE  ELECTRI- 
CAL   FLUID.* 

The  fluid  set  in  motion  by  the  application  of  the  me- 
tals to  each  other,  and  to  animal  bodies,  or  to  water, 
agrees  with  or  resembles  the  electrical  fluid  in  the  fol- 
lowing respects  : 

Like  the  electrical  fluid,  it  communicates  the  sense 
of  pungency  to  the  tongue. 

Like  the  electrical  fluid,  it  is  conveyed  readily  by  water, 
blood,  the  bodies  of  animals,  the  metals  ;  and  is  arrest- 
ed in  its  course  by  glass,  sealing-wax,  &c. 

It  passes  with  similar  rapidity  through  the  bodies  of 
animals. 

Like  the  electrical  fluid,  it  excites  the  activity  of  the 
vessels  of  a  living  animal ;  as  the  pain  it  gives  and 
hemorrhagy  it  produces  seem  to  prove.  Hence,  perhaps, 
it  might  be  employed  with  advantage  in  amenorrhcea. 
It  excites  convulsions  of  the  muscles,  in  the  same  man- 
ner, and  with  the  same  effects  as  electricity. 

When  the  metals  and  animal  are  kept  steadily  in  con- 
tact with  each  other,  the  convulsions  cease,  or  an  equi- 
librium seems  to  be  produced,  as  after  discharging  the 
Leyden  phial. 


GENERAL    OBSERVATIONS. 

A  view  of  the  great  agents  in  nature  naturally  leads 
us  to  consider  the  opinions  of  those  who  wish  to  set 
religion  and  reason  in  opposition  to  each  other,  and  to 
suppose  that  philosophy  and  revelation  can  never  agree. 
But,  in  opposition  to  such  insidious  attempts,  attempts 
which  never  were  designed  to  enlarge  the  mind  or  to 
improve  the  heart,  it  may  easily  be  made  to  appear  that, 
take  philosophy  in  its  most  improved  state,  enriched 


*  Munro's  Experiments  on  the  Nervous  System,  p.  25. 
VOL.  IV.  a  B 


370  GENERAL    OBSERVATIONS. 

by  the  discoveries  of  ages,  examined  by  the  test  of  the 
closest  reasoning,  elevated  above  the  fallacies  of  the 
senses  and  of  appearances  ;  and  yet,  in  this  improved 
state,  it  shall  be  found  perfectly  to  correspond  with  the 
philosophy  of  scripture,  rightly  understood.  The  word 
of  God  is  as  perfect  as  his  works.  Both  proceed  from 
the  one  fountain  of  truth,  who  cannot  contradict  him- 
self. His  word  and  his  works  mutually  illustrate  each 
other :  the  one  is  not  to  be  understood  without  the  other ; 
for  both  are  the  offsprings  of  divine  love,  manifested  in 
wisdom,  and  exercised  in  power. 

Creation  may  be  considered  as  the  grand  chain  of 
causes  and  effects,  intimately  connected  together.  It  is 
the  work  of  Omnipotence,  guided  by  infinite  wisdom, 
and  excited  to  work  by  communicative  goodness.  But, 
do  we  not  obtain  wrong  ideas  on  this  important  subject, 
if  we  imagine  that  any  part  of  this  grand  system  stands 
unconnected  ?  or,  as  if  the  Great  Master  Builder  was 
obliged  to  collect  discordant  materials  from  different 
parts,  and,  overcoming  the  repugnance  of  their  natures, 
to  form  one  whole  out  of  these  heterogeneous  sub- 
stances  ?  Whereas,  the  truth  appears  to  be,  that  in 
his  divine  hand,  the  one  naturally  and  orderly  produces 
the  other ;  that,  which  was  the  effect  of  a  prior  princi- 
ple, becomes  the  cause  of  that  which  follows  it  immedi- 
ately ;  and  again,  chis  effect  becomes  an  instrumental 
cause  in  its  turn ;  and  is  thus  extended  in  a  long  series, 
until  all  are  completed  in  outward  nature. 

Let  us  examine  how  this  will  agree  with  the  Mosaical 
account  of  the  creation  ;  for,  although  we  may  readily 
allow,  that  that  book  contains  more  interesting  and  im- 
portant subjects  than  the  detail  of  the  mere  creation  and 
formation  of  this  material  system  ;  yet  the  natural  ac- 
count, when  rightly  understood,  may  be  found  to  be 
most  accurate,  philosophical,  and  just.  The  great  and 
spiritual  truths,  conveyed  under  that  form,  may  yet  be 
delivered  down  to  us  in  a  vehicle  of  the  most  accurate 
philosophical  truth  ;  the  stricter  the  truth,  the  greater 
and  more  perfect  the  analogy  and  correspondence ;  but 
it  seems  to  have  been  the  peculiar  fate  of  these  sublime 
and  ancient  writings  of  the  Hebrew  Sage,  that  they  have 


GENERAL    OBSERVATIONS.  371 

been  supposed  to  contain  what  they  did  not,  whilst  their 
real  and  most  important  contents  have  been  greatly  over- 
looked. The  ideas  of  the  Divine  Mind  disclosed,  the 
energies  of  his  almighty  will  exerted,  produced  motion 
in  different  degrees,  as  the  instrumental  cause  for  future 
productions.  Hence  the  motion  of  spirits,  of  minds, 
of  life,  of  thought,  of  light,  of  the  heavenly  bodies, 
of  blood,  and  of  the  sap.  Hence  this  motion,  depend- 
ent and  continued  from  one  source  of  life  and  motion, 
may  be  considered  as  the  key  of  natural  knowledge, 
which  opens  the  temple  of  physical  truth.  Motion  is 
the  visible  discovery  of  the  divine  hand  ;  motion  is  the 
grand  connecting  link  between  the  spiritual  and  natural 
worlds ;  by  this  the  energies  of  the  one  are  impressed 
on  the  other. 

This  motion,  proceeding  from  a  pure  and  superior 
system,  was  at  first  most  perfect  and  full,  unencumber- 
ed by  matter,  unimpeded  by  obstructions. 

Now,  in  the  first  day  (or  in  the  first  state  of  creating 
things,  for  as  yet  there  was  no  sun  and  earth,  and  there- 
fore no  measure  of  day  and  night),  in  this  first  state  of 
things,  the  Scripture  says,  light  was  formed,  or  rather 
the  matter  of  light,  by  the  means  of  pure  original  mo- 
don.  Now  the  matter  of  light  is  elementary  fire.  This 
is  evident  from  the  most  intimate  relation  between  fire 
and  light ;  light  being  only  an  effect,  an  outward  visible 
manifestation  of  latent  fire. 

This  pure  elementary  fire,  the  matter  or  substance  of 
light,  produces  that  rapid  motion  of  light  from  the  sun 
or  stars  to  the  earth,  travelling  with  such  amazing  ve- 
locity. Fire  and  light  combined,  produced  air,  or  the 
first  and  purest  etherial  particles  ;  and  therefore,  in  the 
Mosaical  account,  the  firmament,  the  expanse,  or  the 
atmosphere  of  the  air,  was  the  second  day's  work,  or 
the  second  state  of  things  in  their  progress  to  perfection 
and  fulness.  This  elementary  principle  is  not  so  subtile 
and  active  as  its  parents,  fire  and  light ;  yet  it  is  more 
subtile  and  active  than  vapour  or  water  ;  therefore  it 
holds  the  intermediate  rank  between  these,  and  is  a  con- 
necting link  in  the  great  chain,  as  it  is  produced  by  fire 
and  light;  so  again,  when  partly  deprived  of  these. 


372  GENERAL    OBSERVATIONS. 

it  is  the  instrumental  cause  to  form  the  vapours  and 
Water. 

That  fire  and  light  produce  air  may  be  illustrated  by 
Various  experiments  ;  the  respiration  of  plants,  and  the 
purity  of  the  air,  which  they  produce,  when  exposed 
to  the  agency  of  light ;  and  the  great  quantities  of  dif- 
ferent airs  produced  in  various  chemical  experiments  by 
the  activity  of  fire. 

Air  condensed,  exposed  to  obstructions,  and  thus  de- 
prived of  the  greatest  portion  of  its  etherial  fire,  be- 
comes first  vapour ;  and  as  the  fire  dissipates,  and  the 
motion  ceases,  it  becomes  water  in  the  various  forms  of 
mist,  dew,  rain,  &c.  In  this  state,  it  is  almost  entirely 
deprived  of  its  original  motion  ;  is  less  subtile,  and  more 
gross  ;  is  become  an  object  of  the  outward  senses,  and 
is  subject  to  the  laws  of  gravitation. 

Water  is  the  great  support  of  animal  and  vegetable 
substances,  which  at  length  are  reduced  to  earth  in  their 
various  changes,  from  the  first  principles  of  active  na- 
ture, down  to  the  lowest,  grossest  material  form  ;  from 
the*  fountain  of  life,  from  the  architypal  ideas  of  the 
Divine  Mind,  through  spirits  to  fire,  light,  ether,  air, 
water,  earth,  down  to  sluggish  inert  matter. 

Fire,  light,  air,  and  water,  may  then  be  considered 
as  the  grand  agents  in  nature.  The  earth  is,  as  it  were, 
a  basis  for  them  to  rest  and  to  work  upon.  In  these, 
the  circulation  of  motion  in  its  descent  and  degrees  is 
preserved,  and  the  earth  is  a  nidus  where  they  rest,  and 
where  their  effects  are  manifested.  Thus  there  was  a 
regular  and  beautiful  descent  from  the  spiritual  to  the 
natural  world,  from  motion  to  resr.  The  wonderous 
fabric  of  the  earth  was  not  built  of  discordant  materi- 
als, of  jarring  elements,  forcibly  restrained  by  the  di- 
vine hand  continually  checking  them  ;  but  the  homoge- 
neous substance  arose  in  a  wise  and  orderly  series.  Each 
part  being  preparatory  for  that  which  was  to  succeed ; 
^every  thing  being  a  link  in  the  great  chain  of  order  and 
usefulness  ;  and  instrumental  cause  to  produce  the  suc- 
ceeding effect,  until  all  was  finished  and  complete,  na- 
ture stood  perfect  in  outward  matter  :  creation  was  no 
longer  all  fire,  light,  air,  or  water,  but  each  retained  its 


GENERAL    OBSERVATIONS.  373 

respective  rank;  and  the  gross  material  world  was 
produced,  able  to  sustain  minerals,  plants,  animals,  and 
man. 

Thus  did  the  Divine  Architect  accomplish  this  great 
and  stupendous  work  by  the  most  simple  means ;  by 
a  regular  descent  from  the  spiritual  to  the  natural  world  -> 
a  continued  series  proceeding  from  the  highest  to  the 
lowest,  from  the  purest  motion  to  inactivity,  from  the 
highest  principles  of  intelligent  mind  down  to  the  low- 
est, grossest  heaviest  matter.  Thus  were  all  things 
ordered  in  infinite  wisdom  :  causes  were  employed  most 
simple  and  prolific,  to  accomplish  the  end  designed. 
Creation  was  accomplished  ;  the  earth  stood  complete ; 
the  work  of  divine  power  resulting  from  divine  wisdom 
and  mercy.  It  was  made  the  theatre  of  his  goodness, 
on  which  he  might  display  it,  and  communicate  it  to  his 
various  creatures,  who  thus  might  rejoice  in  their  ex- 
istence ;  and  manifest  his  praise,  by  enjoying  happiness, 
and  rising  in  perfection  through  endless  ages. 

Thus  was  the  earth  designed  to  be  the  repository  of  the 
human  race,  the  seminary  of  men  ;  until,  full  of  years 
and  wisdom,  they  were  ripe  for  a  happier  change  ;  were 
prepared  to  quit  the  perishing  body,  and  to  be  trans- 
planted into  a  paradise  of  endless  delights. 

The  whole  material  system  was  also  a  volume  of  di- 
vine instruction  opened  to  man,  in  which  he  might  read 
and  understand  and  live  for  ever ;  in  which  he  might 
discover  immense  benevolence,  design,  and  order,  and 
thus  be  led  to  understand  and  adore  him,  who  is  the 
source  of  all  things. 


C      374     ] 


LECTURE  L. 


ON    MAGNETISM.* 


1  HOUGH  the  phenomena  of  the  magnet  have  for 
many  ages  engaged  the  attention  of  natural  philoso- 
phers, both  from  their  singularity  and  importance  ;  we 
are  not  yet  in  possession  of  any  hypothesis  that  will  sa- 
tisfactorily account  for  the  various  properties  of  the 
magnet,  or  point  out  those  links  of  the  chain  that  con- 
nect it  with  the  other  phenomena  of  the  universe 

It  is  known  by  the  works  of  Plato  and  Aristotle,  that 
the  ancients  were  acquainted  with  the  attractive  and  re- 
pulsive powers  of  the  magnet ;  but  it  does  not  appear, 
that  they  knew  of  its  pointing  to  the  pole,  or  the  use  of 
the  compass.  That  property  of  the  magnet,  whereby, 
when  properly  suspended,  it  turns  towards  the  north, 
renders  it  of  the  utmost  service  to  mankind  in  general, 
but  more  particularly  to  an  Englishman;  the  riches 
and  power  of  whose  country  depend  on  navigation. 

The  powers  of  the  magnet  excited  the  wonders  of 
the  ancients  ;  they  were  to  them  inexplicable,  and  still 
remain  so.  Posterity,  instead  of  being  able  to  remove 
the  difficulties,  have  only  by  their  researches  found  out 
new  wonders  equally  inexplicable.  All,  therefore,  that 
I  shall  be  able  to  do,  will  be  to  relate  to  you  the  prin- 
cipal qualities  of  this  curious  phenomenon.  "  The 
magnet  is  a  proof,  that  nature  has  many  secrets,  and  that 
philosophy,  if  contented  with  present  knowledge,  fore- 
goes most  valuable  and  interesting  discoveries,  towards 


See  my  Essay  on  Magnetism. 


OF    MAGNETISM.  375 

which,  perhaps,  the  previous  steps  are  already  trod- 
den." From  its  action  on  the  compass  in  all  parts  of 
the  world  it  is  plain,  that  its  influence  is  universal.  From 
our  knowledge  of  this  we  are  naturally  led  to  suppose, 
that  there  may  be  other  invisible  agents  exerting  their  in- 
fluence on  us,  and  on  our  globe. 

Let  the  modern  philosopher,*  who  denies  the  existence 
of  a  God,  because  he  cannot  perceive  him  with  his  cor- 
poreal eyes,  tell  you  what  magnetism  is,  and  how  it  exists. 
Let  him,  who  will  understand  every  thing  that  exists,  be- 
fore he  allows  of  its  existence,  first  employ  himself  here; 
and  when  he  has  given  the  world  a  proof  of  his  powers,  let 
him  attempt  a  higher  subject. 

The  loadstone,  leading-stone,  or  natural  magnet,  is  an 
iron  ore  or  ferruginous  stone,  found  in  the  bowels  of  the 
earth,  generally  in  iron  mines,  of  all  forms  and  sizes,  and 
of  various  colours.  It  is  endowed  with  the  property  of 
attracting  iron ;  and  of  both  pointing  itself,  and  also  ena- 
bling a  needle,  touched  upon  it,  and  duly  poised,  to  point 
towards  the  poles  of  the  world. 

Loadstones  are  in  general  very  hard  and  brittle,  and 
for  the  most  part  more  vigorous  in  proportion  to  their 
degree  of  hardness.  Considerable  portions  of  iron  may 
be  extracted  from  them.  Newman  says,  that  they  are  al- 
most totally  soluble  in  spirit  of  nitre,  and  partially  in  the 
vitriolic  and  marine  acids. 

Mr.  Kirwan  says,  that  the  magnet  seems  to  contain  a 
small  quantity  of  sulphur,  it  is  often  contaminated  with  a 
mixture  of  quartz  and  argil ;  it  is  possible  it  may  contain 
nickel,  for  this,  when  purified  to  a  certain  degree,  ac- 
quires the  properties  of  a  magnet ;  but  its  constitution 
has  not  as  yet  been  properly  examined  f 

Artificial  magnets,  which  are  made  of  steel,  are  now 
generally  used  in  preference  to  the  natural  magnet ;  not 
only  as  they  may  be  procured  with  greater  ease,  but  be- 


*  Condorcet,  and  many  of  his  school,  have  laughed  at  mankind  for  be- 
lieving in  an  invisible  Being. 

t  Kirwan's  Elements  of  Mineralogy.  In  the  second  edition,  1796,  p.  158, 
of  thisexcellent  work,  are  giveu  further  and  more  correct  particulars  of  this 
curious  mineral E.  Edit* 


376  OF    MAGNETISM. 

cause  they  are  far  superior  to  the  natural  magnet  in 
strength,  and  communicate  the  magnetic  virtue  more 
powerfully,  and  may  be  varied  in  their  form  more  easily, 
so  that  the  natural  magnet  is  now  very  little  esteemed, 
except  as  a  curiosity. 

The  power  of  attracting  iron,  &c.  possessed  by  the 
loadstone,  which  is  communicable  to  iron  and  steel,  is 
called  magnetism*  It  has  been  supposed,  that  iron  and  the 
loadstone  were  the  only  two  bodies  which  could  be  ren- 
dered magneticai ;  but  it  now  appears,  that  nickel,  when 
purified  from  iron,  becomes  more  instead  of  less  magne- 
tic, and  acquires,  what  iron  does  not,  the  properties  of  a 
magnet.5* 

A  rod  or  bar  of  iron  or  steel,  to  which  a  permanent 
polarity  has  been  communicated,  is  called  a  magnet. 

The  points  in  a  magnet  which  seem  to  possess  the 
greatest  power,  or  in  which  the  virtue  seems  to  be  con- 
centrated, are  termed  the  poles  of  the  magnet. 

The  magneticai  meridian  is  a  vertical  circle  in  the  hea« 
vens,  which  intersects  the  horizon  in  the  points  to  which 
the  magneticai  needle,  when  at  rest,  is  directed. 

The  axis  of  a  magnet  is  a  right  line,  which  passes  from 
one  pole  to  the  other. 

The  equator  of  a  magnet  is  a  line  perpendicular  to  the 
axis  of  the  magnet,  and  equally  distant  from  the  two 
poles.  < 

The  distinguishing  and  characteristic  properties  of  a 
magnet,  are. 

1.  Its  attractive  and  repulsive  powers. 

2.  The  force  by  which  it  places  itself,  when  suspend- 
ed  freely,  in  a  certain  direction  towards  the  poles  of  the 
earth. 

3.  Its  dip  or  inclination  towards  a  point  below  the  hori- 
zon. 

4.  The  property  which  it  possesses  of  communicating 
the  foregoing  powers  to  iron  or  steel. 


Kirwan's  Elements  of  Mineralogy,  second  edition,  1796,  p.  2ft  J. 


[  377  ] 


OF  THE  TENDENCY  OF  IRON  AND  A  MAGNET  TO  AP- 
PROACH EACH  OTHER. 

This  curious  property  of  the  magnet  was  that  by  which 
it  was  first  discovered,  and  by  which  it  engaged  the  at- 
tention of  the  curious. 

Every  substance  that  contains  iron  is  more  or  less  at- 
tracted by  the  magnet.  And  so  universally  is  this  metal 
disseminated,  that  there  are  very  few  substances  that  are 
not  in  some  degree  capable  of  being  attracted  by  the  mag- 
net. You  will  find  it  in  animals,  vegetables,  minerals, 
and  even  in  the  air.* 

Iron  is  attracted  with  different  degrees  of  force,  ac- 
cording to  the  different  states  of  its  existence  ;  but  it  ne- 
ver becomes  quite  insensible  to  the  magnetic  power.  Even 
the  purest  calx,  or  the  completest  solution  ever  made  of 
the  metal,  when  accurately  examined,  is  found  to  be  in 
some  degree  obedient  to  the  magnet. 


TO   ASCERTAIN    WHETHER    A 

OR    IS    CAPABLE    OF    BEING    ATTRACTED    BY     THE 
MAGNET. 

If  the  given  body  contain  evidently  a  large  quantity 
of  iron,  on  bringing  a  magnet  in  contact  therewith,  you 
will  find  it  adhere  so  strongly  as  to  require  a  certain 
degree  of  force  to  separate  them.  If  the  body  be  not  sen- 
sibly attracted  by  the  magnet  in  this  way,  then  you  may 
Boat  it  by  a  piece  of  wood  or  cork  on  water;  in  this  situa- 
:ion  it  is  more  easily  acted  on,  and  consequently  small 
quantities  of  iron  are  readily  discovered.  The  magnet 
should  be  presented  sidewise  to  the  body,  and  when  it  is 
it  rest,  it  is  sometimes  necessary  to  bring  the  magnet 
within  one-tenth  of  an  inch  distance  from  the  swimming 
x>dy  in  order  to  perceive  the  attraction. 


*  Cavaii'o  on  Magnetisrr,  p.  66, 
VOL.  IV,  3  C 


S78  TO    ASCERTAIN    WHETHER 

A  still  smaller  degree  of  attraction  may  be  discovered 
by  placing  the  given  body  upon  quicksilver,  and  then  pre- 
senting a  magnet  to  it.  The  vessel,  in  which  the  quick- 
silver is  contained,  should  be  at  least  six  inches  in  dia- 
meter, otherwise  the  curvature  of  the  fluid  will  be  per- 
petually carrying  the  body  towards  the  sides  of  rhe  ves- 
sel. The  quicksilver  should  be  pure,  and  occasionally 
cleared  by  passing  it  through  a  funnel  of  clean  writing- 
paper  ;  the  smaller  the  aperture  of  the  funnel,  the  better 
wiil  it  answer  the  purpose.  The  air  should  be  agitated  as 
little  as  possible.  Attending  to  these  precautions,  you 
will  seldom  fail  to  discover  whether  a  body  contains  any 
ferruginous  particles.* 

I  place  a  piece  of  iron  on  a  cork,  and  put  the  cork  into 
a  bason  of  water.  I  present  a  magnet  to  it,  and  it  is  at- 
tracted thereby,  and  follows  the  magnet,  so  that  I  can 
move  it  without  touching,  wherever  I  please.  On  this 
principle,  many  ingenious  and  entertaining  pieces  of  me- 
chanism have  been  contrived. 

The  tendency  between  the  magnet  and  the  iron  is  reci- 
procal ;  for,  if  the  magnet  be  put  on  the  cork,  it  will  fol- 
low the  iron  in  the  same  manner  as  this  followed  the  mag 
net.  And  this  attraction  takes  place,  although  a  piece  of 
paper,  glass,  brass,  &c.  be  interposed  between  the  mag- 
net and  the  iron. 

The  reciprocal  tendency  of  iron  to  a  magnet,  and  of  a 
magnet  to  iron,  is  pleasingly  illustrated  by  suspending  a 
magnet  under  the  scale  of  a  balance,  and  counterpoising 
it  by  weights  in  the  other  scale  ;  when  thus  counterpois- 
ed, bring  a  piece  of  iron  towards  it,  and  the  magnet  will 
immediately  descend.  Reverse  the  experiment  by  sus- 
pending the  iron  from  the  scale,  and  the  iron  will  now  de- 
scend and  follow  the  magnet. 

I  place  a  magnet  upon  a  stand,  to  raise  it  some  distance 
irom  the  table ;  I  shall  bring  a  small  sewing-needle  to- 
wards it,  keeping  the  thread  which  is  in  the  needle  in  my 
hand,  to  prevent  the  needle  from  fixing  itself  to  the  mag- 


*  A  fine  magnetic  needle,  about  four  or  five  inches  long,  suspended  on  a 
pointed  stand,  as  shown  at  plate  2,  Jig.  16,  1  think  thj  m  jst  sensible  and 
convenient  apparatus  for  this  purpose E.  Edit. 


A    BODY    HAS    ANY    IRON.  S79 

net ;  and  the  needle  endeavouring  on  one  hand  to  fly  to 
the  magnet,  and  being  withheld  on  the  other  by  the 
thread,  remains  pleasingly  suspended  in  the  air. 

Mathematicians  have  endeavoured  to  compute  the 
force  with  which  the  magnetic  attraction  acts  at  different 
distances,  but  hitherto  without  success.  No  law  has 
been  ascertained,  upon  which  any  dependence  can  be 
placed. 

Though  many  experiments  have  been  made  to  disco- 
ver, whether  the  force  by  which  two  magnets  are  repel- 
led or  attracted,  acts  only  to  a  certain  distance ;  whether 
the  degrees  of  its  action  within,  and  at  this  distance,  is 
uniform  or  variable,  and  in  what  proportion,  to  the 
distances  it  increases  or  diminishes  ;  yet  we  can  only  in- 
fer from  them,  that  the  magnetic  power  extends  further 
at  some  times  than  it  does  at  others,  and  that  the  sphere 
of  its  action  is  variable. 

The  smaller  the  loadstone  or  the  magnet  is,  the  great- 
er is  its  force,  ceteris  paribus,  in  proportion  to  its  size. 
When  the  axis  of  a  magnet  is  short,  and  of  course  its 
poles  very  near,  their  action  on  each  other  weakens  the 
magnetic  force.  A  variety  of  other  causes  will  also  oc- 
casion great  irregularity  in  the  attraction  of  magnetism. 
The  attraction  is,  as  I  shall  show  you,  always  strongest 
at  the  poles  of  the  magnet ;  and  most  so  when  the  body 
is  near  the  magnet,  but  diminishes  as  either  recedes  from 
the  other.  It  appears  also  from  experiment,  that  a  mag- 
net attracts  another  magnet  with  less  force  than  it  does 
a  piece  of  iron. 


OF    THE    POLES    OF    A    MAGNET. 

It  has  been  already  observed  to  you,  that  there  are 
certain  points  of  a  magnet  called  the  poles,  which  are  pos- 
sessed of  the  greatest  magnetic  force,  and  in  which  its 
virtues  seem  as  it  were  to  be  concentrated.  This  I  shall 
prove  by  an  easy  experiment :  here  are  several  small  iron 
balls  ;  I  shall  try  what  number  of  these  the  magnetic  bar 
will,  sustain  at  different  places,  and  you  find  that  it  sup- 
ports  the  greatest  number  near  the  ends  \  this  will  an* 


380  THE    ACTION    OF    THE    MAGNETIC 

swer  our  purpose  in  the  first  instance  ;  you  will  find  this 
further  confirmed  by  the  subsequent  experiments,  de- 
signed to  point  out  with  accuracy  the  situation  of  the 
poles  of  a  magnet. 

I  have  covered  a  pane  of  glass  with  writing  paper,  that 
the  difference  in  colour  may  enable  us  to  discern  more 
distinctly  what  effect  a  magnet  has  on  steel  filings  strewed 
over  the  paper ;  I  place  this  pane  over  a  magnet,  and 
sift  some  fine  steel  filings  thereon  ;  these  you  see  arrange 
themselves  in  a  very  curious  manner  ;  those  points  from 
which  the  curves  seem  to  rise,  and  over  which  the  filings 
stand  in  an  erect  position,  are  the  poles  of  the  magnet. 

Here  is  a  small  needle  inclosed  in  a  glass  ball ;  move 
this  over  a  magnetic  bar,  and  the  needle  will  be  perpen- 
dicular to  the  bar,  when  it  is  over  either  of  the  poles. 

The  poles  of  a  magnet  may  be  ascertained  with  great 
accuracy  by  means  of  a  small  dipping  needle,  plate  2, 
Jig.  7,  (Electricity ) ;  place  this  on  a  magnet,  and  move 
it  backwards  and  forwards  till  the  needle  is  perpendicu- 
lar to  the  magnet,  it  will  then  point  directly  to  one  of 
the  poles.  When  it  is  between  the  north  and  south 
poles,  so  that  their  mutual  actions  balance  each  other, 
the  centre  of  the  needle  will  stand  over  what  is  called 
the  equator  of  the  magnet,  and  the  needle  will  be  ex- 
actly parallel  to  the  bar  ;  between  this  situation  and  the 
poles,  it  inclines  to  the  bar  in  different  angles,  accord- 
ing to  its  distance  from  the  poles. 


OF    THE    ACTION    OF    THE    MAGNETIC    POLES    ON 
EACH    OTHER. 

in  the  action  of  the  magnetic  virtue  at  the  poles, 
there  is  a  strong  similarity  with  that  of  electricity  ;  thus 
the  contrary,  or  north  and  south  poles  of  two  magnets 
attract  each  other,  but  poles  of  the  same  name,  as  two 
north  or  two  south  poles,  repel  each  other. 

Suspend  on  a  point  a  touched  needle,  then  present 
towards  its  north  pole  the  south  pole  of  a  magnet,      d 
it  will  be  attracted   by,  and  fly  towards  ir ;  present  l 
other  pole  of  the  magnet,  and  the  needle  will  fly  from 


POLES    ON    EACH    OTHER.  381 

Strew  a  few  steel  filings  upon  a  pane  of  glass,  put 
either  the  north  or  south  pole  of  one  of  the  bars  under 
the  pane ;  the  filings  will  rise  upon  the  glass  as  the 
magnet  approaches.  Bring  the  same  pole  of  the  other 
bar  directly  over  that  under  the  glass,  and  when  it  is  at 
a  proper  distance,  the  steel  filings  will  drop  flat  on  the 
pane. 

Fix  two  needles  horizontally  in  two  pieces  of  cork, 
and  put  them  in  water ;  if  the  poles  of  the  same  name 
be  placed  together,  they  will  mutually  repel  each  other ; 
if  the  poles  of  a  contrary  denomination  be  turned  to- 
wards each  other,  they  will  be  attracted  and  join. 

Dip  the  north  or  south  ends  of  two  magnets  in  steel 
filings,  which  will  hang  in  clusters  from  the  end  of  the 
bars ;  bring  the  ends  of  the  bars  towards  each  other, 
and  the  steel  filings  on  one  bar  will  recede  from  those 
on  the  other.  Dip  the  south  pole  of  one  magnet,  and 
the  north  pole  of  the  other,  into  steel  filings,  and  bring 
the  ends  near  to  each  other,  and  the  tufts  of  filings  will 
unite,  forming  small  circular  arches. 


THE    ACTION    OF    THE    MAGNETIC     POLES    RENDERED 
VISIBLE    BY    STEEL    FILINGS. 

I  place  the  glass  pane  covered  with  paper  over  a  mag- 
netical  bar,  and  strew  it  over  with  steel  filings ;  on 
striking  the  glass  gently,  the  filings  dispose  themselves 
in  such  a  manner,  as  to  represent  with  exactness  the 
course  of  the  magnetic  matter.  The  curves,  by  which 
it  seems  to  go  from  pole  to  pole,  are  pleasingly  indicated 
by  the  arrangement  of  the  filings  ;  the  larger  curves  rise 
from  one  polar  surface  and  extend  to  the  other ;  they 
are  larger  in  proportion  as  they  rise  nearer  the  axis  or 
centre  of  the  polar  surface  ;  the  interior  curves  are  smal- 
ler and  smaller  in  proportion  to  their  distance  from  the 
end  ;  see  plate  $.,  fig.  8.  The  greater  the  distance  be- 
tween the  poles  of  a  magnet,  the  larger  are  the  curves 
which  arise  from  the  polar  surface. 

Let  two  magnets  be  placed  in  a  straight  line  at  a  small 
distance  from  each  other,  the  south  pole  of  the  one  op* 


582  ACTION    OF    THE    MAGNETIC    POLES 

posed  to  the  north  of  the  other  ;  lay  a  pane  of  glass  over 
them,  sprinkle  it  with  steel  filings,  and  then  strike  the 
pane  gently  with  a  key,  and  the  filings  will  arrange 
themselves  in  the  direction  of  the  magnetic  virtue  ;  those 
that  lie  between  the  two  polar  surfaces,  and  near  the 
common  axis,  are  disposed  in  straight  lines,  going  from 
the  north  pole  of  one  to  the  south  pole  of  the  other,  as  if 
uniting  and  joining  together;  plate  2,  Jig.  9. 

Place  two  north  or  two  south  poles  under  a  pane  of 
glass,  on  which  iron  filings  have  been  strewed,  and  the 
tilings  will  be  disposed  into  curves,  which  seem  to  turn 
back  and  avoid  each  other;  plate  2,  Jig.  10. 

In  magnetism,  as  well  as  in  electricity,  it  is  not  the 
mere  matter  that  is  attracted,  but  the  state  of  the  mag- 
netic fluid  therein,  so  that  the  body  always  becomes 
magnetic  before  it  is  attracted ;  and  hence  there  is  no 
magnetic  attraction  but  between  the  contrary  poles  of  two 
magnets. 

When  a  piece  of  iron,  or  any  other  substance  that 
contains  iron,  is  brought  within  a  certain  distance  of  a 
magnet,  the  powers  thereof  are  separated,  and  it  becomes 
itself  a  magnet,  having  poles,  attractive  power,  and  every 
property  of  a  real  magnet.  That  part  which  is  nearest 
the  magnet  has  a  contrary  polarity. 

The  magnetism  that  soft  iron  acquires,  when  placed 
within  the  influence  of  a  magnet  only  lasts  while  it  con- 
tinues in  that  situation,  but  disappears  as  soon  as  it  is  re- 
moved. But  with  hard  iron,  and  particularly  with  steel, 
the  case  is  quite  different.  For  the  harder  the  iron,  or 
the  steel,  the  more  permanent  is  the  magnetism  it  ac- 
quires ;  but  it  is  also  more  difficult  to  render  it  mag- 
netic. 

Thus  if  two  pieces,  one  of  soft  iron,  the  other  of  hard 
steel,  but  both  of  the  same  shape  and  size,  be  brought 
within  the  influence  of  a  magnet,  and  at  the  same  dis- 
tance, you  will  find  the  iron  appear  more  magnetical 
than  the  steel ;  but  when  the  magnet  is  removed,  the 
soft  iron  instantly  loses  its  magnetism,  whereas  the  steel 
will  preserve  it  for  a  long  time. 

A  magnet  will  therefore  attract  soft  iron  more  forcibly 
than  hard  iron,  because  it  can  render  it  more  strongly 
magnetical. 


RENDERED    VISIBLE    BY    STEEL    FILINGS.  383 

In  the  foregoing  experiments,  the  steel  filings  became 
so  many  little  magnets,  with  contrary  poles.  On  the 
same  principles,  a  large  key,  or  any  other  untouched 
piece  of  iron,  will  attract  and  support  a  small  piece  of 
iron,  while  it  is  near  the  pole  of  a  magnet,  but  will  let  it 
fall  when  removed  therefrom. 

A  ball. of  soft  iron,  in  contact  with  a  magnet,  will  at- 
tract a  second  ball,  and  that  a  third,  till  the  influence 
becomes  too  weak  to  suppost  a  greater  weight. 

Here  is  a  small  spinner,  plate  2,  fig.  1 1 ,  with  an  iron 
axis ;  I  spin  the  spinner,  and  then  take  it  up  by  a  mag- 
net, and  you  will  not  only  find  that  it  will  continue  spin- 
ning longer  than  if  it  were  left  to  whirl  on  the  table, 
but  a  second  and  a  third  whirligig  may  be  suspended 
one  under  another,  and  yet  continue  in  motion.  The 
number  suspended  depends  on  the  strength  of  the  mag- 
net. 

OF    MAGNETIC    CENTRES. 

There  is  a  point  between  the  two  poles,  where  the 
magnet  has  no  attraction  nor  repulsion ;  this  point  is 
called  the  magnetic  centre^  though  it  is  not  always  exact- 
ly between  the  two  poles. 

Pass  the  dipping  needle,  plate  2,  fig.  7,  over  a  mag- 
netic bar,  and  you  will  find  a  place  between  the  two 
poles,  where  the  needle  will  be  parallel  to  the  bar  ;  but 
if  you  move  it  ever  so  little  from  thence,  it  immediately 
inclines  towards  the  poles,  and  when  over  either  pole, 
is  perpendicular  to  the  bar. 

This  effect  is  also  pleasingly  exhibited  by  surrounding 
a  magnet  with  small  compass  needles.  I  place  the  nee- 
dles on  these  brass  stands,  so  that  they  may  be  nearly  in 
the  same  place  with  the  bar,  and  you  see  those  near  the 
ends  incline  towards  the  pole,  but  that  the  two  needles 
near  the  middle  of  the  bar  are  parallel  thereto,  not  in- 
clining to  either  pole ;  see  plate  2,  fig.  1 2.  You  may 
also  observe,  that  the  north  pole  of  the  magnet  attracts 
the  south  poles  of  all  the  needles,  and  the  south,  the 
north  of  the  needles. 


384  OF    MAGNETIC    CENTRES. 


Lay  a  number  of  magnetic  bars  in  a  straight  line  with 
the  north  and  south  poles  together,  pass  the  dipping 
needle  over  them,  and  you  will  find  a  magnetic  centre 
at  each  place  of  contact,  the  union  of  the  two  powers 
destroying  their  action ;  separate  them,  and  you  have 
the  north  and  south  poles,  as  at  first. 

Upon  the  same  principles,  if  a  magnetic  bar  be  broken 
into  any  two  parts,  each  part  becomes  a  magnet,  having 
two  poles  ;  the  ends  of  which  next  to  where  it  was  bro- 
ken acquiring  a  polarity  contrary  to  the  other  end.  Place 
a  magnetic  needle  upon  one  of  the  stands,  and  when  the 
needle  is  steady,  place  an  iron  bar  about  eight  inches  long 
and  between  a  quarter  of  an  inch  and  one  inch  in  thick- 
ness, upon  the  stand,  so  that  one  end  of  it  may  be  on 
one  side  of  the  north  pole  of  the  needle,  and  so  near  it  as 
to  draw  it  a  little  way  out  of  its  natural  direction.  In 
this  situation,  approach  gradually  the  north  pole  of  a 
magnet,  to  the  other  extremity  of  the  bar,  and  you  will 
see  that  the  needle's  north  end  will  recede  from  the  bar 
more  and  more,  in  proportion  as  the  magnet  is  brought 
nearer  to  the  bar.  If  the  experiment  be  repeated,  with 
only  this  difference,  viz.  that  the  south  pole  of  the  mag- 
net be  directed  towards  the  iron  bar,  then  the  north  end 
of  the  needle  will  advance  nearer  and  nearer  to  the  bar, 
in  proportion  as  the  south  extremity  of  the  magnet  is 
brought  nearer  to  the  iron. 

The  reason  of  this  phenomenon  is,  that,  by  the  ap- 
proach of  the  north  pole  of  the  magnet,  in  the  first  case, 
the  extremity  of  the  iron  bar  which  lies  next  to  it  ac- 
quires a  south  polarity,  and,  of  course,  the  opposite  ex- 
tremity acquires  the  north  polarity  ;  in  consequence  of 
which  the  needle  is  repelled,  because  magnetic  poles  of 
the  same  name  repel  each  other  ;  but  in  the  second  case, 
when  the  south  pole  of  the  magnet  is  brought  near  the 
bar,  the  end  of  the  bar  which  is  next  to  it  acquires  the 
north  polarity,  and  the  opposite  end  acquiring  the  south 
polarity,  attracts  the  north  end  of  the  needle. 

If,  whilst  the  pole  of  the  magnet  stands  contiguous  to 
one  end  of  the  bar,  a  small  magnetic  needle  be  presented 
within  a  certain  distance  to  various  parts  of  the  surface 
of  the  latter,  it  will  be  observed,  by  the  attraction  and 


TO    RENDER    IRON    AND    STEEL    MAGNETIC.    385 

repulsion  of  the  needle,  that  that  half  of  the  bar  which  is 
next  to  the  magnet  possesses  the  contrary  polarity,  and 
the  other  half  the  same  polarity  with  the  pole  of  the 
magnet  that  is  applied  to  the  iron. 

The  magnetic  centre,  however,  or  the  limit  between 
the  polarities,  is  not  always  in  the  middle  of  the  bar ;  it 
is  generally  nearer  that  end  which  is  presented  to  the 
magnet.  This  difference  is  greater  as  the  magnet  is 
weaker,  and  the  length  of  the  bar  increases ;  but  when 
the  bar  exceeds  a  certain  length,  which  depends  on  the 
strength  of  the  magnet,  then  the  bar  acquires  several 
successive  poles,  viz.  when  the  north  pole  of  the  mag- 
net is  contiguous  to  one  of  its  extremities,  that  extre- 
mity becomes  a  south  pole  ;  a  few  inches  farther  on  you 
will  have  a  north  polari-y,  then  a  south  polarity,  and  so 
on.  In  this  case,  the  first  magnetic  centre  comes  very 
near  that  end  of  the  bar  which  stands  next  to  the  mag- 
net, and  other  magnetic  centres  are  formed  between  every 
pair  of  successive  poles. 

Those  successive  poles  become  weaker  and  weaker  in 
power  according  as  they  recede  from  that  end  of  the  bar 
which  is  contiguous  to  the  magnet ;  so  that  in  a  pretty 
extended  bar,  they  quite  vanish  long  before  they  come 
to  the  farther  end  of  it  ;  hence,  if  one  pole  of  a  magnet 
be  applied  to  the  end  of  a  long  bar,  the  other  end  of  the 
bar  will  not  thereby  acquire  any  magnetism.  This  will 
happen,  when  a  magnet,  capable  of  lifting  about  two 
pounds  weight  of  iron,  is  applied  to  one  extremity  of  an 
iron  bar  about  one  inch  square  and  about  five  feet  long. 
On  removing  the  magnet,  the  bar,  if  of  soft  iron,  will 
immediately  lose  all  its  magnetism  ;  otherwise  it  will  re- 
tain it  a  longer  or  shorter  time,  in  proportion  to  its  hard- 
ness. 

TO    RENDER    IRON    AND    STEEL    MAGNETIC. 

The  communication  of  the  magnetic  power  to  iron 
and  steel  bars,  is  termed  by  artists,  touching  a  needle,  a  bar9 
&c.  To  give  a  detail  of  the  various  processes  used  by 
ditferent  artists  for  communicating  magnetism  to  iron, 
would  take  up  too  much  of  our  time  •>  I  shalL,  therefore, 

vol.  IV,  3  d 


386  TO    RENDER    IRON    AND 

only  mention  two  methods,  which  you  will  find  ade- 
quate to  every  common  purpose. 

I  first  place  two  magnets,*  A,  B,  plate  2,  Jig,  13,  in 
a  straight  line,  the  north  end  of  one  opposed  to  the 
south  end  of  the  other,  but  at  such  a  distance  that  the 
bar  to  be  touched  may  rest  upon  them,  taking  care  that 
the  end  I  designed  for  the  south  be  laid  upon  the  north 
end  of  one  bar,  and  the  north  end  on  the  south  pole  of 
the  other  bar. 

I  now  take  two  other  bars,  D  and  E,  and  apply  the 
north  end  of  Dt  and  the  south  end  of  E  to  the  middle 
of  the  untouched  bar  C,  elevating  their  other  ends  so 
as  to  form  an  acute  angle  with  the  said  bar.  I  now  se- 
parate D  and  E,  drawing  them  different  ways  along  the 
surface  of  the  bar  C,  but  preserving  the  same  elevation 
all  the  way  ;  I  remove  D  and  E  to  the  distance  of  a 
foot  or  more  from  the  untouched  bar  0,  and  bringing 
the  north  and  south  ends  in  contact,  I  apply  them 
again  to  the  middle  of  the  bar  C,  and  shall  repeat  the 
process  three  or  four  times  ;  after  which  I  shall  touch 
the  other  three  surfaces  in  the  same  manner,  and  the 
bar  will  thereby  have  acquired  a  strong  and  permanent 
magnetism.  This  was  one  of  the  methods  used  by  Dr. 
Knight,  who  first  taught  us  the  great  advantage  that 
might  be  obtained  from  the  use  of  magnetic  bars,  giv- 
ing  by  their  means  a  magnetism  to  compass  needles 
double  in  force  to  that  which  the  strongest  natural 
loadstone  could  communicate.  He  was  the  first  also 
who  found  the  way  of  working  on  the  natural  magnet, 
so  as  to  increase  its  power  in  a  great  degree,  and  of 
inverting  its  poles  at  pleasure. 

You  may  readily  communicate  the  virtue  to  un- 
touched bars  by  a  horse-shoe  magnet,  shown  at  plate 
2,  Jig.  15,  either  single  or  compound;  the  bar  to  be 
touched  should  be  laid  on  two  other  magnets,  as  in  the 


*  The  longer  and  stronger  these  are,  the  better  will  they  answt  r  the 
purpose. 

f  The  north  ends  of  magnetic  bars  are  generally  marked  by  a  line  cut 
across  them,  as  are  also  the  north  ends  of  horse-shoe  or  other  shaped  mag- 
nets, E.  Edit. 


STEEL    MAGNETIC.  387- 

preceding  case  ;  the  horse-shoe  magnet  must  be  placed 
on  the  middle  of  the  untouched  bar,  with  the  north 
end  towards  that  you  design  to  be  the  south ;  you  are 
then  to  draw  it  backwards  and  forwards  over  the  bar 
five  or  six  times,  but  be  careful  when  you  remove  it 
that  it  be  at  that  time  over  the  middle  of  the  bar.  The 
same  operation  is  to  be  used  with  the  other  surfaces  of 
the  bar. 

A  small  compass  needle  may  be  touched  by  being 
put  between  the  opposite  poles  of  two  magnetic  bars  ; 
while  it  is  receiving  the  magnetism,  it  will  be  violently 
agitated,  moving  backwards  and  forwards  as  if  it  were 
animated  :  when  it  has  received  as  much  magnetism  as 
it  can  acquire  in  this  way,  it  becomes  quiescent.* 

TO    TOUCH    A    HORSE-SHOE    MAGNET. 

Place  a  pair  of  magnetic  bars  against  the  ends  of  the 
horse-shoe  magnet,  with  the  south  end  of  the  bar 
against  that  end  of  the  horse-shoe  which  is  intended  to 
be  the  north,  and  the  north  end  of  the  other  bar  to 
that  which  is  to  be  the  south  :  the  contact  or  lifter  of 
soft  iron  to  be  placed  at  the  other  end  of  the  bars.  In 
this  situation  the  magnetic  fluid,  which  circulates 
through  the  bars,  will  endeavour  to  force  a  passage 
through  the  horse-shoe  magnet,  and  thus  facilitate  the 
further  communication  of  the  magnetic  virtue  to  the 
horse-shoe  magnet :  to  this  end,  rub  the  surfaces  of 
the  horse-shoe  with  a  pair  of  bars  placed  in  the  form  of 


*  Magnetism  is  best  and  most  conveniently  communicated  to  compass 
needles  by  the  two  following  methods  :  1.  By  a  pair  of  magnetic  bars  not 
less  than  six  inches  in  length.  Fasten  the  needle  down  on  a  board,  and 
with  a  magnet  in  each  hand  draw  them  from  the  centre  upon  the  needle 
outwards  ;  then  raise  the  bars  to  a  considerable  distance  from  the  needle, 
and  bring  them  perpendicularly  dowu  upon  the  centre,  and  draw  them 
over  again.  This  repeated  about  twenty  times  will  magnetize  the  needle, 
and  its  ends  will  point  to  the  poles  contrary  to  those  that  touched  them. 

2.  Over  one  end  of  a  combined  horse-shoe  magnet,  of  at  least  two  in 
number  and  six  inches  in  length,  draw  from  its  centre  that  half  of  the  nee- 
dle which  is  to  have  the  contrary  pole  ;  from  a  considerable  distance  draw 
the  needle  over  it  again.  This  repeated  about  twenty  times  at  least,  and 
the  same  for  the  other  half,  will  sufficiently  communicate  the  power. 

E.  Edit.. 


$SS  TO    MAKE    A    MAGNETICAL    BAR 

a  compass,  or  with  another  horse-shoe  magnet,  turn- 
ing the  poles  properly  towards  the  poles  of  the  horse- 
shoe magnet,  being  careful  that  these  bars  never  touch 
the  ends  of  the  straight  bars,  as  this  would  disturb  the 
Current  of  the  magnetic  fluid,  and  injure  the  operation. 
If  the  bars  be  separated  suddenly  from  the  horse-shoe 
magnet,  its  force  will  be  considerably  diminished ;  to 
prevent  this,  slip  on  the  lifter  or  support  to  the  end  of 
the  horse-shoe  magnet,  but  in  such  a  manner,  however, 
that  it  may  not  touch  the  bars ;  the  bars  may  then  be 
taken  away,  the  support  slid  to  its  place,  and  left 
there  to  strengthen  the  circulation  of  the  fluid. 

TO  MAKE  A  MAGN2TICAL  BAR  WITH  SEVERAL  POLES. 

Place  magnets  at  those  parts  where  the  poles  are  in- 
tended to  be,  the  poles  to  be  of  a  contrary  name  to 
those  required  ;  and  if  a  south  pole  be  fixed  on  one 
part,  the  two  next  places  must  have  north  poles  set 
against  them  ;  consider  each  piece  between  the  suppor- 
ters as  a  separate  magnet,  and  touch  it  accordingly. 

The  difference  in  the  nature  of  steel  with  respect  td 
its  receiving  magnetism,  is  exceedingly  great,  as  is  ea- 
sily proved  by  touching  in  the  same  manner  and  with 
the  same  bars  two  pieces  of  steel  of  equal  size,  but  of 
different  kinds.  With  some  sorts  of  steel  a  few  strokes 
are  sufficient  to  impart  to  them  all  the  power  they  arc 
capable  of  retaining  ;  other  sorts  require  a  longer  ope- 
ration ;  sometimes  it  is  impossible  to  give  them  more 
than  just  a  sensible  degree  of  magnetism. 

Steel  that  is  hardened  receives  a  more  perfect  mag- 
netism than  soft  steel,  though  it  does  not  appear  that 
they  differ  from  each  other  in  any  thing  but  the  ar- 
rangement of  the  parts  ;  perhaps  the  soft  steel  contains 
phlogiston  in  its  largest  pores,  while  hardened  steel 
contains  it  in  the  smaller.  Iron  and  steel  have  very  lit- 
tle air  incorporated  in  their  pores  ;  when  they  are  se- 
parated from  the  ore,  they  are  exposed  to  a  most  in- 
tense degree  of  heat ;  and  most  of  the  changes  to 
which  they  are  afterwards  submitted,  are  effected  in  a 
red-hot  state.     A  piece  of  spring-tempered  steel  will 


WITH    SEVERAL    POLES.  389 

not  retain  as  much  magnetism  as  hard  steel,  soft  steel 
still  less,  and  iron  scarce  retains  any.  From  some  ex- 
periments of  Mr.  Musschenbroek,  it  appears  that  when 
iron  is  united  with  an  acid,  it  will  nut  become  magne- 
tical ;  but,  if  the  acid  be  separated,  and  the  phlogiston 
restored,  it  will  become  as  magnetical  as  ever. 

In  communicating  magnetism,  it  is  best  to  use  weak 
magnets  first,  and  those  that  are  stronger  afterwards  ; 
but  you  must  be  very  careful  not  to  use  weak  magnets 
after  strong. 

A  magnet  can  never  communicate  a  greater  power 
than  itself  possesses,  or  even  of  an  equal  degree  ;  but, 
as  several  magnets  of  nearly  an  equal  degree  of  magne- 
tism, by  being  joined  together,  have  a  stronger  power 
than  either  of  them  singly  ;  in  order  to  impart  a  stron- 
ger magnetic  power  to  a  given  body  A,  by  means  of  a 
weak  magnet  B,  you  must  first  render  several  bodies, 
C,  D,  E,  F,  &c.  weakly  magnetic,  and  then  by  pro- 
perly joining  C,  D,  E,  F,  together,  you  may  commu- 
nicate to  another  body,  or  several  bodies,  a  stronger 
magnetism ;  and  thus  by  degrees  be  able  to  communi- 
cate to  A  the  desired  degree  or  magnetic  power. 

A  magnet  loses  nothing  of  its  own  power  by  commu- 
nicating to  other  substances,  but  is  rather  improved 
thereby. 

If  bars  of  iron  be  heated,  and  then  cooled  equally  in 
various  directions,  as  parallel,  perpendicular,  or  in- 
clined to  the  dipping  needle,  the  polarity  will  be  fixed 
according  to  their  position,  strongest  when  they  are  pa- 
rallel to  the  dipping  needle,  and  so  less  by  degrees,  till 
they  are  perpendicular  to  it,  when  they  will  have  no 
fixed  polarity ;  but  if,  upon  cooling  a  bar  of  iron  in 
water,  the  under  end  be  considerably  hotter  than  the 
upper,  and  the  upper  end  be  cooled  first,  it  will  some- 
times  become  the  north  pole,  but  not  always.  If  iron 
or  steel  undergo  a  violent  attrition  in  any  one  particu- 
lar part,  it  will  acquire  a  polarity  ;  if  the  iron  be  soft, 
the  magnetism  remains  very  little  longer  than  while  the 
heat  continues.  Lightning  is  the  strongest  power  yet 
known  in  producing  a  stream  of  magnetism  ;  it  will  in 
an  instant  render  hardened  steel  strongly  magnetical, 
and  invert  the  poles  of  a  magnetic  needle. 


JJ 

! 


390  TO    MAKE    A    MAGNETICAL    BAR 

Every  kind  of  violent  percussion  weakens  the  power 
of  a  magnet.  A  strong  magnet  has  been  entirely  de- 
prived of  its  virtue  by  receiving  several  smart  strokes  of 
a  hammer  ;  indeed,  whatever  deranges  or  disturbs  the 
internal  pores  of  a  magnet,  will  injure  its  magnetic 
force,  as  the  bending  of  touched  iron,  wires,  &c. 

Fill  a  small  dry  glass  tube  with  iron  filings,  press 
them  in  rather  close,  and  then  touch  the  tube  as  if  it 
were  a  steel  bar,  and  the  tube  will  attract  a  light  nee- 
dle, &c.  shake  the  tube  so  that  the  situation  of  the  fil- 
ings may  be  disturbed,  and  the  magnetic  virtue  will 
vanish. 

But,  though  a  violent  percussion  will  destroy  a  fixe 
magnetism,  yet  it  will  give  polarity  to  an  iron  bar 
which  had  none  before  ;  for  a  few  smart  strokes  of  a 
hammer  on  an  iron  bar  will  give  it  a  polarity,  and  by 
hitting  first  one  end  of  the  bar,  and  then  the  other, 
while  it  is  held  in  a  vertical  situation,  the  poles  may  be 
changed.  Twist  a  long  piece  of  iron  wire  backwards 
and  forwards  several  times,  then  break  it  off  at  the 
twisted  part,  and  the  broken  end  will  be  magnetical. 

The  pole  of  a  magnet  always  produces  the  contrary 
polarity  on  a  bar  to  which  it  is  applied  :  therefore,  if 
two  bars  fully  touched  have  the  poles  of  the  same  name 
joined  together,  they  tend  to  produce  on  each  other  a 
force  of  a  contrary  name  to  that  with  which  they  are 
endowed  ;  and  this  effect  will  diminish  the  polar  force 
of  each  bar  ;  consequently,  the  magnetic  force  of  each 
longitudinal  element  of  an  artificial  magnet  diminishes 
as  its  bulk  is  increased,  and  the  total  force  of  two  mag- 
nets fully  touched,  and  of  the  same  length,  but  unequal 
in  bulk,  will  be  in  a  less  ratio  than  that  of  their  mass. 

If  the  magnet  do  not  touch  the  bar,  but  be  held  at 
some  distance  from  it,  the  phenomena  will  be  the  same  ; 
but  the  bar  will  acquire  less  magnetism  than  when  it 
was  in  contact  with  the  magnet. 

Each  point  of  a  magnet  may  be  looked  upon  as  the 
pole  of  a  smaller  magnet,  tending  to  produce  on  the 
points  of  the  magnet  a  force  contrary  to  its  own.  The 
effect  of  this  tendency  will  be  greater,  in  proportion  to 
the  force  of  the  point,  and  its  nearness  to  those  points 


WITH  SEVERAL  POLES.  391 

m  which  it  acts  ;  and  the  force  of  a  magnet  will  depend 
m  the  reciprocal  action  of  these  points  on  each  other. 

Hence,  a  narrow  bar  will  in  general  be  more  power- 
ful than  a  broader  one ;  and  hence  also  the  exterior 
?dges  and  points  of  a  magnet  will  have  more  power 
han  the  interior  ones  of  the  same  bar. 

Hence,  also,  magnets  should  never  be  left  with  two 
lorth  or  two  south  poles  together  ;  for,  when  they  are 
;hus  placed,  they  diminish  and  destroy  each  other's 
nagnetism,  Magnetic  bars  should  therefore  be  always 
[eft  with  the  opposite  poles  laid  against  each  other,  or 
by  connecting  their  opposite  poles  by  a  bar  of  iron. 
The  magnetic  power  is  increased  in  a  magnet,  by  let- 
ting a  piece  of  iron  remain  attached  to  one  or  both  of 
its  poles.  A  single  magnet  should  therefore  be  always 
thus  left. 

OF    ARMED    MAGNETS. 

As  both  magnetic  poles  together  attract  a  much 
greater  weight  than  a  single  one,  and  as  the  two  poles 
of  a  magnet  are  generally  in  opposite  parts  of  its  sur- 
face, in  which  situation  it  is  almost  impossible  to  adapt 
the  same  piece  of  iron  to  both  at  the  same  time  ;  two 
soft  pieces  of  iron  are  applied  to  the  poles  of  a  loadstone, 
so  as  to  project  on  one  side  the  magnet ;  these  pieces 
being  rendered  magnetic,  another  piece  of  iron  can  be 
conveniently  adapted  to  these  projections,  so  as  to  let 
both  poles  act  at  the  same  time.  The  magnet  in  this 
case  is  said  to  be  armed,  the  pieces  of  iron  are  called 
the  armature,  the  piece  of  iron  that  connects  the  poles  is 
termed  the  lifter.  Plate  2,  Jig.  14,  represents  an  arm- 
ed artificial  magnet.  In  a  similar  manner  the  load- 
stone, or  natural  magnet,  is  advantageously  armed. 

To  avoid  the  expense  and  trouble  of  the  armature, 
artificial  magnets  have  been  made  in  the  shape  of  a 
horse-shoe,  of  which  I  have  already  spoken. 

Gassendi  invented  a  peculiar  kind  of  armour,  by 
piercing  a  loadstone  in  the  direction  of  the  axis,  and 
placing  a  cylinder  of  iron  in  the  hole,  which  augment- 
ed considerably  the  force  of  the  magnet. 


392  OF    THE    MAGNETISM    OF    THE    EARTH, 

Here  is  a  straight  magnetic  bar,  the  north  pole  of 
which  supports  four  ounces.  I  apply  another  magnet 
against  it,  but  so  that  the  north  pole  thereof  is  about 
half  an  inch  from  the  pole  of  the  other,  and  it  will  now 
sustain  near  seven  ounces. 

OF    THE    MAGNETISM    OF    THE    EARTH. 

What  has  been  usually  termed  the  magnetism  of  the 
earth,  might  with  more  propriety  be  termed  the  mag- 
netism  of  the  atmosphere.  Even  the  experiments  usu* 
ally  adduced  to  prove  the  magnetism  of  the  earth,  are 
full  proofs  that  it  is  an  aerial  influence  ;  as  you  will  per- 
ceive by  the  account  I  am  going  to  give  you  of  the  ex- 
periments brought  in  support  of  the  earth's  magnetism. 

Mr.  Savery  has  adduced  several  instances  to  show 
the  force  and  action  of  the  earth's  magnetism  ;  among 
others,  that  it  will  support  small  pieces  of  iron.  He 
hung  up  a  bar  of  iron,  about  five  feet  long,  by  a  loop 
of  small  cord  at  the  upper  end,  and  then  carefully  wip- 
ed the  lower  end,  and  the  point  of  a  nail,  that  there 
might  be  no  dust  or  moisture  to  prevent  a  good  con- 
tact ;  then  holding  the  nail  under  the  bar  with  its  point 
upward,  he  kept  it  close  to  the  bar,  holding  only  one 
finger  under  its  head  for  the  space  of  thirty  or  more 
seconds ;  then  withdrawing  his  finger  gently  down- 
wards, that  the  nail  might  not  vibrate;  if  it  fell  off, 
he  wiped  the  point  as  before,  and  tried  some  other  part 
of  the  plane  at  the  bottom  of  the  bar.  If  the  ends  be 
similar,  and  the  bar  have  no  permanent  virtue,  it  is  in- 
different which  end  is  downwards  ;  if  it  have  an  imper- 
fect degree  of  polarity,  one  end  will  answer  better  than 
the  other. 

The  upper  end,  A,  of  a  long  iron  rod,  which  has 
no  fixed  polarity,  will  attract  the  north  end  of  a  mag- 
netic needle  ;  the  under  end,  B,  repels  the  north  end 
of  the  needle ;  invert  the  iron  bar,  and  the  end  B, 
which  is  now  the  upper  one,  will  attract  the  north  pole 
of  the  needle  it  repelled  before.  The  case  is  the  same, 
if  the  bar  be  placed  horizontally  in  the  magnetic  meri- 
dian ;  the  end  towards  the  south  will  then  be  the  north 
pole. 


THE    MAGNETISM    OF    THE    EARTH.  393 

The  explanation  of  this  curious  phenomenon  is  easily- 
deduced  from  the  foregoing  observations  ;  for,  since  in 
these  northern  parts,  the  earth  is  possessed  of  a  south 
magnetic  polarity,  the  lowest  part  of  the  iron  bar,  by  be- 
ing nearest  to  it,  must  acquire  the  contrary,  namely,  the 
north  polarity  ;  the  other  extremity  of  the  bar  becoming 
a  south  pole. 

It  follows,  likewise,  and  it  is  confirmed  by  actual  ex- 
periment, that  in  the  southern  parts  of  the  earth,  the 
lowest  part  of  the  bar  acquires  the  south  polarity ;  that 
on  the  equator,  the  bar  must  be  kept  horizontal,  in  order 
to  let  it  acquire  any  magnetism  from  the  earth;  and  that, 
even  in  these  parts  of  the  earth,  the  most  advantageous 
situation  of  the  bar  is  not  the  perpendicular^  but  that  a 
little  inclined  to  the  horizon.  In  short,  in  every  part  of 
the  world  it  must  be  placed  in  the  magnetical  line,  viz, 
in  the  direction  of  the  dipping  needle.  If  the  iron  bar* 
instead  of  being  kept  in  the  magnetical  line,  be^placed 
in  a  direction  perpendicular  to  it,  then  it  will  acquire  no 
magnetism,  because  in  that  situation  the  actions  of  both 
poles  of  the  earth  upon  each  extremity  of  the  bar  are 
equal.  If,  instead  of  the  above-mentioned  two  directions, 
the  bar  be  placed  in  any  other  position,  then  it  will  ac- 
quire more  or  less  magnetic  power,  according  as  it  ap- 
proaches nearer  to  the  former  or  to  the  latter  of  the  said 
two  directions. 

Iron  bars  of  windows,  which  have  remained  long  in  a 
vertical  position,  acquire  a  fixed  polarity.  Mr.  Lewen- 
boek  mentions  an  iron  cross,  which  had  acquired  a  very 
strong  polarity.  Mr.  Canton  proposed  to  make  artificial 
magnets  without  the  assistance  of  natural  ones ;  but  in 
this  he  was  mistaken,  for  his  poker  and  tongs  were  natu- 
ral magnets,  and  had  their  verticity  fixed  by  being  heated 
and  cooled  in  a  vertical  position  ;  and  an  iron  or  steel  bar, 
though  without  a  verticity,  while  it  remains  in  that  posi- 
tion exerts  a  polarity,  and  is  able  to  communicate  a  fixed 
verticity  to  the  small  bar,  and  is,  therefore,  for  the  lime 
i  natural  magnet.  And  further,  every  iron  bar,  from 
.he  largest  size  to  a  sixpenny  nail,  will  exert  this  power 
when  treated  as  above-mentioned.  But  how  this  power 
is  raised  so  soon  to  a  degree  greatly  exceeding  that  which 

VCL.  IV.  2  E 


,494  DIRECTIVE    PROPERTY    OF    MAGNETS. 

communicated  it,  we  do  no*  know ;  nor  is  it  more  easy 
to  account  for  the  facility  with  which  the  magnetic  power 
is  withdrawn  by  a  friction  contrary  to  that  which  gave  it. 


OF    THE    DIRECTIVE    PROPERTY    OF    MAGNETS. 

Let  an  iron  rod  be  exactly  balanced  and  suspended  on 
a  point,  so  as  to  revolve  in  a  plane  parallel  to  the  horizon ; 
communicate  the  magnetic  virtue  to  this  rod,  and  one 
extremity  will  be  always  directed  towards  the  north. 

Here  is  an  untouched  magnet,  I  place  it  on  a  point, 
and  you  may  observe  that  I  can  make  it  rest  in  any  given 
situation ;  I  shall  communicate  the  magnetic  virtue  to 
it,  and  you  will  then  find  it  no  longer  indifferent  as  to 
its  situation,  but  it  will  fix  upon  one  in  preference  to  any 
other,  one  end  always  pointing  to  the  north. 

Whenever  a  magnet  can  move  itself  freely,  as  if  it  be 
suspended  by  a  fine  thread,  or  if  it  be  made  to  float  on 
water  by  means  of  a  piece  of  cork,  or  if  it  be  balanced  on 
a  point,  provided  it  be  not  disturbed  by  the  vicinity  of 
iron;  it  will  always  place  itself  so  as  to  direct  its  north 
pole  towards  the  north,  and  the  south  pole  towards  the 
south. 

The  directive  power  of  a  touched  needle  is  of  the  great- 
est importance  to  mankind  ;  it  enables  the  mariner  to  tra- 
verse the  ocean,  and  thus  unites  the  arts,  the  manufac- 
tures, and  the  knowledge  of  distant  countries,  together. 
The  surveyor,  the  miner,  and  the  astronomer,  derive 
many  advantages  from  this  wonderful  property. 

The  mariner's  compass  consists  of  three  parts,  the  box, 
the  card  or  fly,  and  the  needle. 

The  card  is  a  circle  of  stiff  paper  representing  the  ho- 
rizon, with  the  32  points  of  the  compass  marked  on  it; 
the  magnetical  needle  is  fixed  to  the  under  side  of  this 
card  ;  the  centre  of  the  needle  is  perforated,  and  a  cap 
with  a  conical  agate  at  its  top  is  fixed  in  this  perforation ; 
this  cap  is  hung  on  a  steel  pin,  which  is  fixed  to  the  bot- 
tom of  the  box,  so  that  the  card  hanging  on  the  pin  turns 
freely  round  its  centre ;  one  of  the  points  being,  from 
the  property  of  the  needle,  always  directed  towards  the 


DIRECTIVE    PROPERTY    OF    MAGNETS.  395 

north  pole.  The  box  which  contains  the  card  and  needle, 
is  a  circular  brass  box  hung  within  a  square  wooden  one, 
by  two  concentric  rings  called  jimbals,  so  fixed  by  cross 
centres  to  the  two  boxes,  that  the  inner  one  shall  retain 
a  horizontal  position  in  all  motions  of  the  ship.  The  top 
of  the  inner  box  has  a  cover  of  glass,  to  prevent  the  card 
from  being  disturbed  by  the  wind.*  Before  the  compass 
was  invented,  the  navigating  of  ships  was  a  tedious  and 
precarious  operation,  and  seldom  performed  out  of  sight 
of  land ;  but  this  instrument  enables  the  mariner  to  tra- 
vel over  the  seas  almost  in  as  direct  and  true  a  tract,  as 
the  land  carrier  directs  his  carriage  in  a  well-beaten  road. 

It  has  been  already  observed,  that  the  ancients  do  not 
seem  to  have  been  acquainted  with  the  directive  power  of 
the  magnet.  The  only  thing  that  seems  capable  of  being 
mistaken  for  some  such  knowledge,  is  what  Jamblichm 
tells  us  in  his  life  of  Pythagoras ',  "  That  Pythagoras  took 
from  Abaris,  the  Hyperborean,  his  golden  dart,  without 
which  it  was  impossible  for  him  to  find  his  road."  But 
the  authority  of  the  writer,  as  well  as  the  obscurity  of  the 
passage,  prevents  any  conclusion  being  drawn  from  it. 

Paul,  the  Venetian,  is  said  to  have  introduced  the  use 
of  the  compass  in  1260 ;  but  this  is  said  not  to  have  been 
his  own  invention,  but  borrowed  from  the  Chinese.  P. 
Gaubil  says,  the  directive  power  of  the  needle  was  known 
to  the  Chinese  as  early  as  the  year  A.  D.  223,  under  the 
dynasty  of  Haz.  But  the  Abbe  Renaudot,  in  his  Disserta- 
tion on  the  Stone,  when  the  Mahomedans  went  first  to 
China,  has  adduced  strong  reasons  to  prove,  that  the  Chi- 
nese knew  nothing  of  the  mariner's  compass  till  it  was  in- 
troduced there  by  the  Europeans.  Vertomanus  affirms, 
that  A.D.  1500,  he  saw  an  East-Indian  pilot  direct  his 
course  by  a  compass,  framed  and  fastened  like  those  used 
in  Europe ;  but  this  must  be  received  with  some  caution, 
as  M.  Barlow,  in  1597,  says,  that  in  a  personal  confer- 
ence with  two  East-Indians  he  was  told  by  them,  that 


_  *  This  is  called  simply  the  steering  comfiass;  with  the  addition  of  sights, 
divided  circles,  &x,.  for  observing  azimuths  and  amplitudes  of  the  heave  nlv 
bodies,  it  is  called  the  azimuth  ro^C5s...,.E.EsrT. 


396  DIRECTIVE    PROPERTY    OF    MAGNETS. 


instead  of  our  compass  they  made  use  of  a  magnetical 
needle  of  six  inches  or  longer,  set  upon  a  pin  in  a  dish 
of  white  China  earth  filled  with  water;  that  in  the  bot- 
tom of  the  dish  they  had  two  cross-lines  to  mark  the 
four  principal  winds,  *and  that  the  rest  of  the  divisions 
were  left  to  the  skill  of  the  pilot.  But  to  return  to  Eu- 
rope, Mr.  Pcrrault,  in  his  parallel  between  the  ancients 
and  the  moderns,  has  cited  some  verses  of  Guyot  de  Pro- 
vim,  who  wrote  in  1 1 80,  which  show  distinctly,  that  the 
mariner's  compass  was  known  in  the  south  of  France  at 
that  time. 

By  most  writers  the  invention  of  the  compass  is  as- 
cribed to  Flavio  Gain,  of  Analsi  in  Campanee,  who  lived 
about  the  year  1300;  and  he  is  said  to.  have  been  the 
first  that  applied  it  to  navigation  in  the  Mediterranean. 

Mr.  de  Lalande  informs  us,  that  in  Le  Tresor  de  Bru- 
nei, a  manuscript  in  the  French  king's  library,  there  is 
a  passage  which  proves  that  the  compass  was  made  use 
of  about  the  year  1260. 

Here  however  it  may  be  observed,  that  though  a  mag- 
net, which  has  only  two  poles,  will  always,  when  freely 
suspended,  place  itself  in  the  magnetic  meridian,  or  in 
the  same  plane  with  other  good  magnets ;  yet  when  a 
magnet  has  more  than  two  poles,  these  may  be  so  situ- 
ate that  the  magnet  will  not  traverse,  that  is,  will  have 
no  directive  power. 

Thus,  suppose  an  oblong  magnetic  needle  to  have  a 
north  polarity  equally  strong  at  each  end,  and  a  south 
polarity  in  the  middle  ;  it  is  plain,  that  as  each  has  an 
equal  tendency  towards  the  north,  neither  of  them  can 
be  directed  towards  the  north  in  preference  to  the  other ; 
consequently,  the  needle  cannot  traverse.  Though  this 
case  very  seldom  occurs,  yet  there  are  many  others  where 
a  needle,  when  fixed  to  a  card  on  which  the  points  of  a 
compass  are  drawn,  may  occasion  considerable  errors; 
this  has  been  clearly  proved  by  Dr.  Knight  and  Capt. 
Greaves.  Mr.  R.  Walker,  of  Jamaica,  has  also  clearly 
proved,  that  the  only  proper  shape  for  magnetic  com- 
pass needles,  is  that  where  the  line  of  direction  is  in  the 
edge  of  the  bar ;  each  end  of  the  bar  should  be 
pointed. 


[     397     ] 


OF    THE    VARIATION    OF    THE    COMPASS. 

Though  the  north  pole  of  the  magnet  is,  in  every  part 
of  the  world,  directed  nearly  towards  the  north,  yet  it 
very  seldom  points  exactly  thereto,  and  consequently  the 
south  pole  of  the  magnet  seldom  points  towards  the  south. 
In  other  words,  the  magnetic  meridian  seldom  coincides 
with  the  meridian  of  the  place,  but  generally  varies  from 
it  some  degrees  eastward  or  westward. 

This  variation  is  different  in  different  places  on  land 
as  well  as  at  sea,  and  is  continually  varying  in  the  same 
place.  For  instance,  the  variation  is  not  the  same  in  Lon- 
don as  at  Paris,  or  at  the  Cape  of  Good  Hope  ;  and  the 
declination  at  London,  or  at  any  other  place,  is  not  the 
same  now  that  it  was  twenty  years  ago. 

This  variation  is  always  reckoned  from  the  north  ;  that 
is,  if  the  north  end  of  a  needle  vary  to  the  east  of  the 
north,  the  variation  is  said  to  be  easterly  ;  and  if  it  vary 
to  the  west,  the  variation  is  said  to  be  westerly. 

The  uncertainty  of  the  quantity  of  this  variation  in  dif- 
ferent parts  of  the  world  is  a  great  impediment  to  the  per- 
fecting of  navigation  ;  and  philosophers  have  earnestly 
endeavoured  to  investigate  its  cause,  and,  if  possible,  to 
correct  the  errors  it  occasions. 

Though  the  directive  power  of  the  compass  was  applied 
to  the  purposes  of  navigation  in  the  fourteenth  and  fif- 
teenth centuries,  it  does  not  appear,  that  there  were  any 
apprehensions  during  that  time  of  its  pointing  otherwise 
than  due  north  and  south. 

The  variation  of  the  compass  is  said  to  have  been  first 
discovered  by  Columbus,  the  latter  end  of  the  fifteenth 
century.  But  the  first  person  who  discovered  that  it 
was  real,  and  was  the  same  with  all  needles  in  the  same 
place,  is  generally  allowed  to  be  Sebastian  Cabot.  This 
was  about  the  year  1497. 

After  the  variation  was  discovered  by  Cabot,  it  was 
thought,  for  a  long  time,  to  be  invariably  the  same  at  the 
same  places  in  all  ages  ;  but  Mr.  Gellibrand,  about  the 
year  1625,  discovered,  that  it  was  different  at  different 
times  in  the  same  place. 


398  VARIATION    OF    THE    COMPASS. 

From  successive  observations  made  afterwards  it  ap- 
pears, that  this  deviation  was  not  a  constant  quantity, 
but  that  it  gradually  diminished,  and  at  last,  about  16.57, 
it  was  found,  that  the  needle  pointed  due  north  at  Lon- 
don, and  has  ever  since  been  increasing  to  the  westward 
of  the  north.  So  that  in  any  one  place  the  variations 
have  a  kind  of  libratory  motion,  traversing  through  the 
north  to  unknown  limits  eastward  and  westwards.  The 
present  variation  at  London  is  about  two  points,  or  23 
degrees  west  of  the  north. 

Dr.  Halley  supposed,  that  the  earth  has  within  it  a 
large  magnetic  globe,  not  fixed  within  to  the  external 
parts,  having  four  magnetic  poles,  two  fixed  and  two 
moveable,  and  by  this  he  has  endeavoured  to  account  for 
the  phenomena  of  the  needle.  His  application  of  this 
theory  to  facts  is  in  many  respects  inadequate,  in  all  la- 
boured and  unnatural.  Mr.  Eider  has  shown,  that  he 
can  with  two  magnetic  poles  placed  on  the  surface  of  the 
earth,  account  for  all  the  phenomena  as  well  as  Dr.  Halley 
with  four  ;  but  his  theory  has  also  various  imperfections. 

The  variation  of  the  needle  may  be  illustrated  by 
placing  several  touched  needles  round  a  magnetic  bar; 
see  plate  2,  fig,  12.  Now,  if  the  earth  be  a  great  mag- 
net, or  if  it  have  only  a  magnetic  atmosphere,  it  is  clear 
from  this  experiment,  that  magnetic  needles  placed  on 
its  surface  would  have  different  directions  in  different 
places,  which  is  conformable  to  experience  ;  and  the 
apparent  irregularities  in  the  variation  of  the  needle 
must  be  occasioned  by  the  situation  of  the  magnetic 
poles  of  the  earth. 

If  the  magnetic  poles  agreed  with  those  of  the  earth, 
there  would  be  no  variation,  and  the  magnetic  needle 
would  point  to  the  true  north  and  south.  If  the  axis  of 
the  magnetic  poles  passed  through  the  centre  of  the 
earth,  it  would  be  easy  to  assign  the  quantity  of  the  va- 
riation at  every  place  ;  but  as  this  is  not  the  case,  to  ac- 
count regularly  for  the  variation,  it  is  necessary  to 
know  the  exact  situation  of  the  magnetic  poles  of  the 
earth,  their  number,  force,  and  distance  from  the  real 
poles  ;  whether  they  shift  their  place,  and  if  they  move, 
the  quantity  of  motion  every  year. 


[     399     ] 


OF    THE    DIURNAL    VARIATION    OF    THE    NEEDLl. 

About  the  years  1722  and  1723,  Mr.  George  Graham 
made  a  number  of  observations  on  the  diurnal  varia- 
tions of  the  magnetic  needle.  In  the  year  1 750,  Mr. 
Wargentin  took  notice  of  the  regular  diurnal  variation 
of  the  needle  ;  and  also  of  its  being  disturbed  at  the 
time  of  an  aurora  borealis.  About  the  latter  end  of  the 
year  1756,  Mr.  Canton  began  to  make  observations  on 
the  variation,  and  in  1759,  communicated  several  va- 
luable experiments  to  the  Royal  Society. 

The  observations  were  made  by  him  for  603  days ; 
on  574  out  of  these  the  diurnal  variation  was  regular. 
The  absolute  variation  of  the  needle  westward  was  in- 
creasing, from  about  eight  or  nine  o'clock  in  the  morn- 
ing till  about  one  or  two  in  the  afternoon,  when  the 
needle  became  stationary  for  some  time  ;  after  that,  the 
variation  westward  was  decreasing ;  and  the  needle 
came  back  again  ro  its  former  situation  in  the  night,  or 
by  the  next  morning. 

The  diurnal  variation  is  irregular  when  the  needle 
moves  slowly  eastward  in  the  latter  part  of  the  morning, 
or  westward  in  the  latter  part  of  the  afternoon  ;  also 
when  it  moves  much  either  way  after  night,  or  suddenly 
both  ways  in  a  short  time.  These  irregularities  seldom 
happen  more  than  once  or  twice  in  a  month,  and  are 
always  accompanied  with  an  aurora  borealis.  The  diur- 
nal variation  in  the  months  of  June  and  July  is  almost 
double  that  in  January  and  December. 

Mr.  Canton  supposes,  that  the  diurnal  heat  of  the  sun 
acts  upon  the  magnetic  parts  of  the  earth,  or  rather 
upon  the  magnet  included  in  the  earth.  But  Mr.  JEfi- 
mis  has  shown,  that  this  supposition  is  inadmissible,  be- 
cause agreeably  to  the  hypothesis  the  magnetic  nucleus 
must  be  very  profound,  and  it  is  well  known,  that  the 
solar  heat  does  not  penetrate  to  very  great  depths; 
there  are  caves  at  no  great  distance  from  the  surface  of 
the  earth,  in  which  a  thermometer  remains  always  at 
the  same  height.  The  diurnal  heat  does  not  penetrate 
even  these,  there  is  therefore  no  probability  of  its  ef- 
fects extending  to  still  greater  depths. 


[     400     ] 


OF    THE    DIP    OF    THE    NEEDLE, 


If  a  needle,  which  is  accurately  balanced  and  sus- 
pended, so  as  to  turn  freely  in  a  vertical  plane,  be  ren- 
dered magnetical,  the  north  pole  will  be  depressed,  and 
the  south  pole  elevated  above  the  horizon :  this  pro- 
perty is  called  the  inclination  or  dip  of  the  needle.  As  it 
is  very  difficult  to  balance  a  needle  accurately,  the  poles 
are  generally  reversed  by  a  magnet,  so  that  its  two  ends 
may  dip  alternately,  and  the  mean  of  the  two  is  taken.* 

This  property  was  discovered  by  Robert  Norman,  about 
the  year  1576.  I  shall  give  the  account  of  the  discovery 
in  his  own  words  : 

"  Having,  says  he,  made  many  and  divers  compasses, 
and  using  always  to  finish  and  end  them  before  I  touch- 
ed the  needle,  I  found  continually  that  after  I  had  touch- 
ed the  yrons  with  the  stone,  that  presently  the  north 
point  thereof  would  bend  or  decline  downwards  under 
the  horizon  in  some  quantity ;  insomuch,  that  to  the 
ilie  of  the  compass,  which  before  was  made  equal,  I  was 
still  constrained  to  put  some  small  piece  of  wax  in  the 
south  part  thereof,  to  counterpoise  this  declining,  and  to 
make  it  equal  again. 

"  Which  effect  having  many  times  passed  my  hands 
without  any  great  regard  thereunto,  as  ignorant  of  any 
such  property  in  the  stone,  and  not  before  having  heard 
nor  read  of  any  such  matter;  it  chanced  at  length  that 
there  came  to  my  hands  an  instrument  to  be  made,  with 
a  needle  of  six  inches  long,  which  needle  after  I  had 
polished,  cut  off  at  just  length,  and  made  to  stand  level 
upon  the  pin,  so  that  nothing  rested  but  only  the  touch- 
ing of  it  with  the  stone :  when  I  had  touched  the  same, 
presently  the  north  part  thereof  declined  down  in  such 


*  The  dipping  needle  represented  at  plate  2,  Jig.  7,  is  one  of  the  com- 
monest and  smallest  kind.  The  mo>t  complete  and  accurate  dipping  or 
rather  universal  magnetic  nadle,  showing  at  the  same  time  the  horizontal 
and  vertical  directions  of  the  magnet,  was  contrived  by  the  late  ii.genious 
Dv  Larimer,  a  philosopher,  who,  among  the  moderns,  has  perhaps  n 
the  greatest  variety  of  experiments  and  discoveries  in  the  science.  See  Ilia 
Concise  Essay  on  Magnetism,  4to.  1795. — E.  Edit. 


INFLUENCE  OF  THE  AURORA  BORCJALIS,  &C.       401 

sort,  that  being  constrained  to  cut  away  some  of  that 
part  to  make  it  equal  again,  in  the  end  I  cut  it  too 
short,  and  so  spoiled  the  needle  wherein  I  had  taken  so 
much  pains. 

"  Hereby  being  stroken  into  some  cholar,  I  applied 
myself  to  seek  further  into  this  effect,  and  making  cer- 
tain learned  and  expert  men  (my  friends)  acquainted 
in  this  matter,  they  advised  me  to  frame  some  instru- 
ment, to  make  some  exact  trial,  how  much  the  needle 
touched  with  the  stone  would  decline,  or  what  great- 
est angle  it  would  make  with  the  plane  of  the  horizon/1 
Thus  far  Mr.  Norman. 

The  dip  is  said  to  be  subject  to  a  variation.  At  this 
time  in  London  it  is  about  72  degrees  ;  from  some  late 
obvervations  it  appears  to  diminish  about  fifteen  minutes 
in  four  years.  The  nature  of  this  phenomenon  is  plea- 
singly illustrated  by  carrying  a  small  dipping  needle 
from  one  end  of  a  magnetic  bar  to  the  other  ;  when  it 
stands  over  the  south  pole,  the  north  end  of  the  needle 
will  be  directed  perpendicularly  to  it ;  as  the  needle  is 
moved,  the  dip  will  grow  less,  and  when  it  comes  to 
the  magnetic  centre  it  will  be  parallel  to  the  bar ;  after- 
wards the  south  end  will  dip,  and  the  needle  will  stand 
perpendicular  to  the  bar,  when  it  is  directly  over  the 
north  pole.* 

OF    THE    INFLUENCE    OF    THE    AURORA    B0REALIS  ON 
THE    MAGNETIC    NEEDLE. 

Messrs.  Wilcke  and  Van  Swinden  have  clearly  prov- 
ed, that  there  is  a  connexion  between  the  aurora  bo- 


*  Plate  2,  Jig.  17,  represents  an  instrument  called  a  magnetometer* 
or  an  instrument  to  ascertain  the  comparative  strength  of  magnetical  bars. 
A  is  a  brass  quadrant  divided  into  90°  ;  B,  a  magnetic  needle  vertically 
suspended  and  balanced  ;  C,  a  brass  base  divided  into  inches  and  tenths. 
The  bars  to  be  examined  are  laid  on  the  base,  and  their  respective  powers 
are  shown  by  the  distance  of  the  ends  of  the  bars  on  the  base  from  0  on  the 
ire,  and  the  number  of  degrees  on  the  arc,  up  to  which  the  needle,  B,  is 
repelled. 

Collections  of  the  several  magnetical  articles,  part  of  which  are  shown 
in  plate  2,  to  illustrate  the  general  principles  of  magnetism,  are  selected 
and  packed  by  us  in  cases,  and  which  form  either  to  the  lecturer  or  student 
d  new  and  useful  collection  of  curious  instruments.  E,  Edit, 

VOL.  IV.  3  F 


402  OF    THE    THEORY   OF    MAGNETISM. 

realis  and  the  magnetic  needle  ;  they  have  shown  it  to 
be  so  evident,  so  general,  and  so  constant,  that  no  one, 
who  examined  the  affections  of  the  one  and  the  other 
with  attention,  could  have  any  doubts  on  the  subject. 
It  remained,  however,  for  Mr.  Dalton*  to  give  a  com- 
plete and  satisfactory  account  of  this  connexion,  and  it 
is  with  great  pleasure  I  take  this  opportunity  of  recom- 
mending his  work  to  your  attentive  perusal. 

From  various  observations  he  has  demonstrated,  1. 
When  the  aurora  appears  to  rise  only  about  5°  10',  or 
\5°  above  the  horizon,  the  needle  is  very  little  dis- 
turbed, and  often  insensible.  2.  When  it  rises  up  to 
the  zenith,  and  passes  it,  there  never  fails  to  be  a  con- 
siderable disturbance.  3.  This  disturbance  consists  in 
a  regular  oscillation  of  the  horizontal  needle,  sometimes 
to  the  eastward,  then  to  the  westward  of  the  mean  daily 
position,  in  such  sort,  that  the  greatest  excursions  on 
each  side  are  nearly  equal,  and  amount  at  Manchester 
to  about  half  a  degree  on  each  side.  4.  When  the 
aurora  ceases,  or  soon  after,  the  needle  returns  to  its 
former  station. 

From  these  facts  alone,  says  Mr.  Dalton,  indepen- 
dent of  other  observations,  we  cannot  avoid  inferring, 
that  there  is  something  magnetic  constantly  in  the  high- 
er regions  of  the  atmosphere,  that  has  a  share  at  least 
in  guiding  the  needle ;  and  that  the  fluctuations  of  the 
needle,  during  the  aurora,  are  occasioned  by  some 
mutations  that  then  take  place  in  this  magnetic  matter 
in  the  incumbent  atmosphere. 

OF    THE     SIMILARITY     BETWEEN    ELECTRICITY    AND 

MAGNETISM. 

The  powers  of  magnetism,  like  those  of  electricity, 
are  excited  and  separated  by  friction.  This  effect  is 
wonderful  in  both,  but  more  so  in  magnetism,  where 
two  powers,  naturally  attracting  each  other,  remain 
separated  in  the  steel  bar  for  many  years,  and  yet  they 


*  Meteorological  Observations  and  Essays,  by  John  Dalfcrn,  1~93. 


HYPOTHESIS.  403 

may  be  reduced  to  their  natural  state  by  the  friction  of 
two  other  magnets,  acting  in  contrary  order  to  that  by 
which  the  poles  were  originally  separated. 

Magnetism  and  electricity  act  powerfully  at  corners, 
edges,  and  points. 

Magnetism  may  be  communicated  to  a  small  steel 
needle,  by  passing  the  discharge  of  a  large  electrical 
battery  through  it. 

The  discharge  of  an  electrical  battery  through  a 
small  magnetic  needle  will  sometimes  destroy  the  mag- 
netism, and  sometimes  invert  the  poles  of  the  magnet. 
Similar  effects  have  been  produced  by  lightning. 


OF    THE    THEORY    OF    MAGNETISM. 

Here,  as  in  other  parts  of  natural  philosophy,  we 
must  content  ourselves  with  mere  conjecture.  Of  the 
various  hypotheses  that  have  been  formed  to  account 
mechanically  for  the  phenomena  of  magnetism,  that  of 
Mr.  Prevost*  is  undoubtedly  the  best ;  but  as  it  de- 
pends on  a  knowledge  of  Mr.  le  Sage's  mechanical  sys- 
tem of  the  universe,  it  will  be  impossible  for  me  to  lay 
it  before  you  in  a  satisfactory  manner ;  you  must  there- 
fore be  contented  with  a  very  imperfect  sketch  thereof. 


HYPOTHESIS. 

There  exists  in  and  about  our  globe  a  very  subtile 
fluid,  possessing  the  following  properties  : 

1 .  It  is  expansive,  and  consequently  discrete. 

2.  The  molecules  of  this  fluid  are  formed  by  the 
union  of  two  kinds  of  elements,  -A,  B,  united  by 
affinity. 

3.  The  elements  of  the  different  kinds  have  a  great- 
er tendency  to  each  other  than  those  of  the  same  kind. 

4.  That  excepting  the  preceding  property,  these  at- 
tractions follow  the  same  laws  as  universal  gravitation. 


*  Prevent  de  L'Origine  lies  Forces  Magnetiques  a  Geneve,  17&8. 


404  HYPOTHESIS. 

5.  This  fluid  has  an  affinity  with  the  particles  of  iron, 
and  which  probably  acts  only  at  contact,  or  when  near- 
ly in  contact.  This  fluid  is  decomposed  by  iron,  and 
seldom  by  any  thing  else. 

The  foregoing  properties  of  the  magnetic  fluid  may 
be  all  mechanically  explained  on  the  principles  of  Mr. 
le  Sage. 

To  explain  the  magnetic  phenomena  of  the  earth, 
it  is  sufficient  to  suppose  that  one  aliment  of  the  magne- 
tic fluid  is  furnished  by  nature  in  a  greater  abundance 
in  one  hemisphere  than  the  other ;  or  that  a  small  por- 
tion thereof  is  decomposed  by  some  of  the  causes  per- 
petually  acting  in  nature,  by  which  means  the  terres- 
trial globe  is  maintained  in  a  charged  state,  having  a 
greater  abundance  of  one  element  in  one  hemisphere 
than  in  the  other.  This  accumulation  may  principally 
exist  in  the  atmosphere. 

On  considering  that  the  aurora  borealis,  the  zodia- 
cal light,  electricity,  and  heat,  all  in  some  measure 
affect  the  magnetic  needle,  there  seems  ground  for 
supposing  that  one  or  other  of  the  elements  of  the 
magnetic  fluid  is  furnished  by  the  solar  rays. 

When  we  consider  the  extent  occupied  by  the  mag- 
netic fluid,  we  are  naturally  led  to  enquire  whether  its 
effluvia  course  incessantly  over  land  and  sea,  only  to  turn 
here  and  there  a  mariner's  compass  ?  Being  assured 
that  God  governs  by  a  long  subordination  of  second 
causes  ;  that  he  not  only  employs  a  concurrence  of 
causes  to  produce  one  effect,  but  likewise  produces  va- 
rious effects  from  one  and  the  same  cause  ;  we  may 
safely  answer,  that  there  are  other  uses  of  the  magne- 
tic effluvia,  besides  those  we  discern.  Here  again,  as 
in  every  other  part  of  philosophy,  we  have  a  further 
confirmation  of  the  littleness  of  human  knowledge, 
and  see  how  much  pains  God  has  taken,  so  to  speak, 
to  hide  pride  from  man. 


[     405     ] 


LECTURE    LI. 


ON      METEOROLOGY. 


I  HERE  is  scarce  any  subject  in  which  mankind 
feel  themselves  more  interested,  than  in  the  state  of 
the  weather,  that  is,  in  the  temperature  of  the  air,  the 
influences  of  wind,  rain,  &c.  It  forms  a  principal 
topic  of  common  conversation.  By  the  weather,  the 
traveller  endeavours  to  regulate  his  journies,  and  the 
farmer  his  operations  ;  by  it  plenty  and  famine  are  dis- 
pensed, and  millions  are  furnished  with  the  necessaries 
of  life.  It  is  intimately  connected  with  the  health  of  the 
human  body,  and  with  every  part  of  natural  history, 
and  more  particularly  with  agriculture.  You  will 
therefore  find  this  branch  of  philosophy  peculiarly  in- 
teresting ;  the  more  so  as  it  will  lead  you  to  consider 
the  great  operations  in  nature. 

"  Here  you  may  see  and  admire  the  changes  in  the 
elements,  which  present  us  with  all  that  is  great  and 
wonderful  in  nature,  and  which,  with  a  variety  little 
less  than  infinite,  work  together  for  the  good  of  man, 
and  the  preservation  of  the  world." 

I  have  long  since  observed  to  you,  how  improperly 
the  science  of  natural  philosophy  has  been  treated  by  its 
most  zealous  advocates  and  ablest  professors  ;  it  is  high 
time  for  them,  after  so  much  labour  in  vain,  to  return 
to  the  point  from  whence  they  should  have  set  out, 
and  now  begin  to  consider  the  great  agency  of  the  ele- 
ments. It  is  by  this  agency,  that  all  the  phenomena 
we  perceive  are  performed  ;  by  it  the  growth  of  plants, 
the  life  of  individuals,  are  supported  and  preserved  ; 
by  it  the  planets  are  maintained  in  their  respective  si- 
tuations, and  made  to  revolve  in  their  orbits. 


406  ON    METEOROLOGY, 

"  There  is  no  hope  in  the  present  mode  of  philoso- 
phizing, but  of  seeing  experiments  varied,  and  facts 
multiplied  ;  and  they  may  be  thus  multiplied  and  varied 
to  eternity  without  advancing  us  one  step  towards  a 
knowledge  of  the  causes  operating  in  nature.  The  in- 
defatigable  experimentalist  may  proceed  for  ever,  and 
flounder  like  the  mole  in  the  dust  he  raises  about  him- 
self ;  but  by  continually  heaping  up  of  facts,  or  mak- 
ing experiments,  he  will  never  be  able  to  trace  either 
the  nature  or  design  of  the  operations  carried  on  in  this 
system  of  things. "  For  the  universe  is  a  system,  in 
which  all  the  parts  are  connected  and  related,  and  mat- 
ter, as  a  part  of  the  created  world,  has  motion ;  but 
he  who  would  understand  the  nature  of  motion,  by 
considering  motion  abstractedly,  as  is  the  case  with 
many  modern  philosophers,  is  studying  motion  from 
that  which  has  no  motion  belonging  to  it.  There  are, 
as  I  have  before  observed  to  you,  no  insulated  facts  in 
nature  ;  they  are  all  systematic,  or  mechanical,  having 
a  double  reference  ;  as  effects  to  their  causes,  and  as 
causes  to  their  effects.  The  material  world  is  an  im- 
mense body,  composed,  like  our  own,  of  an  infinite 
number  of  parts,  so  interwoven  together,  as  to  unite 
in  one  common  centre.  It  is  the  business  of  philoso- 
phers to  point  out  these  connexions,  and  to  explain 
when  they  appear  to  us  as  separated,  and  thus  lead  us 
to  that  principle  of  unity  which  harmonizes  and  con- 
nects all  the  works  of  creation. 

But  alas,  you  find  the  philosopher  continually  losing 
sight  of  the  true  construction  of  nature,  and  endea- 
vouring to  build  systems  upon  matter  independently 
considered,  "  upon  which  he  can  only  raise  such  a 
world  as  never  did  nor  can  exist,  being  as  empty  and 
absurd,  as  it  is  arbitrary."  "  You  find  physicians 
treating  of  the  nature  and  causes  of  diseases,  of  ble- 
mishes, of  preternatural  appearances  in  the  body  ;  but 
wholly  indifferent,  and  altogether  inattentive  to  the 
proceedings  of  the  healthy  economy  :  you  will  find  an 
hundred  dissertations  on  fevers,  for  one  upon  life.  The 
action  of  stimuli,,  and  the  irritability  of  the  living  fibre, 
have  been  the  subjects  of  many  ingenious  discussions : 


ON    METEOROLOGY.  407 

the  regular  and  uniform  action  of  the  fibre,  but  of  few. 
It  is  the  same  with  philosophy  ;  we  have  treatises  on  light, 
as  separated  and  divided  by  the  prism  ;  on  heat,  as  mea- 
sured by  the  thermometer ;  but  none  on  that  ocean  of 
the  solar  fluid,  in  which  all  bodies  are  as  it  were  immers- 
ed -,  none  upon  the  various  influences  of  the  sun,  upon 
which  the  natural  life,  and  the  activity  of  all  things  in 
the  natural  world  depend.* 

If  we  look  into  artificial  nature,  we  shall  every  where 
find  a  want  of  known  agents.  Hence  the  variety  and 
changes  of  opinion  with  respect  to  a  great  number  of 
phenomena  that  are  observed  in  our  laboratories;  al- 
though we  can  there  multiply  and  vary  the  processes,  and 
thus  subject  our  conjectures  to  experiment ;  but  the  phe- 
nomena being  all  on  a  small  scale,  we  are  often  but  very 
little  struck  with  circumstances,  that  may  in  themselves 
be  very  important,  and  which  are  daily  perceived.  We 
are  diffident  of  the  exactness  of  our  measures  and  weights; 
we  suspect  some  foreign  influence  from  the  vessels  used, 
from  the  disparity  in  substances  of  the  same  kind  and 
name,  or  from  some  unknown  action  of  the  air  and  va- 
pour ;  and  yet,  unless  we  have  learned  not  to  be  satisfied 
with  vague  conjecture,  we  seldom  attend  to  the  notices 
which  result  from  the  imperfections  and  inaccuracies  of 
our  theories.  But  in  the  laboratory  of  the  atmosphere, 
all  the  phenomena  are  carried  on  upon  a  scale  propor- 
tioned to  their  importance  among  the  operations  of  na- 
ture, which  can  be  disturbed  by  nothing  foreign  to  these 
operations,  without  producing  some  characteristic  pheno- 
mena ;  every  thing  has  a  reference  to  the  vessel  itself,  u  e. 
to  the  surface  of  our  globe,  whose  distinct  parts,  as  mi- 
nerals, vegetables,  and  animals,  offer  masses  perpetually 
changing ;  here,  therefore,  the  disagreement  of  theory 
with  facts  must  give  us  great  and  important  lessons.f 


*  Young's  Essay  on  the  Powers  and  Mechanism  of  Nature.  Jones's 
Physiological  Disquisitions.     Adajns's  Dissertation  on  the  Barometer,  &x. 

t  Ue  Luc,  ldees  sur  la  Meteorologie,  a  work  that  should  be  fully  consi- 
dered by  all  who  mean  to  understand  the  subject,  and  to  which  I  am  indebt- 
ed for  a  great  part  of  the  Lectures  on  Meteorology. 


408  ON    METEOROLOGY. 

If  you,  however,  compare  attentively  meteorological 
phenomena  with  our  physical  measures,  the  barometer, 
the  thermometer,  hygrometer,  &c.  you  will  find  your- 
self unable  to  reduce  them  to  any  law,  that  can  be  ex- 
pressed by  the  range  of  these  instruments  :  which  shows 
more  evidently  than  any  thing  that  can  be  seen  in  our 
laboratories,  the  necessity  of  admitting  other  combina- 
tions than  those  that  are  known,  and  even  perhaps  other 
ingredients. 

The  meteorological  phenomena,  whose  causes  we  have 
yet  to  explore,  are  those  that  are  most  common  and  the 
most  important  to  terraqueous  physics.  They  are  chan- 
ges of  heat  independent  of  seasons  and  latitude,  those  of 
winds,  and  the  variations  in  the  heights  of  a  local  baro- 
meter ;  the  vicissitudes  of  rain  and  fair  weather  ;  aerial 
electricity  and  magnetism  ;  the  relations  of  the  state  of 
the  air  to  our  sensations ;  the  small  connexion  we  find 
between  vegetation  (as  well  in  general,  as  for  certain  par- 
ticular products),  and  the  different  remarkable  charac- 
ters of  the  seasons.  All  these  grand  lines  in  the  opera- 
tions of  nature  on  our  globe  are  to  us,  with  respect  to 
the  producting  causes,  as  a  sealed  book. 

These  observations  on  the  chemistry  of  nature  are  by 
no  means  designed  to  lessen  your  esteem  for  that  of  our 
laboratories,  which  is  certainly  one  of  the  principal  sour- 
ces of  all  the  true  science  we  possess.  They  are  designed 
to  show  you,  that  the  smallest  meteorological  phenomena 
should  be  studied  with  as  scrupulous  an  attention,  as  that 
we  pay  to  the  modifications  of  a  little  air  in  our  close  ves- 
sels ;  for  it  is  these  phenomena  which  should  guide  us 
in  our  researches  into  the  nature  of  the  latter. 

In  the  atmosphere  every  cause  produces  its  effects  :  the 
subtile  fluids  are  there  distributed  according  to  their  na- 
tural tendencies ;  and  can  form  or  destroy  themselves 
differently,  in  different  times,  different  soils,  different 
climates :  the  winds,  which  have  seldom  been  consider- 
ed, but  as  more  or  less  violent,  warm,  or  humid,  may, 
with  respect  to  these  fluids,  answer  more  important  pur- 
poses than  what  have  yet  been  attributed  to  them.  In  a 
word,  great  general  causes  act  in  the  atmosphere  from 
which  our  confined  air  is  secluded.    It  is  therefore  only 


ON    METEOROLOGY.  409 

by  letting  meteorology  and  chemistry  go  hand  in  hand, 
that  we  can  hope  to  be  secure  from  error  in  either  pur- 
suit, or  be  enabled  to  make  advances  towards  real  know- 
ledge. 

Our  researches  into  the  nature  of  these  interesting  ob- 
jects will  be  ever  vague,  and  of  course  attended  with  but 
little  success,  till  we  have  some  certain  theory  on  the  na- 
ture of  expansible  fluids,  and  in  general  of  all  physical 
agents.  Here  it  may  be  proper  to  observe,  that  we  com- 
monly join  to  physical  laws  the  idea  of  their  being  an  ex- 
planation of  the  phenomena ;  but  this  notion  is  errone- 
ous, for  all  physical  laws,  not  excepting  even  those  of 
gravity,  are  only  generalisations.  We  are  now  acquaint- 
ed with  many  phenomena,  where  the  real  agents  mani- 
fest themselves,  and  whose  laws  flow  from  the  nature  of 
these  agents :  it  is  to  such  as  these  only  that  the  idea  of 
a  physical  explanation  can  be  attached.  Thus,  to  assign 
to  phenomena  a  real  and  determined  agent,  acting  in  a 
certain  manner,  and  from  which  certain  effects  are  the 
natural  consequences;  and  to  show  that  the  laws  of  these 
phenomena  correspond  thereto,  is  really  to  explain  the 
phenomena.  Now  we  know  phenomena  enough,  thus 
connected  with  real  causes,  to  enable  us  by  analogy  to  ex- 
tend very  far  the  real  links  that  connect  effects  one  with 
another  in  nature ;  remembering  always  to  consider  true 
agents  as  being  more  and  more  general,  the  further  they 
are  removed  from  us ;  or  subordinate,  as  they  approach 
us.  These  are  the  proper  objects  to  occupy  the  attention 
of  a  rational  philosopher,  as  they  alone  can  form  the 
foundation  of  rational  physics,  one  that  will  give  us  effi- 
cacious aid  in  our  experimental  researches.  What  wrould 
become  of  practical  mechanics,  if  in  elementary  mecha- 
nics there  were  no  system  of  its  agents,  no  laws  previously 
determined,  which  they  follow  when  they  operate  ?  And 
yet,  though  all  is  action  in  nature,  philosophers  scarce 
think  of  looking  for  the  true  agents. 

Among  the  means  of  advancing  our  knowledge  in  me- 
teorology, are  the  instruments  that  have  been  contrived 
to  ascertain  the  variations  in  the  weight  of  the  atmos- 
phere ;  its  changes  with  respect  to  temperature  ;  the  de- 

koL.  TV,  3G 


410  ON    METEOROLOGY. 

grees  of  humidity,  &c.  If  every  one  who  is  in  posses- 
sion of  meteorological  instruments  would  keep  a  diurnal 
register  of  their  state,  and  of  the  corresponding  pheno- 
mena of  the  atmosphere,  and  transmit  the  result  of  his 
observations  to  the  public,  he  would  contribute  more  to 
the  advancement  of  this  branch  of  science  than  he  might 
at  first  imagine.  While  he  was  amusing  himself,  and 
gratifying  curiosity,  he  would  be  promoting  knowledge, 
and  probably  procuring  benefits  for  posterity.  Let  no  one 
suffer  the  apparent  improbability  of  success  to  discourage 
him  from  the  attempt.  Remember  that  science  advances 
by  slow  and  gradual  steps,  that  its  progress  depends  on 
the  cultivation  of  the  mind,  the  acquisition  of  facts,  the 
removal  of  obstacles,  and  the  exertion  of  individuals :  the 
present  is  ever  pregnant  with  the  future,  though  the  con- 
nexion between  them  can  only  be  found  by  long  atten- 
tion and  diligent  observation.  A  register  for  preserving 
what  no  memory  can  retain,  becomes  an  authentic  do- 
cument, a  reference  from  which  facts  may  be  combined 
and  compared,  and  thus  one  of  the  purest  sources  of 
practical  knowledge.  To  indulge  ends  so  rational  to  as 
great  an  extent  as  the  human  powers  can  reach,  and  with 
as  much  enjoyment  as  the  human  mind  can  bear,  Divine 
Providence  hath  appointed  the  means  whereby  each  man's 
small  stock  of  knowledge  and  truth  may  be  communi- 
cated to  others  without  loss  to  himself;  and  further, 
how  it  may  be  placed  in  a  common  treasury  for  even" 
man  to  draw  from  thence  whatever  his  occasions  or  in- 
clinations may  require.  These  ends  are  known  to  be  ac- 
complished, the  one  by  speech,' the  other  by  writing  and 
publishing  what  is  written. 

It  may  be  observed  in  general,  that  meteorological  phe- 
nomena, like  all  the  durable  motions  of  the  universe,  de- 
pend upon  a  circulation  of  matter.  Here  it  is  principally 
carried  on  by  changing  of  water  into  a  new  form,  and  a 
regeneration  of  it  again  into  its  primitive  form.  It  goes  off 
from  the  surface  of  the  earth  in  the  form  of  a  rare  invi- 
sible expanded  vapour ;  in  the  atmosphere,  its  state  is 
changed,  from  that  of  vapour  to  an  aeriform  fluid ;  by 
some  unknown  cause  it  is  again  changed  into  mists  and 
clouds,  it  is  then  gathered  into  drops  when  it  fails, 


OF    THE    BAROMETER.  41  i 

and  in  this  form  it  returns  to  the  place  from  whence  it 
came,  to  take  its  turn  once  more  in  the  common  course 
of  evaporation,  and  be  again  and  again  circulated  to  the 
great  promptuary  of  the  world. 

The  principal  objects  therefore  of  inquiry  are,  1.  In 
what  manner  the  atmosphere  is  supplied  with  humidity  ? 
2.  What  causes,  and  what  prevents,  invisible  humidity 
being  formed  into  clouds  ?  And,  3.  What  occasions,  and 
what  prevents,  visible  clouds  being  precipitated  into  rain  ? 
That  is,  to  know  the  various  balancings  of  the  clouds, 
and  learn  how  such  ponderous  materials  are  suspended  in 
the  air ;  and  how  the  waters  are  bound  up  in  the  thick 
clouds. 

As  the  principal  means  of  answering  these  inquiries 
are  the  instruments  used  to  discover  the  changes  in  the 
atmosphere,  I  shall  first  describe  these,  and  then  proceed 
to  give  you  some  account  of  meteorological  phenomena. 
The  instruments  in  general  use  are,  1 .  A  barometer ; 
2.  A  thermometer  ;  3.  A  hygrometer  -,  4.  Arain-gage ; 
.5.  An  electrometer. 


OF    THE    BAROMETER. 

In  treating  of  the  barometer,  I  shall  have  only  to  en- 
large on  some  circumstances  that  were  but  slightly  noti- 
ced, when  I  explained  this  instrument  to  you  in  my  third 
lecture.  The  barometer,  as  I  there  showed  you,  consists 
of  a  straight  glass  tube,  about  thirty-two  inches  long, 
open  at  one  end  and  close  at  the  other  ;  this  tube  is  first 
filled  with  mercury,  and  then  inverted  in  a  bason  of  the 
same  fluid,  by  applying  a  finger  to  the  open  end,  so  as  to 
prevent  any  air  from  entering  the  tube ;  when  the  open 
end  of  the  tube  is  immerged  beneath  the  surface  of  the 
mercury  in  the  bason,  the  finger  is  withdrawn  ;  the  tube 
being  now  set  upright,  the  mercury  descends,  leaving  the 
top  of  the  tube,  and  subsiding  till  it  has  attained  a  certain 
distance  from  the  surface  of  that  in  the  bason;  this  is  more 
or  less,  according  to  the  state  of  the  air  at  the  time  of  mak- 
ing the  experiment.     The  tube  is  then  fixed  to  a  frame 


412  OF    THE    BAROMETER. 

with  a  scale  annexed,  to  show  at  all  times  the  height  of 
the  mercurial  column. 

From  the  method  of  filling  the  tube,  the  air  is  excluded 
from  the  top  of  the  column,  or  that  part  of  the  tube  above 
the  surface  of  the  mercury  in  the  tube.  The  external  air 
presses  upon  the  quicksilver  in  the  cistern,  and  sustains, 
by  its  pressure  in  a  contrary  direction,  a  column  equal  in 
weight  to  itself.  For  it  is  evident,  that  the  mercury  endea- 
vours to  descend  with  a  force  equal  to  that  by  which  its 
descent  is  prevented.  In  other  words,  the  pressure  of  the 
atmosphere  on  a  given  surface  is  equal  to  the  weight  of  a 
column  of  mercury,  whose  base  is  the  given  surface,  and 
height  equal  to  that  of  the  atmosphere.  The  height  of  the 
mercury  is  therefore  an  adequate  measure  of  the  weight 
or  pressure  of  the  air  upon  a  surface  equal  to  the  base  of 
the  tube  containing  the  mercury. 

The  truth  of  this  reasoning  1  have  confirmed  by  seve- 
ral experiments,  exhibited  in  my  third  lecture;  you  there 
saw,  when  a  baromete/  was  placed  under  a  receiver,  that 
in  proportion  as  the  air  was  exhausted  from  the  surface  of 
the  mercury  in  the  bason,  the  column  in  the  tube  descend- 
ed  till  at  last  they  were  nearly  on  the  same  level.  The  ba- 
rometer-gage exhibited  the  reverse  of  this  experiment,  for 
you  there  saw  the  mercury  rise  in  the  tube  in  proportion 
as  the  air  was  exhausted  therefrom,  the  open  air  pressing 
at  the  same  time  upon  the  surface  of  the  mercury  in  the 
baj.on.  What  is  thus  exhibited  by  our  instruments  is  con- 
firmed by  nature,  for  the  higher  we  ascend  in  the  atmos- 
phere the  shorter  is  the  column  of  mercury. 

The  barometer  with  a  straight  tube,  as  originally  con- 
trived by  Torricellius,  is  preferable  to  all  the  subsequent 
variations  in  its  form.  When  it  was  found  that  the  differ- 
ent heights  of  the  mercury  in  the  barometer  were  in  some 
measure  connected  with  the  state  of  the  weather,  the  phi- 
losopher and  artist  endeavoured  to  vary  the  form  of  the 
instrument ;  hence  a  variety  of  constructions  more  or  less 
accurate,  according  to  the  views  of  the  inventors,  and  the 
distance  to  which  they  removed  the  barometer  from  its 
original  simplicity. 

They  thought,  that  by  augmenting  the  scale  of  the  ba- 
rometer, and  thus  rendering  the  variations  of  the  mercury 


OF    THE    BAROMETER.  413 

more  sensible,  they  should  sooner  discover  the  minute 
changes  of  the  atmosphere,  and  the  causes  which  occa- 
sioned them  :  unfortunately,  by  altering  the  construction 
of  this  instrument,  they  only  multiplied  errors,  and  ren- 
dered it  less  capable  of  answering  the  purposes  for  which 
it  was  designed.  Some  of  these  forms  may  appear  more 
elegant  than  the  plain  barometer,  but  none  of  them  can 
be  depended  upon  for  keeping  an  accurate  register  of  the 
weather,  or  for  observing  the  extent  of  the  variation  thereof 
in  any  given  situation,  or  comparing  the  different  changes 
at  one  place  with  corresponding  ones  at  another.  Hence 
it  is  necessary  to  point  out  to  you  what  is  requisite  to  con- 
stitute a  good  barometer. 


SOME    OF    THE    PRINCIPAL    REQUISITES    OF    A    GOOD 
BAROMETER.* 

1.  It  is  requisite  that  the  height  of  the  column  of  mer- 
cury be  altered  by  no  other  causes  but  the  changes  that 
arise  from  the  pressure  of  the  air  ;  and  that  these  changes 
be  truly  indicated. 

2.  That  the  variations  in  the  height  of  the  column  be 
ascertained  by  a  known  measure. 

3.  That  the  column  of  mercury  be  susceptible  of  the 
smallest  alterations  in  the  weight  of  the  air. 

In  order  that  the  column  of  mercury  in  the  tube  may 
be  affected  by  no  other  cause  than  the  pressure  of  the 
lir,  it  is  absolutely  requisite  that  the  upper  part  of  the 
:ube,  and  the  mercury  itself,  be  entirely  free  from  air ; 
For,  if  there  be  any  air  between  the  upper  surface  of 
the  column  of  mercury  and  the  sealed  end  of  the  tube, 
it  will  be  the  source  of  many  errors  and  much  irregularity. 
Ihe  included  air  will  act  as  a  counterpoise  against  the 
weight  of  the  atmosphere,  and  to  a  certain  degree  coun- 
:eract  its  pressure ;  and,  therefore,  render  the  indication 
^f  the  instrument  uncertain  and  erroneous.  This  included 
lir,  being  also  often  combined  with  humidity,  expanded 


1  ^ee  my  Dissertation  on  the  Earometfp  and  Thermometer,  ir96* 


414  TO    BOIL    THE    QUICKSILVER 

by  heat  or  contracted  by  cold,  acts  differently  at  differen 
times  :  the  only  method  of  preventing  these  errors,  aru 
perfectly  excluding  the  air  from  the  barometer,  is  by  boil 
ing  the  mercury  in  the  tube ;  an  operation  which  is  care 
fully  performed  in  all  the  best  instruments. 


TO    BOIL    THE    QUICKSILVER    IN    A    BAROMETER 
TUBE.* 

Choose  a  tube  not  less  than  three  feet  long  with  a  bon 
about  three  or  four  twentieths  of  an  inch  in  diameter,  bu 
not  more  ;  and  that  it  may  be  sufficiently  strong,  it  shoulc 
be  nearly  as  thick  on  all  sides  as  the  diameter  of  the  bore 
Let  this  tube  be  nicely  sealed  or  closed  at  one  end,  anc 
as  clean  as  possible ;  fill  the  tube  with  pure  mercury  tc 
within  two  inches  of  the  top,  and  then  hold  it  with  the 
sealed  end  lowest  in  an  inclined  position  over  a  chafing- 
dish  of  burning  charcoal,  placed  near  the  edge  of  a  table, 
in  order  that  all  parts  of  the  tube  may  be  exposed  suc- 
cessively to  the  action  of  the  fire,  by  moving  it  obliquel) 
over  the  chafing-dish.     The  sealed  end  is  to  be  first  gra- 
dually presented  to  the  fire  ;  as  soon  as  the  mercury  be- 
comes hot,  the  internal  surface  of  the  tube  will  be  studded 
with  an  infinite  number  of  air-bubbles,  giving  the  mer- 
cury a  kind  of  grey  colour  ;  these  increase  in  size  by  rui 
ning  into  one  another,  and  ascend  towards  the  high( 
parts  of  the  tube,  where  meeting  with  a  cooler  part 
the  fluid,  they  are  condensed,  and  nearly  disappear.    In 
consequence,  however,  of  successive  emigrations  towards 
the  upper  parts  of  the  tube  which  are  successively  heated, 
they  finally  acquire  a  bulk  which  enables  them  in  their 
united  form  entirely  to  escape.     When  the  first  part  ol 
the  tube  is  sufficiently  boiled,  move  it  onward  by  little 
and  little  through  the  whole  length  of  the  tube.    When 
the  mercury  boils,  its  parts  strike  against  each  other,  and 
against  the  sides  of  the  tube  with  such  violence,  that  a 
person  unacquainted  with  the  operation  naturally  appre- 
hends the  destruction  of  his  tube. 


*  Dc  Lu:'s  Rec'ierches  sur  les  Modifications  de  FAtmcsphere. 


IN    A    BAROMETER    TUBE.  415 

The  great  advantages  that  result  from  this  operation 
ippear  to  be  these  :  the  whole  body  of  the  mercury,  and 
he  interior  surface  of  the  tube  are  hereby  freed  from  all 
he  minute  and  imperceptible  particles  of  dust  and  mois- 
ure  which  they  generally  contain,  and  of  the  little  at- 
nospheres  that  are  seen  to  surround  them ;  which,  du- 
ing  the  tumultuous  motions  of  the  mercury,  are  visibly 
Iriven  up  towards  its  surface,  and  expelled.  The  tube  and 
he  mercury  are  deprived  likewise  of  all  the  air  that  can 
>e  expelled  from  them,  and  particularly  from  the  surface 
>f  the  former,  by  the  violent  heat  and  agitation  of  boiling 
[uicksilver.  As  that  heat  too  is  a  determinate  or  fixed 
[uantity,  its  effects  in  expelling  the  air  from  different  tubes 
vill  be  nearly  equal  ;  so  that,  though  some  small  portion 
)f  air  may  still  be  left  in  them,  there  can  be  no  difference 
n  the  quantity  of  it  remaining  in  different  tubes  thus  uni- 
brmly  treated.  Accordingly,  the  barometers  thus  pre- 
)ared  not  only  stand  higher  than  those  which  have  not 
mdergone  this  process ;  but  at  the  same  time  they  pretty 
iccurately  correspond  with  each  other. 

M.  de  Luc  observes,  that  the  greatest  part  of  the  air 
vhich  is  expelled  during  the  process,  proceeds  from  the 
nternal  surface  of  the  tube,  where  it  seems  to  have  form- 
ed a  thin  stratum  or  lining  of  air,  which  cannot  be  dis- 
odged  from  thence  by  the  mercury  introduced  into  the 
ube  in  the  common  manner,  but  requires  the  violent  heat 
>nd  agitation  of  boiling  quicksilver  to  detach  it.  But  it  is 
'ery  remarkable,  that,  after  this  aerial  coating  has  been 
>nce  effectually  separated  and  expelled,  if  the  tube  be 
mptied  and  some  other,  even  cold  mercury,  be  intro- 
lucedinto  it,  the  barometer  thus  extemporaneously  made, 
vill  be  nearly  as  perfect  or  as  free  from  air  as  before.  It 
vill  stand  nearly  as  high  as  it  did  when  it  contained  the 
tiercury  that  had  been  boiled  in  it ;  if  the  same  process  be 
iow  repeated,  it  will  not  be  studded  with  bubbles  of  air, 
s  in  the  former  operation  ;  when  the  mercury  has  been 
ompletely  boiled,  the  tubes  may  be  cut  off  to  their  proper 
?ngth  by  a  file. 

When  the  tubes  are  well  boiled,  the  mercury  generally 
emains  suspended  at  top,  mi  will  not  descend  to  its  pro- 
er  level  without  shaking  the  tube  to  bring  it  down. 


416  TO    BOIL    THE    QUICKSILVER 

That  the  changes  in  the  height  of  the  mercurial  column 
may  be  truly  ascertained,  it  is  necessary  to  know  at  all 
times  the  exact  distance  of  the  surface  of  the  mercury  in 
the  tube  from  the  surface  in  the  reservoir  or  cistern. 

The  first  point  of  the  measure  must  commence  from 
the  surface  of  the  mercury  in  the  cistern  ;  but  this  sur- 
face is  variable  ;  for,  when  the  mercury  descends,  a  quan- 
tity of  it  falls  into  the  bason,  and  raises  the  surface  there- 
of, and  on  the  contrary,  when  it  rises,  a  quantity  is  taken 
out  of  the  cistern,  and  the  surface  thereof  is  lowered.  The 
scale  of  inches  to  the  barometer  is  fixed  ;  but  the  surface 
of  the  mercury  in  the  cistern  from  which  it  originates  is 
continually  varying.  To  remedy  this  evil,  it  is  necessary 
that  the  lower  surface  should  be  always  kept  at  the  same 
height  from  divisions  on  the  scale  affixed  to  the  insrru 
ment.  This  is  effected  by  means  of  a  floating  gage,  which 
was  first  applied  to  the  barometer  by  my  Father,  though 
others  have,  since  his  time,  assumed  the  merit  to  them- 
selves. By  means  of  the  floating  gage,  the  same  screw- 
that  renders  the  barometer  portable,  regulates  the  sur- 
face of  the  mercury  in  the  cistern,  so  that  it  is  always  at 
the  place  from  whence  the  divisions  on  the  scale  com- 
mence. This  gage  is  never  applied  to  the  common  port- 
able barometers,  but  only  to  those  of  the  best  kind. 

Another  circumstance  necessary  to  be  attended  to  in 
very  accurate  observations,  is  the  effect  of  heat  and  cold 
on  the  barometer,  as  by  these  the  mercurial  column  is 
either  dilated  or  contracted ;  for,  as  all  bodies  expand  and 
occupy  larger  spaces  when  their  temperature  is  increased, 
the  mercury  in  the  barometer  will,  when  heated,  be  spe- 
cifically lighter,  and  will  consequently  ascend  from 
cause,  though  the  pressure  of  the  air  should  remain  un- 
changed ;  and  therefore,  in  order  to  know  accurate])'  tfi< 
effect  of  the  air's  pressure  on  the  barometer,  it  is  ne 
sary  to  correct  the  height  by  the  addition  or  subtraction 
of  a  quantity  equal  to  the  influence  of  the  temperatui 
the  air  thereon.    In  some  cases,  a  scale  of  correction  i^ 
applied  to  the  thermometer  accompanying  the  barometer, 
and  which  is  indeed  a  necessary  companion  to  it. 

2d.  Condition.  That  the  scale  should  be  of  some  knowr 
measure.  It  would  have  been  totally  unnecessary  to  hav; 


OP    THE    NONIUS.  417 

mentioned  this  condition,  had  it  not  been  to  prevent 
you  from  being  imposed  upon  by  venders  of  imperfect 
instruments.  Some  of  these  instruments  have  no  de- 
terminate scale  affixed  to  them ;  and  those  which  have 
a  scale,  have  one  that  is  in  general  ill-graduated  ind 
erroneously  placed,  so  that  no  comparative  observa- 
tions can  be  made  with  them  ;  and  often,  indeed,  no 
observation  at  all ;  as,  from  the  small  bore  of  the  tube, 
they  act  as  a  thermometer,  as  well  as  a  barometer.  I 
have  already  observed,  that  by  enlarging  the  scale,  er- 
ror is  multiplied,  and  uncertainty  produced. 

3.  Condition.  That  the  smallest  changes  in  the 
height  of  the  column  of  mercury  may  be  discerned. 

To  measure  the  smallest  changes,  a  nonius  division 
moves  with  the  index,  by  which  each  inch  is  subdivided 
into  100  parts,  and  the  height  of  the  mercury  is  accu- 
rately obtained  without  any  danger  from  parallax,  by 
the  peculiar  construction  of  the  index. 


OF    THE    NONIUS. 

The  scale  of  inches  is  affixed  to  the  right  side  of  the 
tube,  the  zero  or  beginning  of  the  scale  being  at  the 
surface  of  the  mercury  in  the  cistern,  the  index  and 
its  nonius  plate  slide  up  and  down  in  a  groove,  which 
is  parallel  to  the  line  of  inches,  that  the  index  may  be 
set  at  any  time  to  the  upper  surface  of  a  column  of 
mercury. 

Each  inch,  or  line  of  inches,  is  divided  into  ten 
parts,  which  are  again  subdivided  into  t^n,  by  means 
of  the  nonius  scale  ;  the  whole  inch  being  thereby  di- 
vided into  100  equal  parts. 


TO  READ  OFF,    OR  ESTIMATE  THE    DIVISIONS    OF    THE 
NONIUS    SCALE. 


I.,  If  that  edge  of  the  nonius  scale,  which  is  in  a  line 
with  the  index,  coincide  exactly  with  any  division  on 
vol.  IV.  s  a 


418        THE    COMMON    PORTABLE    BAROMETER, 


the  line  of  inches,  that  division  expresses  the  height 
of  the  index  from  the  surface  of  the  mercury  in  the  cis- 
tern in  inches  and  tenths  of  inches.  But,  2dly,  When 
the  foregoing  edge  does  not  coincide  with  any  division, 
you  must  look  for  that  division  of  the  nonius,  which 
coincides  with  a  division  in  the  line  of  inches,  and  the 
number  on  the  nonius  shows  how  many  tenth  parts  of 
the  ten  hundredth  part  the  index  or  edge  has  passed  the 
last  decimal  division.  Thus  for  example,  suppose  the 
edge  of  the  nonius  was  to  point  somewhere  between  29 
inches  8  tenths,  and  29  inches  9  tenths ;  then  if  by 
looking  at  the  nonius,  you  observe  the  coincidence  at 
J,  it  shows  the  altitude  to  be  29  inches  8  tenths,  and 
/>  parts  of  another  tenth,  or  29.85. 


OF    THE    COMMON    PORTABLE    BAROMETER. 


This  instrument,  when  made  with  care,  will  answer 
for  general  and  domestic  observation,  but  is  not  suffi- 
ciently accurate  for  philosophical  purposes.  It  consists 
of  a  tube  of  a  proper  length  accurately  filled  w7ith  mer- 
cury ;  the  lower  end  of  the  tube  is  glued  to  a  wooden 
reservoir,  the  bottom  of  which  is  formed  of  leather; 
into  this  reservoir  the  superfluous  mercury  descends, 
and  the  air,  by  pressing  upon  the  flexible  leather  at  the 
bottom  of  the  reservoir,  keeps  the  mercury  suspended 
at  its  proper  height.  This  reservoir  is  concealed  from 
the  eye  by  a  neat  mahogany  cover  or  box.  This  tube 
and  reservoir  are  placed  in  a  frame,  on  the  upper  part 
of  which  is  a  silvered  brass  plate ;  on  the  right  hand 
side  of  this  plate  is  a  scale  of  inches,  reckoned  from 
the  surface  of  the  mercury  in  the  cistern ;  each  inch 
is  divided  into  ten  parts.  Close  to  the  line  of  inches 
there  is  a  slit  or  groove  for  conveniently  sliding  the  no- 
nius  scale  and  index  up  and  down.  The  upper  edge 
of  the  index  and  nonius  scale  are  in  a  line.  It  is  the 
upper  edge  of  the  index  that  is  to  be  set  to  the  uppei 
surface  of  the  mercury.  On  the  left  hand  side  of  the 
plate  the  words  fair,  changeable,  rain,  are  en- 
graved.    At  the  bottom  of  the  frame  there  is  a  screw 


THE    BEST    PORTABLE    BAROMETER.  419 

passing  through  the  mahogany  box  which  covers  the 
reservoir  :  a  flat  round  plate  is  placed  upon  the  end  of 
the  screw  within  the  box  ;  this  end  is  designed  to 
press  upon  the  leather  bag,  and  force  the  mercury  up 
to  the  top  of  the  tube,  and  thus  prevent  it  from  shak- 
ing, or  violently  striking  against  the  top  of  the  tube 
when  transported  from  one  place  to  another. 

TO    USE    THE    PORTABLE    BAROMETER.       \ 

1.  Suspend  it  against  a  wall  or  wainscot,  so  that  the 
tube  may  be  perpendicular  to  the  horizon. 

2.  Unscrew  the  screw  at  the  bottom  of  the  frame  as 
low  as  it  will  go,  and  the  mercury  will  fall  to  its  proper 
height,  and  be  obedient  to  the  changes  in  the  weight 
of  the  air. 

3.  Set  the  upper  edge  of  the  index  so  as  to  coincide 
with  the  surface  of  the  mercury  in  the  tube,  and  the 
nonius  scale  will  point  out  the  height  of  the  column. 

4.  Before  every  observation,  strike  the  frame  gently 
with  the  knuckles  to  disengage  the  quicksilver  from 
the  tube. 

5.  When  the  barometer  is  to  be  moved  from  one 
place  to  another,  turn  the  screw  till  the  mercury  is 
pressed  by  it  against  the  top  of  the  tube. 

DEFECTS    OF    THE    COMMON    PORTABLE    BAROMETER. 

It  is  necessary  here  just  to  mention  some  of  the  de- 
fects of  this  kind  of  barometer,  in  order  to  render  the 
advantages  of  the  better  kind  more  conspicuous. 

1.  It  cannot  be  so  adjusted,  as  ro  be  sure  that  the 
divisions  on  the  scale  are  at  that  height  from  the  mer- 
cury in  the  cistern,  which  is  expressed  by  the  numbers 
affixed  to  them.  As  when  the  mercury  falls  in  the 
tube,  it  rises  in  the  reservoir ;  and  when  it  rises  in  the 
tube,  it  falls  in  the  reservoir  ;  its  distance  is  perpetual- 
ly varying  from  the  divisions  of  the  scale. 

2.  The  tension  of  the  leather  forms  a  considerable 
resistance  to  the  pressure  of  the  atmosphere. 


[    42°    J 

OF    THE    BEST    PORTABLE    BAROMETER. 


I 


This  barometer,  like  the  preceding,  consists  of  a 
glass  tube  properly  filled  with  mercury,  having  the  low- 
er  end  fixed  to  a  wooden  cistern  with  a  leather  bottom, 
and  this  tube  and  cistern  placed  in  a  mahogany  frame 

On  the  upper  part  of  the  frame  a  brass  plate  is  plac- 
ed ;  on  the  right  hand  side  of  the  tube  a  scale  of  inches 
is  graduated  on  the  plate,  the  beginning  of  the  scale 
being  at  the  surface  of  the  mercury  in  the  cistern  :  each 
inch  is  divided  into  ten  parts,  which  are  again  subdi- 
vided into  tenths  by  the  nonius  scale. 

The  nonius  plate  carries  two  indexes  exactly  similar 
to  ach  other,  one  placed  before  the  tube,  the  other 
behind  it.  The  indexes  may  be  raised  or  depressed  by 
turning  the  key,  which  fits  into  a  small  hole  in  the 
frame,  directly  under  the  groove  of  the  nonius  plate. 

On  the  left  hand  of  the  tube  a  small  thermometer 
is  placed,  with  Fahrenheit's  scale  ;  there  is  an  index  to 
the  thermometer,  which  may  be  set  by  the  same  key  as 
the  barometer,  only  putting  it  into  the  small  hole  un- 
der the  thermometer,  and  turning  it  round  till  the  in- 
dex points  to  the  mercury  in  the  thermometer.  A 
scale  for  correcting  the  expansion  of  the  mercury  in  the 
barometer  is  often  graduated  close  to  the  scale  of  the 
thermometer. 

The  upper  part  of  the  barometer  is  covered  with  a 
glass  piate,  to  prevent  the  silvering  of  the  plate  from 
being  injured  b\  dirt,  or  being  corroded  by  the  action 
of  the  air. 

OF  THE  LOWER  PART  OF  THE  BAROMETER. 

The  lower  end  of  the  tube  is  immersed  in  th< 
tern  which  contains  the  mercury  ;  the  cistern  is  cover- 
ed with  a  mahogany  box  ;  at  the  bottom  of  the  frame 
is  a  screw,  to  raise  or  lower  the  surface  of  the  mercun  ; 
at  the  top  of  the  cistern  is  a  hole,  which  is  fitted  with 
an  ivory  screw,  to  be  placed  there  occasionally  for  the 
conveniency  of  transporting  the  instrument  safely  from 
one  place  to  another.     . 


OF    THE    SCALE    OF    CORRFCTION.  421 

The  gag°  consists  of  a  small  stem  of  ivory,  arising 
from  a  float  of  the  same  substance ;  a  circular  division 
is  cut  round  this  stem  ;  the  stem  passes  through  a  short 
cylinder  of  ivory,  which  is  cut  open  in  front ;  on  this 
front  two  small  divisions  are  cut :  at  the  bottom  of  this 
c)  linder  is  a  male  screw,  to  fit  the  female  screw  of  the 
cistern  ;  the  upper  part  of  the  gage  is  protected  by  a 
tube  of  glass  perforated  at  top. 

TO    USE    THIS  BAROMETER. 

1 .  The  barometer  being  fixed  in  a  perpendicular  posi- 
tion, unscrew  the  screw  at  bottom  as  far  as  it  will  go 
without  forcing  it. 

2.  Take  out  the  ivory  screw  at  the  top  of  the  cistern, 
and  place  it  between  the  scroles  on  the  upper  part  of 
the  frame. 

3.  Screw  the  gage  into  the  place  from  whence  the 
ivory  screw  was  taken. 

4.  Screw  up  that  screw  which  is  at  the  bottom  of 
the  frame,  until  the  line  on  the  float  exactly  coincide 
with  the  two  lines  on  the  front  of  the  ivory  cylinder. 

5.  btrike  the  barometer  gently  with  the  knuckles, 
and  then  so  set  the  lower  edge  of  the  front  index  to  the 
convex  surface  of  the  mercury,  that  it  may  be  at  the 
same  time  in  a  line  with  the  edge  of  the  index  behind 
the  tube  ;  and  the  nonius  will  then  give  the  true  height 
of  the  mercurial  column,  from  the  surface  of  the  mer- 
cury in  the  cistern. 

6.  The  preceding  rule  for  setting  the  gage  must  be 
complied  with  previous  to  every  observation. 

7.  When  the  barometer  is  to  be  transported  from 
one  place  to  another,  the  gage  must  be  removed,  and 
the  solid  ivory  screw  inserted  in  its  place  ;  after  w7hich, 
the  mercury  in  the  tube  may  be  forced  gently  up  to 
the  top  thereof,  by  the  screw  at  the  bottom  of  the  frame. 

OF    THE    SCALE    OF    CORRECTION. 


1  his  scale  is  placed  close  to  that  of  the  thermometer  ; 
but  on  the  right-hand  side,  the  zero,  or  O  degree  of 


422  OF    THE    THERMOMETER. 


this  scale,  corresponds  to  the  55th  degree  of  the  ther- 
mometer. 

1.  If  the  barometer  be  at  30  inches,  and  the  ther- 
mometer at  55  degrees,  no  correction  is  necessary. 

2.  But  if  the  thermometer  be  under  55^  and  the  ba- 
rometer at  30  inches,  you  must  add  to  the  height  of 
the  barometer  as  many  of  the  lOOths  of  inches  as  are 
on  the  scale  of  correction  opposite  to  the  degree  of  the 
thermometer. 

3.  If  the  thermometer  be  above  559  and  the  barome- 
ter at  30  inches,  you  must  subtract  as  many  lOOths  as 
are  indicated  by  the  given  degree  of  the  thermometer 
on  the  scale  of  correction. 

4.  The  scale  applied  to  the  thermometer  answers  for 
the  general  range  of  meteorological  observations ;  but 
if  the  height  of  the  barometer  be  very  far  distant  from 
30  inches,  it  will  be  necessary  to  make  use  of  the  rule 
of  three,  in  order  to  obtain  the  true  correction  :  for  in- 
stance, let  the  barometer  be  at  28  inches,  which  we 
call  P,  c  the  correction  indicated  by  the  thermometer, 

x  the  true  correction' :  then  as  30  :  P  :  :  c  :  x  ;  or  — rz  x, 

which  is  to  be  added  to  the  height  of  the  barometer 
whenever  the  thermometer  is  under  55  degrees,  but  to 
be  subtracted  when  it  is  above  55. 


OF    THE    THERMOMETER. 


No  instrument  is  of  more  importance  for  making 
discoveries  in  meteorology  than  the  thermometer,  as  it 
points  out  the  temperature  or  degree  of  heat  of  the  air 
and  other  bodies.  Heat  and  cold  are  perceptions,  the 
ideas  of  which  we  acquire  by  our  senses.  Our  sensa- 
tions are,  however,  inadequate  measures  of  heat  and 
cold,  for  they  depend  not  only  on  the  substances  which 
excite  them,  but  on  the  actual  state  of  our  bodies  at 
the  time :  we  cannot,  therefore  infer  the  exact  iden- 
tity or  similarity  of  the  cause,  from  the  sameness  of 
the  sensations,  unless  we  can  be  assured  that  our  bo- 
dies are  in  the  same  state  ;  if  they  be  not,  the  same 
objects  will  produce  very  different  sensations.  Thus, 
if  the  hand  be  plunged  into  lukewarm  water,  this  water 


OF    THE    THERMOMETER.  423 

will  appear  cold,  if  the  hand  be  warm ;  but,  if  t he- 
hand  be  cold,  the  water  will  appear  to  be  warm  ;  though 
in  both  cases  it  possesses  the  same  temperature. 

Our  senses  are,  therefore,  both  imperfect  and  de- 
ceitful measures  of  heat  ;  and  we  cannot  ascertain,  by 
their  means  only,  the  state  of  the  surrounding  bodies, 
with  respect  to  heat  and  cold.  This  occasioned  philo- 
sophers to  seek  for  some  method,  by  which  they  might 
determine  the  temperature  of  bodies  with  more  certain- 
ty. This  they  found  in  the  property  of  fire  to  dilate 
or  expand  all  bodies,  whether  solid  or  fluid ;  and  of 
cold  to  contract  or  condense  them.  This  expansion 
and  contraction  is  considered  as  a  measure  infinitely 
more  certain  of  the  degrees  of  heat  and  cold  than  the 
senses. 

It  would  appear  from  this  expansion,  that  fire,  when 
it  is  agitated  by  that  motion  which  we  call  heat,  always 
acted  as  if  it  wanted  more  room  ;  and  this  in  such  a 
wonderful  manner,  as  if  every  particle  of  the  space  in 
which  it  exists  were  a  radiant  point  or  centre,  from 
whence  it  spreads  forcibly  outwards  in  every  direction  ; 
and,  consequently,  when  fire  thus  acting  is  admitted  in- 
to the  pores  of  bodies,  their  parts  must  be  stretched 
out,  and  their  dimensions  every  way  increased,  accord- 
ing to  the  degree  of  fire  by  which  they  are  acted  on. 
Some  idea  of  the  force  of  this  expansion  may  be  gained, 
by  considering  how  vast  a  weight  may  be  suspended 
from  a  bar  of  iron  or  brass  in  a  vertical  position,  with- 
out separating  the  parts  of  the  metal,  or  overcoming 
the  force  with  which  they  cohere.  Now,  this,  fire  easily 
executes,  so  far  relaxing  the  texture  of  brass  and  iron, 
that  their  parts  will  fall  asunder  with  nothing  but  the 
force  of  gravity. 

Thermometers  are  instruments  which  measure  the 
degree  of  heat  by  the  expansion  of  bodies.  Fluids  are 
those  generally  used,  because  they  are  dilated  more  rea- 
dily than  solids ;  and  quicksilver  is  preferred  to  other 
fluids,  because  its  expansibility  is  not  affected  by  the 
different  circumstances  in  which  it  is  placed  ;  it  does 
not  soil  the  tube  like  many  other  fluids,  and  at  the  sam^ 
time  affords  an  extensive  scale  of  divisions. 


42*  OF    THE    THERMOMETER. 

A  thermometer  is  a  tube  of  glass,  the  end  of  which 
is  blown  into  a  ball  or  cylinder ;  the  ball  and  part  of 
the  tube  is  filled  with  mercury.  The  fluid  in  the  ball 
dilates  by  the  heat,  and  contracts  by  the  cold,  which 
occasions  the  fluid  in  the  tube  to  rise  and  fall ;  and  the 
smaller  the  bore  of  the  tube  is  in  proportion  to  the  bail, 
the  more  visible  will  be  the  rise  of  the  fluid  by  a  small 
expansion.  We  may,  therefore,  consider  this  instru- 
ment as  a  convenient  measure  of  the  changes  of  heal 
and  cold,  which  is  shown  by  the  scale  to  which  the 
tube  is  affixed. 

But  it  is  not  sufficient  to  have  found  a  measure  of 
heat ;  .it  must  be  universal,  always  speaking  the  same 
language,  and  awaking  the  same  ideas  in  the  mind,  in 
all  places,  and  at  all  times. 

To  this  end  it  is  necessary,  1.  That  this  measure 
should  begin  from  a  known  and  determinate  point. 
That  another  point,  equally  certain  as  the  first,  but 
some  distance  from  it,  be  fixed  upon.  And,  3.  That 
the  space  between  them  be  divided  into  a  certain  num. 
ber  of  parts,  which  in  all  instruments  will  have  a  con- 
stant proportion. 

It  has  been  fully  proved,  that  the  temperature  of 
freezing  water,  or  melting  ice,  is  constantly  the  same 
in  all  places,  and  at  all  times.  The  same  may  be  said 
of  boiling  water,  under  a  given  pressure  of  the  atmos- 
phere. If,  therefore,  the  ball  of  a  thermometer  be 
plunged  into  melting  ice,  and  afterwards  into  boiling 
water,  and  left  in  each  till  it  acquire  their  temperature, 
and  marks  be  made  at  the  respective  heights  at  which 
the  mercury  stands  in  each,  two  fixed  points  will  be 
obtained.     To  be  more  particular : 

When  ice  is  at  the  melting  temperature,  whatever 
be  the  heat  you  apply  to  it,  it  does  not  become  hotter; 
a  thermometer  in  the  middle  of  the  mass  continually 
stands  at  the  thawing  point  as  long  as  any  of  the  ice  re- 
mains about  it,  so  that  the  same  cause,  wrhich  in  other 
circumstances  would  produce  heat,  here  only  produces 
liquifaction.  Hence  it  is,  that  melting  ice,  or  freezing 
water,  is  so  well  adapted  for  giving  one  of  the  fixed 
points  of  a  thermometer.     The  quantity  of  fire  absorb- 


OF    THE    THERMOMETER.  42$ 

ed  by  ice  in  melting,  is  such  as  would  increase  the  tem- 
perature of  the  water  about  140  degrees:  conversely, 
water  may  be  cooled  1 8  degrees  below  the  freezing 
point,  without  freezing  :  congelation  cannot  take  place 
till  a  certain  portion  of  the  combined  or  latent  fire  be 
disengaged ;  when  any  part  does  congeal,  the  fire  let 
loose,  raises  the  thermometer  to  the  freezing  point,  and 
it  continues  there  till  the  water  be  frozen  ;  after  which, 
as  the  water  in  the  first  case,  so  the  ice  m  the  latter, 
obeys  the  external  temperature. 

Continual  accession  of  fire  arrives  at  water  when 
boiling,  without  increasing  the  heat  thereof;  for  ebul- 
lition, under  any  given  pressure,  cannot  take  place,  till 
the  vapour  produced  in  the  liquid  has  obtained  a  de- 
gree of  expansive  force  sufficient  to  raise  the  liquor  into 
bubbles ;  under  that  pressure,  and  so  long  as  the  va- 
pour retains  this  heat,  it  must  continue  capable  of  re- 
sisting the  same  pressure  ;  as  the  heat  abates,  a  decom- 
position takes  place,  which  occasions  the  opake  steam 
over  boiling  water. 

These  principles  explain  the  fixity  of  the  boiling 
point,  for  vapours  cannot  be  formed  within  the  mass, 
unless  they  have  sufficient  expansive  force  to  displace 
or  raise  it  into  bubbles :  they  cannot  acquire  this  force 
till  the  heat  has  arrived  at  a  certain  point,  and  as  soon  as 
they  have  acquired  it,  they  escape  in  virtue  of  that  ex- 
pansion :  further  accession  of  fire  passes  off  in  the  same 
manner,  and  only  accelerates  the  evaporation. 

Though  boiling  water  under  the  same  pressure  has 
always  the  same  heat,  it  may  be  made,  before  it  does 
boil,  to  receive  a  greater  heat  than  it  can  retain  when  it 
does  boil.  In  a  vessel  with  a  very  narrow  orifice,  filled 
with  water,  well  purged  of  air,  though  the  water  sus- 
tains no  other  pressure  than  that  of  the  atmosphere,  yet 
its  particles  meet  with  so  much  resistance  to  their  sepa- 
ration, that  M.  de  Luc  found  it  would  receive,  without 
boiling,  a  heat  of  22  degrees  above  the  boiling  point  j 
as  soon  as  vapours  could  form  themselves,  their  expan- 
sive force  was  so  great,  that  they  pushed  a  large  quan- 
tity of  water  out  of  the  vessel,  in  the  way  of  explosion, 

vol.  IV.  3  1 


426  OF    THE    THERMOMETER. 

but  the  remainder  was  immediately  reduced  to  boiling 
heat. 

The  vapours  of  boiling  water  arise  from  within  the 
mass,  but  water  may  yield  from  its  surface  vapours  of  an 
equal  expansive  force,  provided  they  be  confined  in  a 
place  of  equal  temperature  with  themselves.  Thus,  if  wa- 
ter be  introduced  above  the  mercury  in  a  barometer,  the 
vapours  it  produces  in  a  temperate  warmth  will  press 
down  the  mercury  nearly  half  an  inch.  In  the  heat  of 
boiling  water,  they  will  depress  it  to  the  level  of  the  mer- 
cury in  the  bason  ;  being  then  become  equivalent  to  the 
pressure  of  the  atmosphere  in  a  greater  heat,  they  will 
depress  it  below  the  level,  and  escape  at  the  bottom  of 
the  tube,  the  water  giving  no  signs  of  ebullition  to  the 
last. 

In  making  thermometers,  care  should  be  taken  that  tl 
tubes  used  for  that  purpose  be  very  clean,  and  very  dry; 
the  next  thing  is  to  examine  whether  the  bore  of  the  tut 
be  equal  and  cylindrical  throughout ;  this  is  easily  pt 
formed,  by  immerging  one  end  of  the  tube  in  mercui 
and  taking  it  out,  previously  stopping  the  other  end  wit 
the  finger  ;  by  this  means  a  small  portion  of  the  mercui 
will  enter  the  tube,  more  or  less  in  proportion  to  the  deptl 
the  tube  is  immerged  ;  measure  the  length  of  this  portioi 
of  mercury,  and  then  slide  it  backwards  and  forwards  ii 
the  tube.    If  the  length  thereof  be  the  same  in  all  parts, 
the  tube  is  a  regular  cylinder  ;  but  if  otherwise,  the  dia- 
meter varies,  and  the  tube  cannot  be  used  to  form  a  gooc 
thermometer,  unless  the  divisions  on  the  scale  be  pro- 
portioned to  the  different  lengths  of  this  mercurial  cylin 
der. 

The  tube  being  chosen,  the  bulb  is  to  be  blown ;  i 
the  tube  be  very  regular,  you  may  now  begin  to  fiil  it : 
if  not  you  must  find  the  proportions  of  the  inequalities  tc 
adapt  the  divisions  thereto  £  to  this  end  tie  a  paper  funne 
over  the  end  of  the  tube,  and  pour  a  small  quantity  o: 
mercury  therein  ;  then  hold  the  bulb  over  the  flame  of  < 
candle  or  lamp,  and  let  some  of  the  air  pass  out  of  thu 
bulb  through  the  mercury  ;  take  it  now  from  the  lamp 
and  as  the  ball  cools  the  mercury  will  begin  to  enter  th( 
fube ;  admit  about  half  an  inch,  take  the  exact  measun 


OF    THE    THERMOMETER.  427 

:hereof,  measure  the  length  of  this  portion  in  different 
parts  of  the  tube,  and  you  will  thereby  obtain  data  for 
proportioning  the  divisions  to  its  inequalities. 

If  you  have  reason  to  suspect  that  there  is  moisture  in 
your  tube,  it  would  be  proper  before  the  preceding  ope- 
ration to  clean  it ;  this  may  be  done  by  laying  the  tube 
on  a  plate  of  iron,  or  over  a  chafing-dish  in  which  there 
is  only  a  small  fire  mixed  with  cinders  ;  it  should  be  con- 
tinued there  till  it  be  so  hot  that  you  must  use  a  glove  or 
a  small  pair  of  pincers  to  hold  it,  taking  care  not  to  warm 
the  bulb.  This  process  dilates  the  included  air,  consumes 
small  particles  of  dirt  imperceptible  to  the  eye,  and  eva- 
porates moisture.  While  things  are  in  this  state,  sud- 
denly heat  the  bulb,  and  the  air  being  thereby  dilated, 
drives  before  it  all  these  impurities,  and  leaves  the  tube 
as  clean  as  you  can  desire. 

To  fill  the  bulb  and  tube,  tie  on  a  paper  funnel  as  be- 
fore, and  put  somewhat  more  mercury  therein  than  you 
think  will  fill  the  thermometer ;  hold  the  bulb  over  the 
flame  of  a  lamp  or  a  small  candle  newly  snuffed,  this  will 
expand  and  force  part  of  the  air  from  the  bulb ;  when 
you  think  a  sufficient  quantity  is  expanded,  withdraw  the 
tube  from  the  candle ;  in  proportion  as  the  bulb  cools, 
the  remaining  air  will  be  condensed,  and  the  space  it  oc- 
cupies will  be  occupied  by  the  mercury ;  by  thus  alter- 
nately cooling  and  heating  the  bulb,  it  is  at  last  completely 
filled. 

When  the  bulb  is  nearly  filled,  you  must  boil  the  mer- 
cury therein,  by  applying  it  over  the  flame  of  a  lamp,  or 
that  of  a  snuffed  candle.  The  air  included  in  the  mercury, 
and  that  which  lines  the  tube,  dilates  itself,  is  collected  in 
small  bubbles,  and  expelled  by  the  first  ebullition  ;  when 
the  mercury  boils  violently,  a  great  part  of  the  contents 
will  rush  up  the  tube  into  the  paper  reservoir.  Remove 
the  bulb  from  the  flame,  and  repeat  the  operation,  till  the 
diminished  noise  and  agitation  show  that  it  is  deprived 
of  its  air  and  moisture. 

After  the  boiling  is  completed  and  the  tube  cool,  plunge 
it  in  melting  ice  or  snow,  which  gives  the  temperature  of 
32°.  Take  off  the  funnel,  and  hold  the  bulb  in  the  hand, 
and  afterwards  in  the  mouth ;  the  heat  thereof  will  cause 


42B  OF    THE    THERMOMETER. 

some  of  the  mercury  to  drop  out  of  the  tube ;  cool  it 
again  to  32°,  and  mark  where  the  mercury  stands.  The 
distance  between  this  mark  and  the  top  of  the  tube  mea- 
sures the  interval  between  freezing  and  blood  heat,  or  32 
and  95,  that  is  63°,  and  will  consequently  point  out  whe- 
ther the  degrees  will  be  large  or  small,  and  what  extent 
your  scale  is  capable  of.* 

When  the  number  of  degrees  to  which  the  length  of 
the  tube  will  extend  is  thus  known,  you  may  settle  where- 
abouts you  will  have  the  freezing  point,  which  may  be 
ftearer  or  further  from  the  bulb,  according  as  your  instru- 
ment is  designed  to  measure  great  or  small  degrees  of 
heat  or  cold.  Now  prepare  the  upper  part  of  the  tube  for 
sealing,  by  drawing  it  out  to  a  fine  capillary  tube ;  then 
heat  the  bulb  in  the  candle  till  a  few  particles  of  mercury 
have  fallen  off  the  top  of  the  tube,  and  afterwards  try  if 
the  freezing  point  be  sufficiently  near  the  bulb  ;  if  it  be 
not,  you  must  repeat  the  operation,  being  careful  how- 
ever not  to  throw  out  too  much  mercury  at  a  time.  Have 
two  candles  now  ready,  one  to  heat  the  ball,  the  other  to 
close  the  tube.  The  blow-pipe  being  in  readiness,  the  up* 
per  part  of  the  tube  near  the  flame  of  one  candle,  and  the 
bulb  near  the  flame  of  the  other,  the  mercury  will  rise, 
and  at  last  begin  to  form  a  globule  at  the  point  of  the  ca- 
pillary tube  ;  at  this  instant  the  bulb  must  be  withdrawn 
from  the  flame  of  the  lower  handle,  and  the  flame  of  the 
upper  one  be  directed  by  the  blow-pipe  upon  the  point 
of  the  tube.  This  will  be  immediately  ignited,  and  will 
close  by  the  melting  of  its  parts  before  the  mercury  has 
perceptibly  subsided.  When  the  mercury  has  fallen,  the 
sealing  may  be  rendered  more  secure  by  fusing  the  whole 
point  of  the  tube  till  it  becomes  sound. 

To  settle  the  freezing  point,  you  have  only  to  immerge 
the  thermometer  so  deep  in  melting  snow  or  ice,  that  the 
mercury  may  be  scarcely  visible  above  the  surface,  and 
then  carefully  mark  the  place  at  which  it  stands.  For  the 
boiling  point,  the  Royal  Society  advise  a  vessel  to  be  pro- 
vided somewhat  longer  than  the  thermometer,  with  a  cover 


*  JVic/idson's  First  Principles  of  Chemistry,  p.  26,  27, 28. 


OF    THE    THERMOMETER.  429 

and  two  holes  therein  ;  one  about  an  inch  in  diameter  for 
the  steam  to  escape,  the  other  smaller  to  hold  the  ther- 
mometer tube.  When  this  is  used,  the  thermometer  must 
be  fastened  in  the  cover,  so  that  the  estimated  place  of  the 
boiling  point  may  be  just  above  the  hole;  water  then  must 
be  put  into  the  vessel,  but  so  as  not  to  touch  the  bulb  of 
the  thermometer  when  the  cover  is  placed  on.  The  cover 
being*  put  on,  and  a  thin  plate  of  metal  laid  on  the  steam- 
hole,  you  are  to  make  the  water  boil  bv  heat  applied  to 
the  bottom  only  ;  the  thermometer  will  thus  be  surround- 
ed by  steam  which  will  raise  its  temperature  to  the  boiling 
point,  and  this  must  be  carefully  marked  on  the  tube. 

Fahrenheit* s  scale  is  that  which  is  used  in  England : 
the  freezing  point  is  called  32,  the  boiling  water  point 
212;  so  that  there  are  180  degrees  or  divisions  between 
them,  which  may  be  extended  upwards  and  downwards, 
as  far  as  is  necessary. 

Foreigners  generally  use  Reaumur's,  or  rather  de  Luc's 
scale,  where  the  freezing  point  is  marked  O,  and  the  boil- 
ing water  point  80. 

Two  thermometers  are  necessary  for  accurate  observa- 
tion ;  one  to  be  suspended  within  doors,  near  the  baro- 
meter ;  the  other  out  of  doors.  That  without  doors 
should  be  placed  at  the  north  side  of  the  house,  or  where 
it  will  be  sheltered  from  the  rays  of  the  sun. 

I  have  already  shown  you,  that  the  increments  of  ex- 
pansion in  a  mercurial  thermometer  are  nearly  as  the  in- 
crements of  heat ;  or  in  other  words,  that  the  dilatations 
and  contractions  of  the  fluid  are  nearly  proportional  to  the 
quantities  of  fire,  which  are  communicated  to,  or  sepa- 
rated from  the  same  homogeneous  body  as  long  as  they 
remain  in  the  same  state.  Thus  the  quantity  of  fire  re- 
quired to  raise  a  body  four  cfegrees  in  temperature  by  the 
mercurial  thermometer,  is  nearly  double  what  is  required 
to  raise  it  two  degrees,  and  four  times  what  is  necessary 
to  raise  it  one.  This  is  proved  by  putting  a  thermometer 
first  in  cold  water,  and  then  into  water  heated  to  any  de- 
gree, noting  the  altitudes  ;  then  putting  equal  quantities 
of  these  two  waters  together,  which  will  give  a  mean 
heat,  and  the  mercury  in  the  thermometer  will  stand  at 
the  mean  altitude  between  the  two  before  observed  ;  this 


430  OF    THE    RAIN-GAGE. 

is  found  to  be  true,  whatever  be  the  temperature  of  the 
two  parts  of  water. 

Though  in  the  sense  here  explained  the  thermometer 
is  an  accurate  measure  of  heat,  yet  I  have  also  shown  you, 
that  it  can  only  indicate  the  proportions  of  that  action  of 
fire  by  which  bodies  are  expanded,  but  is  by  no  means  a 
measure  of  the  whole  quantity  of  fire  disengaged,  dis- 
placed, or  absorbed;  properly  speaking,  it  is  therefore 
only  a  scale  of  expansion  indicating  certain  translations 
and  transfusions  of  the  igneous  fluid. 

It  may  be  proper  to  observe  to  you,  that  glass  is  dilated 
and  contracted  by  heat  and  cold  together  with  the  fluid, 
and  consequently  the  apparent  variations  in  the  dimen- 
sions of  the  fluid  are  the  difference  between  the  real  co- 
temporary  expansions,  or  sum  of  the  cotemporary  con- 
tractions of  the  glass  and  the  fluid.  The  changes  arising 
from  these  causes  are  too  inconsiderable  to  be  worthy  of 
notice  in  the  general  use  of  this  instrument ;  the  change 
of  dimensions  in  the  glass  is  prior  to  that  in  the  fluid ; 
hence  the  fluid  is  found  to  sink  before  its  rise  upon  an 
increase  of  temperature  ;  and  if  the  bulb  be  large,  some 
time  may  elapse  before  the  fluid  acquires  the  same  tem- 
perature with  the  glass.  The  pressure  of  the  atmosphere  on 
the  outside  of  the  bulb  not  being  counteracted  by  the  air 
within,  will  affect  its  magnitude,  diminishing  it  as  the 
pressure  is  increased.  The  variation  on  the  scale  occa- 
sioned by  this  cause  is,  like  the  preceding  one,  very  small, 
being  never  above  one-tenth  of  a  degree. 


OF    THE    RAIN-GAGE. 

It  is  necessary  towards  forming  a  systematic  idea  of  the 
weather  and  its  various  changes,  to  measure  the  quantity 
of  rain  which  falls  upon  the  earth ;  and  this  is  done  by 
what  is  called  a  rain-gage. 

The  rain-gage  is  a  very  simple  instrument,  consisting 
of  a  square  tin  funnel  of  twelve  inches  diameter,  commu- 
nicating with  a  tube  or  cylinder  of  tin,  into  which  the 
rain  is  conveyed  by  the  funnel.  The  depth  of  the  water 
is  measured  by  a  rule  fixed  to  a  float  \  this  rule  passes 


OF    THE    HYGROMETER.  431 

through  the  centre  of  the  funnel.  The  divisions  on  the 
rule  show  the  number  of  cubic  inches  of  water  that  have 
fallen  in  a  given  time  on  a  surface  of  one  square  foot. 

To  use  the  rain-gage,  so  much  water  should  be  first 
poured  in  as  will  raise  the  float,  so  that  the  zero  on  the 
rule  may  exactly  coincide  with  the  aperture  of  the  funnel. 
The  funnel  is  so  contrived,  as  to  prevent  the  water  from 
evaporating. 

This  gage  should  be  fixed  down  firmly  in  a  place  where, 
whatever  winds  blow,  the  fall  of  the  rain  may  not  be  in- 
tercepted by  the  house,  or  any  other  impediment. 


OF    THE    HYGROMETER. 

The  hygrometer  is  an  instrument  intended  to  discover 
the  moisture  contained  in  the  atmosphere. 

As  the  substances  that  are  affected  by  moisture  are 
very  numerous,  so  are  also  the  contrivances  that  have 
been  executed  to  indicate  the  degrees  of  moisture,  and 
render  sensible  the  smallest  variations  in  the  substances 
influenced  thereby.  Thus,  wood  expands  by  moisture 
and  contracts  by  dryness  ;  on  the  contrary,  cord,  cat- 
gut, &c.  contract  by  moisture  and  lengthen  by  dryness  ; 
consequently,  the  contraction  and  expansion  of  these 
substances  indicate  different  states  of  the  air  with  re- 
spect to  moisture.  The  twisted  beard  of  a  wild  oat, 
with  a  small  index  fixed  to  it,  moveable  against  a  scale, 
makes  a  very  good  hygrometer  ;  for  the  twisting,  being 
affected  by  the  variations  of  moisture,  moves  the  index. 

Mr.  de  Luc>  who  has  laboured  more  on  this  subject, 
and  with  more  success  than  any  other  man,  after  ma- 
king an  immense  number  of  experiments  to  find  out  a 
substance,  whose  expansion  increases  most,  nearly  in 
proportion  to  the  quantity  of  moisture  imbibed,  found 
that  whalebone  and  box,  cut  across  their  fibres,  in- 
creased very  nearly  in  proportion  to  the  quantity  of 
moisture,  and  more  so  than  any  other  substance  which 
he  tried.  He  however  preferred  the  whalebone;  1st. 
On  account  of  its  steadiness,  in  always  coming  to  the 
same  point  at  extreme  moisture  j  2dly,  On  account  of 


432  PRINCIPLES    OF 

its  greater  expansion,  increasing  in  length  above  one- 
eighth  of  its  length,  from  extreme  dryness  to  extreme 
moisture ;  lastly,  Because  it  is  more  easily  made  thin 
and  narrow. 

As  the  whole  atmospheric  economy,  as  far  at  least  as 
relates  to  the  weather,  depends  upon,  or  is  connected 
with  the  state  of  vapour  it  contains,  it  is  rather  surpriz- 
ing, that  we  find  so  few  hygrometrical  observations 
among  ihe  many  meteorological  diaries  that  have  been 
published.  From  time  immemorial,  the  effects  of  moist- 
ure have  been  considered  as  prognostic  of  the  weather, 
as  is  evident  by  the  confidence  the  housewife  places  in 
her  salt-box,  the  carter  in  his  whip  leather  thong,  and 
the  sailor  in  his  shrouds.  But  whether  the  hygrome- 
ter be  a  prognostic  or  not  of  the  weather,  it  is  certainly 
of  the  utmost  importance  to  the  natural  philosopher, 
and  would  probably  prove  a  valuable  oracle  to  the  far- 
mer,  which  is  fully  evinced  by  the  following  observa- 
tion of  Mr.  Marshall,  in  his  minutes  of  agriculture. 
"  Yesterday  morning,"  says  he,  "  while  the  hygrome- 
ter stood  at  two  degrees  moist,  the  peas  were  by  no 
means  fit  for  carrying ;  the  balm  was  green,  and  the 
peas  soft.  About  ten  o'clock  the  hygrometer  fell  to 
one  degree  dry  ;  before  one,  the  peas  were  in  good  or- 
der ;  I  went  into  the  field,  merely  on  the  word  of  the 
hygrometer,  and  found  the  peas  fit  to  be  carried."  It 
is  plain  therefore,  that  on  a  scattered  farm,  in  hay-time 
and  harvest,  a  hygrometer  must  be  peculiarly  useful. 

Before  I  proceed  to  take  further  notice  of  hygrome- 
ters, it  will  be  necessary  to  remind  you  of  the  principles 
I  have  already  laid  down  concerning  evaporation  and 
vapour ;  for,  unless  these  be  properly  attended  to,  you 
will  never  be  able  to  attain  any  fixed  and  certain  no- 
tions of  meteorology.  Atmospherical  fluids  are  divisi- 
ble into  two  classes  vapours  and  aeriform,  the  distinct- 
ive characters  of  which  are  these  :  vapours  are  decom- 
posed by  pressure,  but  aeriform  fluids  bear  the  strong- 
est compression  without  decomposition :  vapours  are 
decomposed  in  vessels  hermetically  sealed  by  the  spon- 
taneous escape  of  fire  ;  but  aeriform  fluids  can  only  be 
decomposed  by  some  substance,  to  which  their  gravi- 


LVAPORATION    AND    VAPOUR.  43cJ 

fating  matter  has  more  affinity  than  to  the  fluid  which 
maintains  them  in  an  aeriform  state.  In  vapours  the 
proportions  of  the  component  parts  are  very  variable, 
according  to  the  subsisting  circumstances  ;  but  aeriform 
fluids,  when  once  formed,  continue  in  the  same  state, 
and  can  only  be  changed  by  chemical  causes :  the  dif- 
ference arises  from  the  weakness  of  the  union  of  water 
in  vapour  with  fire,  so  that  it  can  separate  itself  there- 
from by  the  mutual  tendency  of  its  own  particles,  when 
they  are  brought  within  a  certain  distance  one  of  the 
other,  and  because  fire  can  so  easily  quit  them,  to  re- 
store certain  equilibria  with  respect  to  itself. 

By  watery  vapour,  1  do  not  here  mean  visible  opake 
steam  or  vapour,  because  that  is  vapour  in  a  state  of  de- 
composition ;  I  mean  the  invisible  and  transparent  ex- 
halations, which  constitute  a  peculiar  and  distinct  fluid, 
expansible  and  compressible,  and  thus,  far  from  pos- 
sessing the  mechanical  properties  of  aeriform  fluids, 
and  exercising  these  properties  whether  mixed  with 
them  or  alone* 

The  specific  gravity  of  these  vapours  is  above  one 
half  less  than  that  of  common  air  ;  that  is*  when  they 
exercise  a  certain  expansive  force,  whether  alone  or 
mixed  with  air,  their  mass  is  above  one  half  less  than 
a  like  volume  of  air,  which  would  exercise  the  same 
expansive  force. 

In  the  course  of  our  lectures  on  fire,  I  showed  you, 
that  vapour  consists  of  particles  of  fire,  united  with 
those  of  water,  and  that  there  was  no  foundation  for 
the  hypothesis  which  considered  it  as  a  chemical  solu- 
tion of  water  by  air.  This  is,  however,  a  hypothesis 
that  has  been  adopted  by  so  many  writers,  though  con- 
trary to  every  circumstance  duly  examined,  and  of 
such  consequence  in  meteorology,  that  1  shall  again 
make  a  few  remarks  thereon.  I  shall  first  notice  the 
phenomena  of  air  contained  in  water,  and  show  you* 
that  these  have  no  relation  to  the  common  notions  of 
solution.  If  water  be  placed  in  a  receiver,  and  a  va- 
cuum made,  a  number  of  air-bubbles  are  formed  in  the 
midst  of  the  water,  which  increase  in  size,  and  then 

VOL.  IV.  3  K 


434  PRINCIPLES    OF 


escape.  Now,  there  is  no  principle  in  the  theory  of 
dissolution  which  can  explain,  why  a  menstruum,  be- 
cause it  is  less  pressed,  should  let  go  the  substance  that 
it  had  dissolved  ;  whereas  it  should  hold  it  stronger  if 
the  menstruum  be  thereby  dilated.  When  the  water 
ceases  to  produce  air  by  this  operation,  if  you  agitate 
it  strongly,  more  air  is  disengaged  ;  this  also  is  contra- 
ry to  the  theory  of  dissolution,  for  this  is  promoted  by 
the  agitation  of  the  menstruum.  When  both  these  me- 
thods cease  to  be  efficacious,  more  air  may  be  disengag- 
ed by  heat ;  here  the  hypothesis  is  contradicted  at  its 
very  foundation  ;  the  sole  plausibility  on  which  it  rest- 
ed was  derived  from  the  idea,  that  the  air  could  contain 
more  water  when  its  heat  was  greatest,  which,  of 
course,  must  also  take  place  with  respect  to  air  contain- 
ed in  water,  to  which  you  see  this  fact  is  diametrically 
opposed.  I  have  shown  you  in  the  lecture  on  fire, 
that  the  phenomena  of  aqueous  vapour  are  the  same  in 
vacuo  as  in  open  air,  that  it  may  be  produced  in  vacuo 
without  any  concurrence  of  the  air.  The  density  of  the 
vapour  is  the  same  every  where  at  any  temperature,  pro- 
vided the  particles  thereof  keep  at  a  certain  distance  from 
each  other.  This  density  in  every  space,  and  at  every 
temperature,  is  determined  by  a  certain  minimum  dis- 
tance among  the  particles  of  the  vapour.  It  is  sufficient 
for  their  conservation  as  vapour,  either  in  vacuo  or  in 
air,  that  they  be  not  forced  to  approach  within  this 
distance.  The  product  of  evaporation  is  always  of  this 
nature,  namely,  an  expansible  fluid,  which  either 
alone  or  in  air  affects  the  manometer  by  pressure,  and 
the  hygrometer  by  moisture,  without  any  difference  aris- 
ing from  the  presence  or  absence  of  air.  I  may  again, 
therefore,  repeat  after  M.  de  Luc,  that  every  phenomenon 
proves,  that  the  hypothesis  of  the  solution  of  water  by 
air  is  vague,  without  any  solid  foundation,  unnecessary 
for  the  explanation  of  evaporation,  while  it  involves  every 
branch  of  philosophy  in  obscurity.* 

*  See  M.  dc  Luc's  Letters,  dans  le  Journal  de  Physique,  hisldees  sur  la 
Metidbrologie. 

See  also  further  proofs  of  the  errors  of  the  chemical  idea  of  water  being 
Solved  by  air  in  Vol.  II.  Lecture  xiv. 


EVAPORATION    AND    VAPOUR.  435 

Evaporation  is  a  dissolution  of  water  by  fire.  A 
most  decisive  reason  in  support  of  this  opinion  is,  that 
every  liquid  cools  when  it  evaporates  ;  the  portion  of 
the  liquid  that  disappears,  being  carried  away  by  a 
quantity  of  fire  proceeding  from  the  liquid  itself.  Mr. 
Watt  has  shown,  that  in  the  common  evaporation  of 
water  in  open  air,  the  quantity  of  heat  lost  by  the  mass, 
bears  to  the  quantity  of  water  carried  away,  a  propor- 
tion still  greater  than  that  which  is  found  in  the  steam 
produced  by  boiling  water. 

As  vapour  consists  of  fire  and  water  united,  zrA 
forming  a  new  compound,  the  specific  properties  of 
each  of  the  component  parts  are  in  certain  respects  sup- 
pressed, as  in  other  chemical  operations,  the  water  loses 
its  faculty  of  moistening,  and  the  fire  that  of  producing 
heat ;  hence  the  loss  of  heat  in  the  evaporation  of  li- 
quids, and  the  augmentation  of  heat  in  the  decompo- 
sition of  vapour.  The  particles  both  of  fire  and  water 
still,  however,  retain  the  faculty  of  maintaining  their 
respective  equilibrium  between  the  medium  and  sur- 
rounding bodies.  Thus  the  particles  of  water  still  re- 
tain the  tendency  of  uniting  together,  and  this  union 
takes  place  whenever  they  are  so  near  each  other,  that 
this  tendency  can  surmount  the  effort  of  the  fire  which 
keeps  them  disseminated. 

Of  course,  the  less  the  quantity  of  free  fire,  or  the 
cause  of  heat,  in  a  given  space,  the  greater  is  the  dis- 
tance at  which  the  particles  of  water  can  exert  their 
faculty  of  uniting  together,  and  of  abandoning  their  la- 
tent fire.  The  precipitation  of  water  or  final  union, 
therefore,  takes  place  when  the  density  of  vapour  has 
exceeded  certain  limits,  which  limits  depend  on  the 
temperature ;  for  the*greater  the  quantity  of  free  fire 
in  "any  given  space,  the  nearer  the  particles  of  vapour 
may  approach  each  other  without  being  decomposed, 
that  is,  without  the  watery  particles,  in  consequence  of 
their  natural  tendency,  re-uniting  together,  and  quit- 
ting the  fire  with  which  they  were  associated. 

Thus  there  is  necessarily  a  minimum  of  distance  of 
the  aqueous  particles,  beyond  which  the  vapour  cannot 
be  compressed  without  being  decomposed  j  and  this  is 


436  HYGROMETRY. 

different  in  different  degrees  of  heat,  but  constant  in 
the  same.  When  vapours  are  mixed  with  air,  thev 
can  sustain  a  much  greater  pressure  than  they  can  bv 
themselves,  because  the  air  supports  the  pressure,  and 
prevents  the  particles  from  being  forced  within  their 
minimum  distance ;  and  it  is  thus  that  vapours  subsist 
in  the  atmosphere  without  being  decomposed  by  its 
pressure. 

Vapours  are  decomposed  not  only  by  the  mutual  ap- 
proach of  the  particles,  but  also  in  virtue  of  the  affinity 
of  water  to  those  substances  that  are  called  hygroscopic, 
of  which  fire  may  be  reckoned  one.  The  principal  law 
of  this  affinity  is,  that  the  water  distributes  itself  to  all 
the  substances  of  the  class  that  are  within  its  reach,  to 
every  one  alike,  proportional  to  its  capacity  of  reten- 
tion. If  new  fire  be  introduced  into  a  given  space, 
where  there  is  no  superabundant  water,  it  will  take 
away  some  of  the  water  from  all  the  hygroscopic  sub- 
stances, and  diminish  their  humidity.  If  some  of  the 
fire  be  taken  away,  the  water  that  was  united  thereto 
will  be  divided  among  all  the  rest ;  and  if  any  other 
hygroscopic  substances  be  introduced,  containing  a 
greater  or  less  quantity  of  humidity  than  those  already 
there,  the  surplus  of  humidity  will  be  divided  among 
them.  It  is  by  fire  that  this  disri  ibution  is  effected  ; 
the  particles  of  this  element  being  always  in  motion, 
take  up  the  water  from  one  that  has  more  than  its  share, 
acid  give  it  out  to  another  that  has  less.  Thus  hygros- 
copic substances  have  their  humidity  always  propor- 
tional to  the  places  they  are  in. 

Hygroscopic  substances  are  of  three  distinct  classes  : 
1.  Those  that  seize  on  the  water  of  vapour  by  a  che- 
mical affinity  with  that  liquid  ;  among  these  are  acids, 
salts,  and  calces.  2.  Those  that  imbibe  the  water,  by 
the  tendency  it  has  to  propagace  itself  in  capillary  pores, 
but  from  their  nature  receive  no  sensible  increase  oi 
bulk  by  its  introduction  ;  such  are  porous  stonesi  3. 
Those  that,  imbibing  a  certain  quantity  of  water,  are 
thereby  expanded  ;  and  these  are  most  of  the  solids  of 
the  vegetable  and  animal  kingdoms.  M.  de  Luc,  by  a 
long  series  of  experiments,  to  which  I  must  refer  you. 


HYGROMETFY.  437 

hows,  that  the  substances  of  the  last  class  are  the  only 
m  s  proper  for  hygrometers,  and  that  even  in  this 
iass,  to  avoid  fallacy  in  respect  to  the  most  important 
>henomena,  we  must  use  those  that  cease  to  lengthen, 
mly  when  they  can  not  be  penetrated  with  more  water. 

ilc-re,  however,  it  will  be  necessary  to  define  in  what 
ense  we  use  the  words  moisture  and  humidity ,  for  in 
he  manner  they  are  commonly  used,  they  sometimes 
tnply  a  cause,  sometimes  an  effect ;  this  ambiguity  is 
lot  peculiar  to  these  words,  you  will  find  many  others 
ised  in  philosophy  as  ambiguous,  particularly  when 
hey  have  oeen  applied  to  certain  phenomena,  the  causes 
)f  which  are  not  determined. 

Moisture,  in  a  general  sense,  may  be  considered  as  in- 
visible water,  producing  observable  phenomena. 

i  nus  in  hygroscopic  bodies,  the  quantity  of  water 
vhich  expands  them,  and  increases  their  weight,  is 
:oi  coaled  within  their  pores  ;  and,  in  the  ambient  me- 
lium,  that  water  which  affects  hygroscopic  bodies,  be- 
rig  th-.--.re  under  the  form  of  vapour,  is  as  invisible  as 
he  air  itself. 

Bur  in  respect  to  hygrometry,  where  moisture  is  con- 
id  ored  as  having  correspondent  degrees  in  the  medium, 
he  word  requires  a  more  particular  determination, 
VIoisture  may  be  either  totally  absent  or  absolutely  ex- 
reine,  both  in  the  hygroscopic  bodies  and  in  the  ambi- 
:nt  medium  ;  hence,  both  in  the  whole  and  in  corres- 
)ondent  parts,  moisture  assumes  in  the  medium  the  cha- 
racter or  a  cause,  and  in  hygroscopic  bodies,  that  of  an 
ffect.  These  two  circumstances  furnish  us  also  with  a 
ixed  module  for  determining  correspondent  degrees. 

Moisture  is  totally  absent,  first,  in  the  medium  when 
t  contains  no  vapour ;  and  then  as  a  consequence  in 
ngroscopic  bodies,  because  they  contain  no  more  wa- 
er  jhat  can  evaporate,  without  a  decomposition  of  their 
'.omponent  parts.  The  case  here  supposed  that  is, 
vhen,  by  some  adequate  cause,  no  sensible  quantity 
A  vapour  is  permitted  to  remain  in  the  medium,  as  in 
he  lime  vessel  used  by  M.  de  Luc  to  obtain  the  point 
)f  extreme  dryness. 

Moisture  is  extreme,  first,  in  the  medium,  whether 
ir  or  a  space  free  from  air,  when  no  more  vapour  can 


438  HYGROMETRY. 

be  introduced  therein,  without  a  part  being  decompos 
ed  ;  and  then,  as  a  consequence  in  hygroscopic  bodies 
because  no  more  water  can  be  admitted  in  their  pores. 

Here  it  is  to  be  observed,  that  from  the  nature  of  the 
last  of  these  maxima  the  quantity  of  water  which  pro 
duces  it,  i.  e.  extreme  moisture,  in  a  given  body  is  fix 
ed,  because  it  is  determined  by  the  actual  capacity  o; 
its  pores ;  but  the  quantity  of  water  which  produce: 
extreme  moisture  in  a  medium  of  a  given  extent,  is  a: 
variable  as  the  temperature. 

The  equilibrium,  therefore,  between  the  mediun 
and  hygroscopic  bodies  in  different  stages  of  moisture 
which  equilibrium  is  the  object  of  hygrometry  as  a  sci 
ence,  does  not  depend  on  certain  quantities  of  wate: 
contained  in  the  medium  of  which  bodies  may  receivi 
their  share  ;  it  depends  on  different  aptitudes  of  th< 
vapour  contained  in  the  medium,  to  communicate  wate 
to  those  bodies  ;  which  aptitudes  vary  not  only  witl 
the  different  densities  of  that  fluid,  but  also  in  vapou; 
of  the  same  density  according  to  the  temperature.* 

From  the  hygrometer  we  have  learned,  that  in  th> 
phenomenon  of  dew,  the  grass  often  begins  to  be  we 
when  the  air  a  little  above  it  is  still  in  a  middle  state  o 
moisture  ;  and  that  extreme  moisture  is  only  certain  ii 
that  air,  when  every  solid  exposed  thereto  is  wet.  I 
has  taught  us,  that  the  maximum  of  evaporation  in ; 
close  space  is  far  from  being  identical  with  the  maxi 
mum  of  moisture  ;  this  depending  considerably,  thougi 
with  the  constant  existence  of  the  other,  on  the  tempe 
rature  common  to  the  space  and  the  water  that  evapo 
rates.  It  has  shown,  that  the  case  of  extreme  moistur 
existing  in  the  open  transparent  air  in  the  day,  eve: 
when  it  rains,  is  extremely  rare ;  M.  de  Luc  has  onl 
found  it  once  iu  this  state,  the  temperature  being  39' 
Messrs.  de  Saussure  and  de  Luc  have  proved  by  th 
hygrometer,  that  the  air  is  dryer  and  dryer  as  we  a? 
cend  in  the  atmosphere  ;  so  that  in  the  upper  attainabl 


*  See  M.  de  Luc's  -paper  on  Evaporation,  from  which  the  remarks  < 
Hygrometers,  Sec.  u  an  extract,  "Phil.  Trans,  for  1<"91,  part  9. 


HYGROMETRY.  439 

•egions,  it  is  constantly  very  dry,  except  in  the  clouds. 
VI,  de  Saussure  has  shown,  that  if  the  whole  atmos- 
)here  passed  from  extreme  dryness  to  extreme  incis- 
ure fhe  quantity  of  water  thus  evaporated  would  not 
•aise  the  barometer  half  an  inch.  Lastly,  in  chemical 
merations  on  the  air,  the  greatest  quantity  of  evaporat- 
ed water  that  may  be  supposed  in  them  at  the  common 
emperature  of  the  atmosphere,  even  if  they  were  at 
extreme  moisture,  is  not  so  much  as  the  one  hundredth 
Dart  of  their  mass.  The  two  last  very  important  pro- 
positions have  been  demonstrated  by  M.  de  Saussure* 


LECTURE  LIL 


OF    RAIN. 


IN  a  science  so  very  difficult  as  that  of  the  weather  5 
it  is  not  to  be  supposed  that  any  thing  like  a  certain  and 
established  theory  can  be  laid  down  :  our  utmost  know- 
ledge in  this  respect  goes  no  further  as  yet  than  the  es- 
tablishment of  a  few  facts  ;  and  in  reasoning  upon  these, 
we  are  involved  every  moment  in  questions  which  seem 
scarcely  within  the  compass  of  human  wisdom  to  resolve. 
To  treat  it  in  a  satisfactory  manner,  we  ought  to 
have  an  intimate  acquaintance  with  the  constitution  of 
the  atmosphere,  and  the  nature  of  those  powerful 
agents,  fire,  light,  and  electricity,  by  which   it  seems 


*  See  M.  de  Luc's  second  paper  on  Hygrometry,  Phil.  Trans. 


440  OF    RAIN. 

to  be  principally  influenced  ;  with  their  peculiar  i^flu. 
ences  upon  one  another  and  upon  the  atmosphere,  a  d 
this  in  every  possible  variety  of  circumsranees.  Many 
of  the  qualities  of  air,  earth,  water,  and  fire,  have 
been  indeed  discovered  and  estimated  ;  but  when  thji>e 
come  to  be  united  by  nature,  they  often  produce  a  re- 
sult which  no  artificial  combinations  can  imitate.  Every 
cloud  that  movris, — 3very  shower  that  falls,  serves 
to  mortify  the  philosopher,  and  to  show  him  hidden 
qualities  in  air  and  water  that  he  is  unable  to  explain. 

The  greater  part  of  the  received  notions  on  meteoro- 
logy are  vague  and  incorrect,  not  only  those  which  re- 
late to  the  nature  of  the  causes,  but  those  also  which 
concern  the  laws  of  their  effects.  The  same  may  be 
said  of  our  notions  of  the  elasticity  of  the  air,  of  heat 
when  applied  to  this  fluid,  of  both  igneous  and  aqueous 
meteors,  of  sudden  and  partial  winds  ;  they  are  all  so 
many  enigmas  to  the  philosopher. 

Indeed,  till  we  were  in  possession  of  a  good  hy- 
grometer, it  was  impossible  to  form  any  certain  conclu- 
sions concerning  the  moisture  of  the  air  :  this  difficulty 
is  removed  ;  M.  de  Luc  has  by  numerous  experim 
and  observations  furnished  us  with  a  comparative  hy- 
grometer, by  which,  together  with  a  thermometer,  the 
air  can  neither  lose  nor  acquire  moisture  without  our 
being  advertised  thereof. 

By  the  use  of  this  hygrometer  we  have  obtained 
clear  and  certain  ideas  of  the  causes,  by  which  water, 
simply  evaporated  in  air,  may  be  precipitated  there- 
from. These  causes  are  the  same  with  those,  which  in 
air,  where  the  quantity  of  evaporated  water  remains  the 
same,  always  produce  an  increase  of  moisture,  the  ne- 
cessary forerunner  of  the  precipitation  of  water  ;  and 
these  are  two,  viz.  the  compression  of  the  air,  and  its 
being  cooled  :  no  other  causes  are  indicated  by  experi- 
ment. Some  philosophers  have  thought  that  the  air, 
when  rarefied,  quitted  a  portion  of  the  water  which, 
according  to  them,  it  held  in  solution  ;  but  I  have 
shown  you  that  this  idea  is  erroneous,  and  that  rarefac- 
tion occasioned  dryness  instead  of  moisture. 


OF    RAIN.  441 

The  great  question,  therefore,  in  the  inquiry  con- 
cerning the  immediate  cause  of  clouds,  &c.  is,  What 
becomes  of  the  water  that  rises  as  vapour  into  the  at- 
mosphere ?  What  is  the  state  in  which  it  subsists  there, 
between  the  time  of  its  evaporation  and  the  time  of  its 
falling  down  again  in  rain  ? 

If  it  continue  in  a  state  of  watery  vapour,  or  such 
as  is  the  immediate  product  of  evaporation,  it  must 
possess  the  distinctive  characters  essential  to  that  fluid. 
It  must  make  the  hygrometer  move  towards  humidity 
in  proportion  as  the  vapour  is  more  or  less  abundant  in 
the  air.  On  a  diminution  of  heat,  the  moisture,  as 
shown  by  the  hygrometer,  would  increase  ;  but,  on  an 
increase  of  heat,  the  humidity  would  decrease.  Again, 
on  this  supposition,  if  hygroscopic  substances  dryer 
than  the  air  be  introduced  therein,  they  must  have  the 
same  effect  as  an  augmentation  of  heat :  for,  Such  are 
always  the  properties  of  aqueous  vapour  on  every  hypo- 
thesis of  evaporation.  If,  therefore,  water  exists  in 
the  atmosphere  without  these  properties,  it  is  no  longer 
vapour,  it  must  have  changed  its  nature.  M.  de  Luc 
has  shown,  that  the  water  which  forms  rain,  does  not 
possess  these  properties  ;  it  must,  therefore,  have 
passed  into  another  state. 

Repeated  observations  have  shown,  that  the  upper 
regions  of  the  atmosphere,  notwithstanding  the  conti- 
nual ascent  of  vapours  there,  are  dryer  than  the  infe- 
rior regions  ;  on  the  submits  of  high  mountains  a  de- 
gree of  dryness  prevails  unknown  on  the  plains. 

If  rain  be  the  immediate  product  of  evaporation,  it 
ought  always  to  be  preceded  and  accompanied  by  a 
diminution  of  heat,  in  that  stratum  of  air  where  it  ori- 
ginated ;  and  this  diminution,  to  produce  its  effect, 
should  be  greater  in  proportion  as  the  moisture  was 
further  removed  from  its  extreme  term  in  this  stratum  \ 
but,  in  a  great  storm  on  the  mountain^  of  Sixt,  M. 
de  Luc  found  that  the  heat  had  increased  instead  of  di- 
minished ;  this  cause  could  not  operate  here,  and  it 
was  therefore  impossible  that  the  quantity  of  water 
which  was  then  precipitated  from  the  air   could  have 

VOL.  IV.  3L 


442  OF    RAIN. 

been  contained  there  in  the  form  of  the  immediate  pro- 
duct of  evaporation. 

On  every  hypothesis  of  the  formation  of  rain  from 
vapour,  it  is  heat  that  produces  the  evaporation,  and  a 
diminution  of  heat  that  occasions  the  return  of  vapour 
into  water,  and  therefore  rain  should  happen  only  in 
the  night,  or  at  the  coldest  time  of  the  day  ;  whereas 
experience  shows,  that  it  has  no  connexion  with  heat 
or  cold.  We  have  rain  as  often  in  the  day  time,  when, 
according  to  the  natural  course  of  things,  the  heat  of 
the  atmosphere  should  be  the  greatest,  as  at  night, 
when  the  heat  ought  to  diminish  ;  besides,  the  heat 
often  diminishes  in  the  day,  without  producing  rain. 
Whatever  be  the  degree  of  heat,  the  air  can  only  part 
with  so  much  of  its  water,  as  it  is  unable  to  retain  in 
that  degree  of  heat ;  no  rain  should  therefore  be  form- 
ed, unless  the  air  were  saturated,  or  at  extreme  mois- 
ture, but  this  also  is  contrary  to  fact. 

Thus,  when  M.  de  Luc  and  his  brother  were  on  the 
Sixt,  though  the  hygrometer  was  66\  degrees  from  ex- 
treme humidity,  thick  clouds  formed  around  them,  which 
obliged  them  to  think  of  retreating  ;  in  a  little  time  thi 
summit  of  the  mountain  was  surrounded  by  them,  they 
spread  and  covered  the  whole  horizon,  a  premature  night 
surprized  them  in  a  very  dangerous  road,  and  a  most 
violent  storm  of  wind,  rain,  hail,  and  thunder,  lasted 
the  greater  part  of  the  night ;  it  extended  over  all  the 
neighbouring  mountains  and  plains :  after  the  storm 
ceased,  the  rain  continued  with  very  few  intermissions 
till  the  next  day  at  noon.  The  hygrometer  being  exa- 
mined in  one  of  these  intervals,  only  showed  1 A  more 
moisture  than  before ;  and  even  this  increase  was  no 
other  than  what  the  difference  of  heat  was  sufficient  for 
producing  ;  nevertheless,  new  clouds  were  formed,  and 
the  rain  began  again,  accompanying  our  travellers  by 
fits  to  the  bottom  of  the  mountain  ;  when  arrived  there, 
the  clouds  entirely  dispersed,  the  hygrometer  was  again 
observed  in  the  open  air,  and  though  the  earth  was 
drenched  with  water,  and  the  heat  much  less,  the  hygro- 
meter was  ItV  dryer  than  it  had  been  two  days  before, 
after  a  course  of  fine  weather.   Now,  where  was  all  this 


OF    RAIN.  443 

water,  and  all  the  ingredients  of  the  storm,  while  the 
hvgrometer  showed  such  a  degree  of  dryness  in  the  stra- 
tum where  it  was  formed  ? 

The  reasoning  of  M.  de  Luc  is  confirmed  by  the  phe- 
nomena of  fair  weather  ;  continued  evaporation  from 
the  inexhaustible  source  of  vapour,  the  ocean,  and  from 
the  earth  after  it  has  been  soaked  with  rain,  would,  if 
vapour  did  not  change  its  nature  in  the  atmosphere,  ren- 
der it  more  and  more  humid,  and  bring  it  at  last  to  a 
maximum  of  humidity,  as  it  does  under  a  glass  receiver. 
But  experience  shows,  that  though  the  evaporation  con- 
tinues for  several  months  together  on  vast  extents  both 
of  seas  and  continents,  the  air  does  not  become  moister, 
but,  on  the  contrary,  more  and  more  dry.  The  diminu- 
tion of  heat  in  the  night  produces  dew  ;  but  this  symp- 
tom of  humidity  diminishes  from  day  to  day,  and  some- 
times ceases  altogether. 

Many  attribute  the  ordinary  occurrences  of  rain  to 
changes  in  the  winds.  When  it  rains  with  a  south  wind, 
it  is  supposed  that  these  winds  are  warm,  because  they 
come  from  the  south,  and  that  they  are  more  humid  be- 
cause the  greater  heats  in  those  climates  from  which 
they  proceed,  ought  to  produce  a  greater  degree  of  eva- 
poration ;  and  that,  consequently,  when  this  air  meets 
with  a  colder  part  of  the  atmosphere,  the  water  it  con- 
tained would  be  precipitated.  When  it  rains  by  a  north 
wind,  it  is  imagined,  that  this  wind  being  colder  than  our 
air,  produces  the  same  effect  that  this  did  upon  the  south 
wind.  There  are,  however,  various  reasons  which  prove, 
that  these  winds  are  not  the  immediate  cause  of  the  phe- 
nomena. 

To  place  this  hypothesis  in  the  most  favourable  light, 
we  will  suppose  that  one  stratum  of  air  is  at  rest  and  the 
other  in  motion,  and  that  both  are  saturated  with  the  im- 
mediate product  of  evaporation.  But  the  quantity  of  eva- 
porated water,  which  constitutes  saturation  or  a  maximum 
of  humidity  in  the  air,  varies  with  the  temperature,  aug- 
menting or  diminishing  with  the  heat.  The  colder  air 
will,  therefore,  contain  proportionally  less  evaporated 
water  than  the  other.  When  these  two  airs  meet,  the 
one  will  be  cooled,  which  should  produce  a  precipitation 


444  OF    RAIN. 

of  water ;  but  the  other  will  at  the  same  time  be  as  much 
heated,  and  therefore  capable  of  receiving  the  superflu- 
ous water :  at  first  a  mist  may  be  formed,  but  this  will 
not  be  durable ;  for,  as  it  is  in  contact  with  the  air  that 
is  growing  warmer,  it  is  soon  dissipated. 

It  sometimes  rains  with  a  south  wind,  which  seems  to 
embrace  the  whole  height  of  the  atmosphere  ;  here  it  has 
been  gratuitously  supposed  that  this  air  proceeded  from 
the  torrid  zone,  saturated  with  water  thrbughout  its  whole 
height.  Granting  this  supposition,  it  will  not  account  for 
the  phenomena  of  rain ;  I  shall  not  consider  here  the  dif- 
ference in  the  seasons,  which  ought  necessarily  to  influ- 
ence these  phenomena,  which  however  is  not  perceived ; 
for  we  have  often  durable  rains  with  this  wind  in  sum- 
mer, when  the  change  of  climate  will  occasion  little  or 
no  variation  in  its  temperature.  Whatever  change  the 
heat  of  this  air  undergoes,  it  will  gradually  take  place  on 
account  of  the  vicissitudes  of  day  and  night;  for,  as  soon 
as  the  rays  of  the  sun  cease  to  act  upon  our  horizon,  the 
heat  in  the  air  decreases  in  as  great  a  degree  as  that  in 
which  it  existed  at  the  same  hour  of  the  day.  If  the  air 
be  thus  cooled  beyond  a  certain  point,  the  excess  is  preci- 
pitated in  dew  :  besides,  moisture  in  the  atmosphere  is 
daily  destroyed  by  some  cause  of  which  we  are  ignorant, 
and  re-appears  as  suddenly  in  vast  abundance  in  some 
strata,  by  causes  of  which  we  are  equally  ignorant.  If 
you  consider  attentively  the  consequences  of  all  these 
facts,  you  will  see  that  there  is  very  little  probability  that 
air,  which  travels  night  and  day  to  come  to  us,  and  which 
must  necessarily  conform,  and  that  successively,  in  all 
the  intermediate  latitudes,  to  the  various  causes  that  de- 
termine their  mean  degrees  of  humidity  and  heat,  can 
ever  occasion  the  phenomena  attributed  thereto. 

The  remarks  I  have  just  made  on  the  effect  supposed 
to  be  deducible  from  different  winds,  are  formed  from 
the  notions  we  gain  by  observations  made  in  plains.  They 
are  strengthened  and  confirmed  when  connected  with 
observations  made  on  mountains,  for  there  these  winds 
are  found  without  those  deceiving  appearances  which 
favour  the  hypothesis  we  are  combating  ;  they  are  found 
to  convey  cold  there,  while  they  are  communicating  heat 


OF    RAIN.  445 

o  the  plains  below.  Now,  if  the  south  wind  derives  its 
leat  from  the  climate  whence  it  proceeded,  why  is  it  not 
,varm  on  the  tops  of  mouncains  as  well  as  in  the  plains? 
[f  it  be  said,  that  it  is  cold  also  on  the  tops  of  high  moun- 
ains  in  the  torrid  zone,  we  reply,  that  if  so,  this  in  it- 
;elf  is  a  great  mystery  ;  and  further,  that  no  one  can  sup- 
Dose  that  the  superior  air  of  this  zone  preserves  its  posi- 
ion  and  degree  during  its  whole  passage,  and  arrives  in 
he  same  state  at  the  tops  of  the  northern  mountains ; 
ind  we  may  conclude,  that  though  the  air  which  pro- 
:eeds  from  donates  warmer  than  our's  be  then  hotter 
han  the  air  surrounding  us,  yet,  the  greatest  heat  we 
ind  therein  does  not  in  general  proceed  from  this  cause, 
)ut  from  some  difference  in  its  nature,  whereby  the  solar 
-ays  are  rendered  more  powerful  and  more  capable  of 
Droducing  heat  near  the  surface  of  the  earth. 

From  observations  that  have  been  made  on  mountains, 
we  may  draw  the  same  conclusions  with  respect  to  the  hu- 
midity that  generally  accompanies  south  winds  near  the 
earth.  For  from  these  we  find,  that  they  do  not  produce 
the  same  effects  in  the  higher  region  of  the  atmosphere, 
but  accord  with  the  usual  dryness  of  these  superior  stra- 
ta ;  they  are  not,  therefore,  in  themselves  the  immediate 
causes  of  these  differences  ;  for,  if  this  were  the  case,  the 
higher  regions  would  be  as  much  affected  as  the  lower, 
or  they  could  not  be  considered  as  an  assemblage  of  the 
same  fluids.  But  in  this  assemblage  there  may  be  un- 
known fluids,  on  which  the  solar  rays  may  in  the  lower 
regions  have  a  different  influence,  arising  from  circum- 
stances of  which  we  are  ignorant,  perhaps  from  a  greater 
or  less  density  in  the  mass,  or  from  a  difference  in  their 
distances  from  the  soil  of  the  earth.  Every  circumstance 
seems  to  indicate,  that  chemical  operations  are  the  gene- 
ral cause  of  the  phenomena,  though  in  a  manner  un- 
known ;  among  the  agents  concerned,  the  solar  rays 
hold  the  first  rank. 

The  necessity  of  the  solar  rays  for  the  fructification  of 
vegetables  has  been  long  established  ;  Messrs.  Priestley, 
Ingenbouz,  and  Se?inebier,  have  proved  that  this  operation 
is  accompanied  by  great  modifications  in  the  air,  modifi- 
cations which  are  essentially  altered  by  the  presence  or 


446  OF    THE    NATURE    OF    CLOUDS. 

absence  of  the  solar  rays.  By  these  operations  we  see  neu 
solids  rising  before  us ;  arid  yet,  if  we  be  scrupulous  ir 
the  connexion  of  causes  with  effects,  we  must  confess  oui 
inability  of  tracing  here  the  combinations  of  this  firsi 
substance,  which  evidently  puts  in  action  every  substanct 
on  our  globe.  We  know  in  this  instance,  that  some  oj 
the  substances  belong  to  the  atmosphere,  some  to  the 
earth,  and  that  both  are  modified  by  the  solar  rays  :  fire 
also  participates,  but  light  is  a  constituent  part  of  fire : 
water  has  its  share,  but  water  contains  fire  and  light ; 
some  ingredients  of  air  are  joined  thereto  ;  but  of  these 
ingredients,  those  that  are  united  depend  on  the  quantity 
of  light. 

Thus,  however,  new  compounds  are  formed,  possessed 
of  different  colours,  consistency,  odour,  flavour,  and  che- 
mical properties.  All  these  wonderful  operations  are  pro- 
duced by  the  medium  of  the  solar  rays  from  the  atmos- 
phere and  from  the  earth  :  and  these  modifications  taking 
place  on  the  earth  and  on  the  waters,  over  the  whole  sur- 
face  of  the  globe,  must  be  considered  as  a  class  of  causes 
which  have  considerable  influence  in  meteorology. 


OF    THE    NATURE    OF    CLOUDS. 

From  considering  the  causes  of  rain,  I  proceed  to  in- 
vestigate the  nature  of  clouds.  As  it  is  from  these  that 
rain  proceeds,  we  must  acknowledge  the  blessings  we  re- 
ceive through  them,  though  we  are  not  able  to  account 
for  their  various  phenomena.  They  are  continually  tra- 
velling over  our  globe,  and  by  a  proper  distribution  of 
moisture,  rendering  the  spacious  pastures  of  the  wealthy 
fruitful,  and  gladdening  the  little  spot  of  the  cottager. 
"  They  satisfy  the  desolate  and  waste  ground,  and  cause 
the  bud  of  the  tender  herb  to  spring  forth  j"  that  the  na- 
tives of  the  lonely  desert,  the  herds  which  know  no  mas- 
ter's stalls,  may  nevertheless  experience  the  care  of  an 
All-supporting  Parent. 

Clouds  are  composed  of  a  mass  of  vesicles  like  soap- 
bubbles,  which  vesicles  are  easily  perceived  in  proper  situ- 
ations, particularly  on  high  mountains:  these  vesicles  float 


OF    THE    NATURE    OF    CLOUDS.  447 

n  the  air,  rising  or  falling,  till  they  are  in  equilibrium 
vith  the  air,  remaining  suspended  there  as  long  as  they 
reserve  the  same  state.  By  the  nature  of  the  suspen- 
ion  of  these  aqueous  vesicles,  they  do  not  alter  the  pres- 
ure  exercised  by  the  strata  in  which  they  are  inclosed, 
leither  on  itself  nor  on  the  inferior  strata. 

When  the  particles  of  vapour,  properly  so  called,  ap- 
>roach  within  a  certain  distance  of  each  other,  which  is 
letermined  by  the  actual  quantity  of  free  fire,  the  parti- 
:les  of  water,  of  which  they  are  composed,  tend  to  unite, 
md  the  fire  which  quits  them  joins  itself  to  the  remaining 
articles  of  vapour.  From  the  observation  of  M.  de  Luc 
t  appears,  that  vesicles  of  liquid  water  may  be  formed, 
md  exist  when  the  temperature  of  the  air  is  at  freezing, 
have  already  shown  you,  that  there  is  something  else 
>esides  cold,  necessary  to  the  formation  of  ice.  It  is  hence 
ve  see  mists,  fogs,  and  clouds,  when  the  thermometer  is 
mder  32°.  There  is  never,  however,  any  great  cold  in 
bgs  or  in  clouds ;  for  the  cause,  whatever  it  may  be,  that 
>rings  vapour  beyond  its  maximum,  disseminates  also 
leat.  Aqueous  vesicles  never  freeze  without  changing 
heir  state ;  but  if  the  bubbles  be  broken  when  the  air 
s  at  or  under  32°,  the  water  thereof  freezes  :  when  this 
lappens  in  the  midst  of  clouds,  snow  is  the  consequence, 
vhose  duration,  like  that  of  rain,  depends  on  the  quantity 
)f  vesicles  that  are  brought  within  a  certain  distance  of 
:ach  other ;  the  destroyed  vesicles  then  group  themselves 
nto  flakes  of  snow  by  a  crystallization,  somewhat  similar 
o  what  are  termed  by  chemists  sublimations,  u  e.  the  pre- 
stations of  substances  dissolved  by  fire.  If  the  quan- 
ity  of  aqueous  vesicles  be  too  small  to  unite  and  be  de- 
troyed  by  being  brought  near  together,  they  may  be  de- 
troyed  and  frozen  by  causes  similar  to  those  which  fix 
ublimates  to  the  sides  of  the  furnace  and  other  receivers, 
>r  which  determine  congelation  in  water  sufficiently  cool- 
id.     In  this  case  a  hoar  frost  is  formed. 

Clouds  are  always  composed  of  bubbles,  formed  of 
iquid  water,  and  they  are  generally  at  a  temperature 
rery  little  above  the  freezing  point ;  the  existence  of 
hese  vesicles  or  bubbles  is  but  of  short  duration,  they 
ise  and  are  destroyed  successively,  like  the  brilliant 


448  OF    THE    DURATION    OF    CLOUDS. 

sparks  we  often  perceive  rising  from  a  chafing-dish,  when 
the  coals  are  animated  by  a  pair  of  bellows. 


OF    THE    DURATION    OF    CLOUDS. 


swn 


Whenever  we  see  a  mist  or  a  fog  formed  by  a  known 
cause,  we  are  always  certain  that  the  vapours,  from  which 
it  proceeds,  are  passing  rapidly  beyond  their  maximum ; 
and  the  mist  ceases  when  no  fresh  vapour  arrives  for  its 
support.    The  principal  known  causes  of  mists  and  fogs 
are  either  the  ebullition  of  water  in  open  air  at  all  tem- 
peratures, the  transpiration  and  respiration  of  animals  in 
winter,  the  evaporation  from  hot  springs  in  the  sam 
seasons,  and  the  fogs,  properly  so  called,  that  happen  i 
autumn.    In  all  these  cases  we  know,  that  the  vapour  i 
produced  in  too  great  abundance  for  the  temperature  o 
the  neighbouring  air  ;  hence  a  rapid  destruction  of  a  part 
of  those  which  arrive  in  that  space  which  is  occupied  b 
the  fog.     Meanwhile,  the  fog  only  occupies  a  certai 
space,  which  is  nearly  fixed  as  long  as  the  circumstance 
remain  the  same ;  in  a  word,  we  always  find  fogs  an 
mists  to  cease,  as  soon  as  the  cause  producing  the  va- 
pours ceases  to  furnish  them  beyond  the  maximum  suit- 
able to  the  temperature  of  the  air  ;  the  vesicles  are  form- 
ed by  a  rapid  decomposition  of  superfluous  vapours ;  as 
soon  as  this  ceases,  the  vesicles  are  dissipated. 

From  a  review  of  known  facts  you  will  find,  that  the 
following  conclusions  are  well  founded.  1st.  That  vesi- 
cles are  only  formed  in  those  cases  where  vapours  get 
beyond  their  maximum.  2d.  That  these  vesicles  are  con- 
crete water,  subject  to  evaporation  like  any  other  water, 
and  which  always  evaporate  when  the  surrounding  air  is 
not  at  the  extreme  point  of  humidity.  3d.  It  is  this  last- 
mentioned  circumstance  which  determines  the  extent  of 
space  occupied  by  a  cloud  or  fog;  for  these  vesicles  only 
exist  in  that  part,  where  the  source  of  vapours,  whatever 
it  may  be,  having  produced  extreme  humidity,  dissemi- 
nates superfluous  vapours  ;  so  that  beyond  this  space  the 
vesicles  evaporate.  4thly  and  lastly.  This  evaporation  is 
prevented  in  whole  or  in  part,  either  by  obstacles  that 


OF    THE    DURATION    OF    CLOUDS.  440 

oppose  the  expansion  of  the  mist  or  fog,  or  because  the 
source  of  vapours  furnishes  them  so  rapidly,  that  the 
vesicles  approach  near  enough  to  unite,  even  in  the  midst 
of  the  fog,  which  occasions  them  to  unite,  and  the  result 
is  a  distillation  of  water.  From  hence  we  may  conclude, 
that  when  a  cloud  is  formed  in  air,  whatever  be  the 
cause,  it  can  only  subsist  there  while  aqueous  vapours 
continue  to  be  produced  in  the  same  place.  Thus,  the 
extent  occupied  by  a  cloud  is  an  indication  of  the  cause 
which  produces  vapours,  or  of  its  intensity  in  some 
part  of  this  space :  extreme  humidity  exists  but  very 
little  beyond  the  extent  of  the  cloud,  and,  as  soon  as 
the  cause  which  furnishes  the  vapour  ceases,  the  cloud 
dissipates. 

We  have  been  accustomed  to  see  clouds  from  our 
earliest  infancy  ;  they  therefore  neither  excite  attention, 
nor  awaken  admiration  ;  and  yet,  of  ail  the  objects 
which  surround  us,  there  is  none  more  truly  wonder- 
ful, or  more  worthy  of  attention.  Those  also,  who 
have  but  little  studied  the  laws  of  hygrology,  are  very 
little  astonished  at  these  appearances,  because  they 
either  suffer  themselves  to  be  amused  by  words,  or  rest 
satisfied  with  a  few  seducing  glimpses :  those  who 
have  considered  the  laws  of  hygrology,  and  weighed  all 
the  circumstances,  find  they  are  only  carried  to  the 
boundaries  of  known  causes  ;  but  they  also  know  how 
to  stop  and  wait  there,  till  fresh  light  enables  them  to 
proceed  further. 

The  traveller  who  has  frequented  high  mountains 
knows,  that  clouds  are  a  species  of  fog  or  mist,  much 
resembling  those  we  perceive  on  plains  ;  he  has  also  re- 
marked, that  where  clouds  are  scattered  in  the  air,  the 
strata  where  they  are  met  with  are  not  at  extreme  hu- 
midity. Among  other  instances,  M.  de  Luc  mentions 
one  where  he  saw  his  own  shadow,  and  that  of  the  rock 
on  which  he  was  situate,  projected  on  a  cloud  beneath 
him,  in  a  stratum  where  there  were  many  other  similar 
clouds  extending  to  a  considerable  distance.  The  air 
was  very  serene,  and  there  was  not  the  least  symptom 
of  extreme  humidity.  How  are  such  clouds  preserved  ? 
Whence  do  they  increase  to  the  eye  ?  Why,  as  they 

VOL.  IV,  3  M 


450  OF  THE    DURATION    OF    CLOUDS. 

are  continually  evaporating,  are  they  not  immediately 
dissipated  ? 

The  evaporation  of  clouds,  even  while  they  are  in- 
creasing in  size,  is  a  circumstance  of  which  you  may 
easily  be  satisfied,  by  considering  attentively  the  brok- 
en edge  of  a  cloud,  which  has  an  azure  ground  behind 
it.  These  edges  present  to  the  imagination  a  thousand 
grotesque  forms,  which,  by  their  striking  changes, 
will  assist  you  in  your  researches.  Often  you  may 
perceive  the  part  you  are  looking  at  dissipated  in  the 
place  where  it  was  first  observed  ;  often  it  stretches  it- 
self out,  the  cloud  remaining  stationary,  and  vanishes 
while  it  is  thus  extending  itself.  Sometimes,  while  one 
festoon  vanishes  others  are  formed,  by  which  the  cloud 
is  enlarged ;  at  other  times  it  diminishes,  the  festoons 
successively  evaporating,  till  the  whole  disappears.  It 
is  impossible  to  consider  these  various  metamorphoses 
of  the  same  cloud,  without  supposing  that  there  is  in 
the  air  a  source  of  vapours,  which  are  produced  in  the 
place  where  the  cloud  is  formed,  and  that  it  is  by  the 
continued  production  of  fresh  vapour  that  the  cloud 
subsists  and  increases,  though  continually  evaporating. 
When  they  wholly  disappear,  it  is  because  the  evapo- 
ration is  not  repaired  by  the  formation  of  fresh  vapour. 
These  phenomena  are  independent  of  heat  and  cold, 
for  clouds  are  sometimes  formed  suddenly  in  the  midst 
of  a  hot  day,  and  after  they  have  poured  down  their 
water,  all  is  clear  again.  Sometimes  they  evaporate 
after  sun-set,  gradually  vanishing  in  the  calmest  wea- 
ther, without  change  of  place.  The  appearances,  on  the 
whole,  are  such  as  would  be  produced  by  a  large  mass 
of  water  in  violent  ebullition,  suspended  invisibly  in  the 
atmosphere ;  and  the  similarity  in  the  effect  naturally 
points  out  an  analogy  in  the  cause,  that  is,  a  source  of 
vapour  in  the  atmosphere. 

When  it  rains,  the  source  which  furnishes  vapours 
produces  them  in  such  abundance,  that  the  vesicles  which 
are  formed  are  driven  against  each  other  even  in  the  be- 
som of  the  cloud ;  and  not  having  time  either  to  dis- 
perse or  evaporate,  they  are  united  ;  and  the  water  fall- 
ing to  the  lowest  part,  as  in  soap-bubbles,  they  are  soon 


OF    THE    DURATION    OF    CLOUDS.  451 

burst,  and  fall  as  rain.  It  is  to  these  surcharged  vesicles 
we  must  attribute  the  pendent  fringes  which  are  some- 
times seen  under  the  clouds  towards  the  horizon.  Ex- 
perience has  shown,  that  it  rains  under  those  clouds ; 
not  that  these  fringes  are  rain  itself,  but  the  vesicles 
which  fall  by  the  augmentation  of  their  weight.  As  drops 
of  rain  are  formed  their  vesicles  are  destroyed. 

Lasting  rains  proceed  from  strata  of  clouds  which  co- 
ver the  whole  heavens ;  and  it  is  these  that  have  the 
greatest  connexion  with  the  fall  of  the  mercury  in  the 
barometer.  The  source  of  vapours  comprehending  a 
stratum  of  considerable  extent,  the  barometer,  after  it 
has  announced  these  rains,  generally  rises,  and  continues 
to  rise,  as  long  as  they  last.  This  is  a  fact  observed, 
but  to  us  inexplicable.  It  is  no  doubt  connected  in 
some  way  with  the  primitive  cause  of  rain,  but  with  that 
cause  we  are  unacquainted.  The  relation  of  rain  with 
the  barometer  is  a  subject  as  obscure  as  the  cause  of  rain 
itself. 

In  the  midst  sometimes  of  the  finest  days,  and  while 
ordinary  symptoms  indicate  that  the  air  is  dry,  and  this 
as  well  in  the  vallies  as  on  the  mountains,  bright  and 
heavy  clouds  appear  on  azure  ground,  announcing  sud- 
den rains.  Sometimes  these  clouds  increase  enormously 
and  descend  ;  other  clouds  form  about,  and  unite  to 
them  ;  the  air  is  darkened,  as  if  a  curtain  was  drawn  be- 
tween heaven  and  earth.  From  the  tops  of  high  moun- 
tains, these  clouds  may  be  often  seen  to  accumulate  ra- 
pidly over  the  plains ;  while  from  these  the  azure  ground 
of  the  heavens  disappears  ;  the  wind  often  rises,  and 
blows  from  different  quarters  in  a  kind  of  whirlwind  ; 
and  lastly  it  pours  with  rain.  As  soon  as  the  rain  ceases, 
the  curtain  is  withdrawn,  and  the  calm  is  restored,  the 
sun  re-appears,  and  no  other  vestiges  remain  of  this 
grand  phenomenon  but  the  water  that  is  on  the  ground. 

When  the  air  is  disposed  to  product  this  phenome- 
non, you  will  often  see  the  clouds  rising  from  the  hori- 
zon ;  sometimes  from  the  side  where  the  wind  proceeds, 
sometimes  from  other  quarters.  Often  theLe  heavy 
showers  are  partial ;  sometimes  they  re-commence  at  in- 
tervals, accompanied  with  heavy  squalls.     Sometimes 


452  OF    THUNDER- 


these  heavy  intermitting  showers  are  a  prelude  of  moFe 
lasting  rains ;  in  which  case  the  clouds  unite,  and  the 
wind  goes  down,  and  you  have  one  or  more  successive 
days  of  rain. 

OF    HAIL, 

Sudden  storms,  accompanied  with  hail  and  thunder, 
are  among  the  number  of  phenomena  which  show  how 
ignorant  we  are  of  the  causes  of  those  that  we  observe  in 
the  atmosphere.  Hail  is  a  sign  of  a  great  degree  of 
cold  ;  but  what  is  the  immediate  cause  thereof  ?  Whence 
a  substance,  that  must  require  so  intense  a  cold  for  its 
formation,  in  seasons  so  warm  as  those  in  which  hail  is 
chiefly  formed  ?  It  is  supposed  in  general,  that  hail- 
stones are  drops  of  rain,  which,  falling  through  a  cold- 
er region  of  air,  are  congealed  in  their  passage  into  a 
rarefied  sort  of  ice.  Dr.  Halley  gives  an  account  of 
hailstones  that  weighed  five  ounces  each,  and  says,  it 
is  very  extraordinary  that  such  sort  of  vapours  should 
continue  undispersed  in  so  long  a  tract  as  sixty  miles  to- 
gether ;  and  in  all  the  way  of  its  passage  occasion  so  ex- 
traordinary a  coagulation  and  congelation  in  the  watery 
clouds,  as  to  increase  the  hailstones  to  so  vast  a  bulk 
in  so  short  a  space  as  that  of  their  fall. 


OF    THUNDER. 

All  the  phenomena  of  stormy  clouds  are  obscure,  and 
I  am  afraid  there  is  very  little  probability  of  explaining 
them  independently  of  each  other.  Those  that  are  sa- 
tisfy d  with  conjectures  may  find  enough  at  their  ser- 
vice ;  but  he  who  conducts  himself  by  the  "  scale  and 
chart  of  truth,"  will  find  little  to  depend  upon.  It  is 
thus  with  thunder  and  lightning :  we  can  neither  ac- 
count for  the  immense  quantities  of  electricity  discharg- 
ed by  the  one,  nor  the  rumbling  noise  of  the  other. 

Mr.  Volta  supp  )sed,  that  water,  by  being  changed  in- 
to vapour,  acquired  a  greater  capacity  for  the  electric 
fluid,  and  that  thus  electricity  was  continually  convey- 


OF    THUNDER.  453 

ed  to  the  atmosphere  by  evaporation ;  and  this  he  de- 
duced  from  an  experiment,  in  which  water  being  eva- 
porated from  a  body,  left  that  body  negatively  electri- 
fied. This,  however,  is  by  no  means  satisfactory  ;  for, 
not  to  insist  on  the  fallacy  of  the  terms  positive  and  ne- 
gative ^  as  both  electricities  may  be  produced  by  evapo- 
ration, if  the  electric  fluid  passed  from  the  earth  to  the 
atmosphere  by  evaporation,  and  its  return  was  occasion- 
ed by  the  reduction  of  vapour  into  water,  there  would 
always  be  more  or  less  lightning  when  there  was  vio- 
lent and  sudden  rain,  for  in  this  case  it  would  be  rapid- 
ly disengaged  ;  but  there  is  much  oftener  violent  and 
sudden  rain  without  than  with  lightning.  In  this  case 
lightning  also  should  always  be  preceded  by  rain, 
whereas  there  is  often  lightning  among  the  clouds  with- 
out any  rain.  Further,  if  we  are  unable  to  explain 
rain  by  the  vapours  which  existed  in  the  air  before  the 
formation  of  the  clouds,  the  source  of  electricity  exist- 
ing in  the  clouds  ought  not  to  be  sought  for  in  vapour. 
Indeed,  on  this  supposition,  as  soon  as  there  was  a  vio- 
lent rain  the  lightning  would  cease,  and  the  fluid  would 
pass  cff  by  the  drops,  illuminating  the  air  by  its  passage 
from  drop  to  drop. 

There  seems  to  be  no  other  mode  of  considering 
lightning,  than  as  an  explosion,  that  is,  as  a  sudden 
production  of  a  great  quantity  of  the  electric  fluid  ;  the 
fluid  which  is  then  manifested  not  existing  as  such  but 
just  before  we  perceive  its  effects ;  just  as  the  vapour, 
of  which  the  clouds  are  formed,  do  not  exist  as  vapour 
in  the  air  until  the  moment  of  their  appearance :  the 
air,  as  yet  transparent,  contained  neither  the  vapour  of 
which  the  cloud  is  formed,  nor  the  electric  fluids,  but 
the  ingredients  proper  to  give  birth  to  both  of  them. 
By  some  cause,  of  which  we  are  ignorant,  clouds  of  a 
certain  kind  are  formed.  During  the  progress  of  their 
formation,  and  by  fits,  the  electric  fluid  is  produced  in 
great  abundance,  and  explodes  every  time  it  is  thus 
produced.  Observations  made  among  mountains  where 
clouds  are  formed,  point  out  this  to  be  the  result  of  the 
phenomena. 


454?  OF    THUNDER, 

In  a  storm  observed  by  M,  de  Luc  on  the  Buet,  he 
had  an  opportunity  of  observing  this  phenomenon  with 
all  its  modifications.  The  air  of  the  strata  where  he 
was  situate  was  perfectly  transparent  and  dry ;  the 
thermometer  at  6  of  Reaumur.  Notwithstanding  this, 
clouds  formed  here  and  there :  by  degrees  they  aug- 
mented, then  became  united,  embracing  the  summit  of 
the  Buet,  and  supporting  themselves  against  Mont 
Blanc,  and  the  summits  of  the  neighbouring  mountains. 
M.  de  Luc  and  his  companions  were  inundated  with 
rain :  though  the  clouds  and  rain  formed  a  complete 
conductor,  communicating  with  the  ground,  yet  there 
was  a  continuance  of  thunder  for  a  considerable  time, 
and  often  very  violent.  Other  instances  may  be  found 
in  the  works  of  M.  de  Saussure  of  thunder  storms, 
where  the  clouds  formed  a  conducting  communication 
with  the  ground,  and  yet  where  the  thunder  succeeded 
without  interruption.     ' 

The  rumbling  noise  of  thunder  has  been  explained 
by  a  supposed  analogy  between  the  passage  of  lightning 
and  the  electric  spark  through  the  air.  This  explana- 
tion might  have  been  admitted  as  plausible,  if  the  rumb- 
ling noise  of  thunder  had  grown  weaker  and  weaker, 
as  being  a  succession  of  sounds  proceeding  successively 
from  points  more  and  more  distant ;  whereas  the  sound 
of  thunder  often  increases,  and  gives  us  a  distinct  per- 
ception of  its  proceeding  from  points  which  are  nearer 
to  us  than  those  from  which  it  set  out.  It  is  sometimes 
intermingled  with  such  terrible  claps,  as  deprive  the  hy- 
pothesis of  all  probability  ;  or  other  inconsistencies 
therein  might  be  pointed  out.  The  rumbling  and  re- 
peated echoes,  &c.  of  thunder  still  remain  among  the 
phenomena  not  yet  accounted  for. 

In  general,  a  course  of  hot  weather  precedes  a  thun- 
der-storm ;  and  it  seldom  happens,  that  very  hot  wea- 
ther in  the  summer  terminates  without  a  storm  of  thun- 
der. Hence  also  in  the  East  and  West  Indies,  where 
the  climate  is  so  much  hotter,  thunder  and  lightning 
are  not  only  much  more  frequent,  but  much  more  \ io 
lent,  than  in  this  country. 


[     455     ] 


OF    WINDS. 


Of  winds,  the  observation  of  our  Saviour  is  still  just  ; 
we  hear  the  sound  of  the  wind  as  it  passes  by,  but  we 
neither  know  from  whence  it  comes,  nor  whither  it 
goes ;  we  cannot  determine  how  it  originates,  or  why 
it  ceases.  The  great  Bacon  indeed  was  of  opinion,  that 
by  a  close  and  regular  history  of  the  wind,  continued 
for  a  number  of  ages  together,  and  the  particulars  of 
each  observation  reduced  to  general  maxims,  we  might 
at  last  come  to  understand  the  variations  of  this  capri- 
cious element,  and  be  able  to  foretel  the  certainty  of  a 
wind  with  as  much  ease  as  we  now  foretel  the  return 
of  an  eclipse.  Indeed  his  own  beginnings  in  this  ardu- 
ous task  seem  to  speak  the  pcssibility  of  its  success ; 
but  unfortunately  this  investigation  is  the  work  of  ages, 
and  we  want  a  Bacon  to  direct  the  process. 

In  the  Historia  Ventorum,  Bacon  reckons  three  sources 
of  winds ;  one  by  descent  from  the  superior  regions  of 
the  atmosphere,  another  from  the  expansion  of  the  low- 
er air,  and  a  third  by  expiration  from  the  earth  :  of 
which  last  he  proposes  it  as  an  object  of  inquiry,  What 
winds  blow  out  of  subterraneous  caverns  ?  Whether 
they  come  forth  in  a  large  body,  or  blow  insensibly 
here  and  there ;  and  then  unite  in  one  stream,  like  a 
river  formed  out  of  many  different  springs  ?  This  latter 
cause  has  been  but  little  attended  to,  though  this  reci- 
procation between  the  earth  and  air  is  surely  a  very  in- 
teresting part  of  natural  philosophy.  In  the  language 
of  Holy  Writ,  God  is  said  "  to  bring  the  winds  out  of 
his  treasures  jw  as  if  some  hidden  storehouse  were  al- 
luded to,  such  as  that  of  the  waters  and  cavities  in  the 
bowels  of  the  earth. 

The  annual  revolution  of  the  sun  is  doubtless  a  gene- 
ral cause  of  winds ;  but  this  cause,  considered  alone, 
should  produce  regular  winds,  whose  progress  would 
correspond  to,  and  be  connected  with  the  seasons  ;  the 
phenomena  however  observed  by  no  means  enable  us  to 
perceive  this  connexion.  There  is  another  cause,  of  which 
we  may  form  an  imperfect  idea,  by  which  the  winds, 


456  OF    WINDS. 

proceeding  from  the  south,  may  be  south  west  to  us, 
and  those  which  come  from  the  north,  north-easr.  This 
cause  is  the  difference  in  the  velocity  of  the  motion  of 
the  parts  of  the  earth  we  inhabit,  and  that  at  the  equa- 
tor, or  polar  circles.  If  the  air  was  calm  at  the  t  quator, 
that  is,  moved  with  the  same  velocity  as  the  earth,  and 
that  in  coming  from  thence  to  us  in  the  same  direction, 
preserving  at  the  same  time  a  portion  of  its  acquired 
motion,  it  would  gain  upon  the  earth  in  this  direction, 
and  would  thus  become  south-west.  The  same  c<tuse 
inverted  would  change  the  north  for  us  into  a  north- 
east wind.  Another  cause,  though  very  inconsiderable, 
may  be  found  in  the  different  diurnal  positions  of  the 
sun  :  this,  in  calm  weather,  often  occasions  a  gentle 
east  wind  after  sun-rising,  and  a  west  wind  after  his 
setting. 

Of  these  causes  we  have  some  knowledge  ;  but  there 
must  be  many,  and  more  powerful  ones,  to  produce 
those  phenomena  to  which  we  are  continually  witness- 
es, and  to  which  these  seem  to  have  little  or  no  affinity. 
Evaporation  and  rain  have  been  considered  as  causes ; 
but  they  are  also  by  no  means  adequate  to  the  purpose. 
Evaporation  is  constantly  operating ;  it  is  also  more 
abundant  in  those  placts  where  the  heat  is  greatest. 
These  places  are  continually  varying  ;  but  still  the  va- 
riations in  evaporation  are  so  slow,  and  the  differences 
in  heat  so  insensible  from  one  place  to  another,  that  it 
can  never  occasion  any  sudden  and  violent  wind.  Rain, 
which  is  the  inverse  of  evaporation,  operates  with  more 
rapidity :  but  the  same  reasons  which  prove  that  rain 
cannot  be  formed  of  the  immediate  product  of  evapo- 
ration, also  prove,  that  the  precipitation  of  this  product, 
in  any  stratum  of  air,  cannot  make  a  sufficient  vacuum 
to  cause  the  surrounding  air  to  press  in  with  violence. 

We  must  then  have  recourse  to  some  other  cause  to 
explain  the  winds  which  accompany  the  rapid  formation 
and  destruction  of  clouds  ;  and  this  may  be  found  in 
the  return  of  air  to  a  state  of  vapours.  It  is  known 
from  experiment,  that  in  similar  cases  there  is  a  great 
increase  of  volume  in  the  new  fluid  ;  as  in  the  sudden 
explosion  of  inflammable  air  with  vital  or  common  air. 


of  winds.  457 

When  the  air  is  changed  into  aqueous  vapour  in  the 
atmosphere,  there  is  probably  a  considerable  expansion 
of  the  stratum  where  this  change  happens,  and  the  ef- 
fect is  more  or  less  extensive  in  proportion  to  the 
strength  of  the  cause.  If  the  production  of  the  clouds 
be  slow,  if  they  embrace  a  very  great  portion  of  the  at- 
mosphere, and  if  the  operation  be  carried  on  at  a  great 
height,  but  little  agitation  will  be  perceived  in  the  air 
under  these  strata :  the  columns  thereof  extending 
lengthwise,  produce  in  distant  countries  winds,  of  which 
the  inhabitants  can  no  more  perceive  the  causes,  than 
those  near  which  it  originated.  But  if  the  clouds  be 
formed  rapidly,  if  they  occupy  but  a  small  space  and 
be  not  very  high,  violent  winds  may  be  occasioned  by 
the  sudden  expansion  of  the  medium  where  they  are 
formed.  As  the  quantity  of  vapour  that  is  the  imme- 
diate product  of  the  evaporation  is  always  very  small, 
the  formation  of  drops  of  rain,  on  the  common  system, 
would  only  produce  insensible  and  trifling  motions  of 
the  air.  But  in  M.  de  Luc's  system,  the  successive 
production  of  vapour  in  the  midst  of  the  air  is  unlimit- 
ed ':  their  accumulation  in  the  form  of  vesicles  may  be 
immense ;  and  when  they  are  resolved  into  drops,  a 
considerable  vacuum  is  the  natural  consequence. 

From  this  view  of  the  origin  of  winds,  we  may  see 
also  why,  in  a  season  of  storms  and  showers,  a  cold 
heavy  cloud,  passing  over  the  head  with  a  hasty  fall  of 
snow  or  hail,  is  often  attended  with  a  sudden  violent 
gust  of  wind,  such  as  sailors  call  a  squall,  which  sub- 
sides into  a  calm  with  the  departure  of  the  cloud  ;  till 
another  cloud,  coming  in  the  same  direction,  brings  a 
fresh  blast.  No  tempest,  hurricane,  or  whirlwind,  ever 
happens  under  a  cloudless  sky.  We  may  hence  per- 
ceive why  a  whistling  or  howling  noise  of  the  wind  is 
the  most  infallible  prognostic  of  rain,  indicating  the 
formation  of  rainy  clouds.  The  sacred  scripture  seems 
to  agree  with  this ;  for  the  prophet  Elijah,  before  any 
Other  symptom  of  the  weather  appeared,  seemed  to  give 
notice  to  Ahab  from  this  one  :  "  Get  thee  up5  and  eat 
and  drink,  said  he,  for  there  is  a  sound  of  abundance  of 

VOL.  IV.  N 


458  OF    TRADE-WINDS 

rain."     Then  it  follows,  "  that  the  heaven  was  soon 
black  with  clouds  and  wind,  and  there  was  a  great  rain." 


OF    TRADE-WINDS    AND    MONSOONS. 

There  are  many  parts  of  the  world  where  the  winds, 
that  with  us  are  so  uncertain,  pay  their  stated  visits.  In 
some  places,  the  winds  are  found  to  blow  one  way  by- 
day,  another  by  night ;  in  others,  for  one  half  of  the 
year,  they  go  in  a  direction  contrary  to  their  former 
course  :  but  what  is  more  extraordinary,  there  are  some 
places  where  the  winds  never  change,  but  for  ever 
blow  the  same  way.  This  is  particularly  found  to  ob- 
tain between  the  tropics  in  the  Atlantic  ocean  and  great 
Pacific  sea. 

Between  the  limits  of  60  degrees,  namely,  from  30° 
Of  north  latitude  to  30°  of  south  latitude,  there  is  a 
constantly  easterly  wind  throughout  the  year,  blowing 
on  the  Atlantic  and  Pacific  oceans ;  and  this  is  called 
the  trade-wind. 

The  trade-winds  near  the  northern  limits  blow  be- 
tween the  north  and  the  east ;  and  near  the  southern 
limits,  they  blow  between  the  south  and  the  east. 

These  general  motions  of  the  wind  are  disturbed  on 
the  continent,  and  near  the  coasts. 

Beyond  the  northern  limit  of  the  general  wind,  on 
the  Atlantic  ocean,  the  westerly  winds  prevail,  but  not 
with  any  certainty  of  continuance. 

In  the  Atlantic  ocean,  the  S.E.  trade- wind  extends 
as  far  as  three  degrees  north  ;  and  the  N.E.  trade-wind 
ceases  at  the  fifth  degree  N.  In  the  intermediate  space 
are  found  calms  with  rain,  and  irregular  uncertain 
squalls  attended  with  thunder  and  lightning :  but  this 
space  is  shifted  farther  to  the  northward  or  southward, 
according  as  the  sun's  declination  is  more  northerly  or 
southerly. 

In  the  Indian  ocean  there  are  periodical  winds,  called 
monsoons,  that  is,  such  as  blow  six  months  in  one  direc- 
tion, and  the  other  six  months  in  an  opposite  direction ; 
the  change  of  their  direction,  which  is  near  the  aur.um- 


AND    MONSOONS.  459 

rial  and  vernal  equinoxes,  is  accompanied  with  violent 
storms  of  wind,  thunder,  and  lightning.  Voyagers  to 
India  endeavour  to  time  their  voyages,  so  as  to  benefit 
by  these  winds. 

On  or  near  the  coast  of  Guinea  the  winds  blow  almost 
always  from  the  west  and  south-west  points.  Between 
the  Cape  Verd  and  the  easternmost  of  the  Cape  Verd 
islands,  there  is  a  tract  of  sea,  which  is  a  perpetual  calm 
with  respect  to  wind ;  but  the  thunder  and  lightning 
there  are  terrible. 

The  varieties  and  deviations  both  in  general  and  par- 
ticular  winds  are  far  from  being  known ;  you  cannot, 
therefore,  expect  any  theory  that  will  solve  them  all; 
there  are  difficulties  which  perplex  every  hypothesis  that 
has  hitherto  been  suggested,  and  that  cannot  be  cleared 
up  at  present. 

The  best  account  we  have  of  the  trade-winds  is  that  of 
Mr.  Dalton,*  namely,  That  as  the  heat  is  always  greatest 
in  the  torrid  zone,  and  decreases  in  proceeding  north- 
ward and  southward,  with  respect  to  these,  the  poles  may 
be  always  considered  as  centres  of  cold ;  so  that,  ab- 
stracting from  accidental  circumstances,  there  must  be  a 
constant  ascent  of  air  over  the  torrid  zone,  which  after- 
wards falls  northward  or  southward,  whilst  the  colder 
air  below  is  determined  by  a  constant  impulse  towards 
the  equator.  In  general,  where  the  heat  is  greatest,  the 
heated  air  will  ascend,  and  a  supply  of  colder  air  will  be 
received  from  the  neighbouring  parts. 

The  following  effects  may  be  attributed  to  the  diurnal 
motion  of  the  earth ;  the  air  over  any  part  thereof,  when 
calm,  will  have  the  same  rotatory  velocity  as  that  part ; 
but  if  a  quantity  of  air  in  the  northern  hemisphere  receive 
an  impulse  in  the  direction  of  the  meridian,  either  north- 
ward or  southward,  its  rotatory  velocity  will  be  greater 
in  the  fomer,  and  less  in  the  latter  case,  than  that  of  the 
air  into  which  it  moves  ;  consequently,  if  it  move  north- 
ward, it  will  have  a  greater  velocity  eastward  than  the 
air,  or  surface  of  the  earth  over  which  it  moves,  and  will 


*  Ualton's  Meteorological  Observations. 


460  OF    TRADE-WINDS 

therefore  become  a  S.  W.  wind,  or  a  wind  between  the 
South  and  the  west ;  and,  vice  versa,  if  it  move  south- 
ward, it  becomes  a  N.  E.  wind.  Likewise  in  the  southern 
hemisphere  it  will  appear,  that  the  winds,  upon  similar 
suppositions,  will  be  N.  W.  and  S.  E.  respectively. 

From  this  view  of  the  air,  Mr.  Dalton  attempts  to  ex- 
plain the  trade-winds  ;  he  considers  two  general  masses 
of  air,  as  proceeding  from  both  hemispheres  towards  the 
equator  ;  these,  as  they  advance,  are  constantly  deflected 
more  and  more  towards  the  east,  on  account  of  the  earth's 
rotatory  motion.  That  from  the  northern  hemisphere, 
originally  a  north  wind,  is  made  to  veer  more  and  more 
towards  the  east ;  and  that  from  the  southern  hemisphere 
is  made  to  veer  from  the  south  towards  the  east :  these 
two  masses  meeting  in  the  torrid  zone,  their  north  and 
south  velocities  destroy  each  other,  and  they  proceed 
with  their  common  velocity  from  east  to  west  round  the 
torrid  zone.  The  equator  is  not  the  centre  of  the  con- 
course, but  the  northern  parallel  of  4°,  because  the  cen- 
tre* of  heat  is  at  that  place,  the  sun  being  longer  on  the 
north  side  of  the  equator  than  on  the  south-side.  Though 
all  the  parts  of  the  atmosphere  seem  to  conspire  in  pro- 
ducing regular  winds  round  the  torrid  zone,  yet,  from 
the  situation  of  the  land  or  ol!ier  causes,  striking  irregu- 
larities are  produced,  as  is  evident  from  the  monsoons, 
sea  and  land  breezes,  &c.  these  are  deviations  from  the 
general  rule,  but  this  will  ever  be  more  or  less  the  case 
with  all  human  theories. 

To  explain  the  monsoons,  it  is  necessary  to  attend  to 
the  circumstances  that  are  peculiar  to  the  Indian  ocean, 
and  which  are  not  found  in  the  Atlantic  andPacific  oceans. 
They  seem  to  be  these  :  that  the  Indian  ocean  is  bounded 
to  the  northward  by  shores,  whose  latitude  does  not  ex- 
ceed the  limits  of  the  general  trade-wind  ;  and  that  the 
general  trade- wind  falls  on  what  sailors  term  lee  shores  to 
the  westward. 

The  sun  being  twice  a  year  vertical  in  the  equator,  and 
never  departing  thence  more  than  23^  degrees,  causes  the 


*  Prcvost  sur  les  Limites  des  Vents  Atizes. 


AND    MONSOONS.  461 

dr  in  that  climate  to  be  hotter  than  at  any  other  place  in 
he  ocean  :  such  a  rarefied  space  must  extend  across  the 
ndian  ocean,  and  produce  a  S.  E.  wind  to  the  southward, 
,nd  a  N.  E.  wind  to  the  northward  of  the  equator,  over 
vhich,  in  the  upper  regions  of  the  air,  the  winds  return 
q  the  contrary  direction.  This  we  accordingly  see  happen 
q  the  months  of  October,  November,  December,  Janu- 
xy,  February,  and  March.  But  when  the  sun  declines  to 
he  northward,  and  heats  the  land  there,  the  air  contigu- 
ms  to  those  lands  is  rarefied,  and  the  lower  air  has  a  ten- 
lency  to  move  that  way :  this  tendency  increases  as  the 
un  advances  further  north,  so  that  the  whole  body  of 
he  lower  air,  to  the  northward  of  the  equator,  moves  to- 
vards  the  northern  lands,  notwithstanding  the  equatorial 
arefaction.  It  seems  then,  that  the  body  of  the  lower 
ir  in  the  northern  part  of  the  Indian  ocean  is  determined, 
is  to  its  course,  by  the  greater  rarefaction.  If  the  rare- 
action  at  the  surface  of  the  land  be  greater  than  that  at 
he  equator,  the  wind  blows  to  the  north  ;  and  the  con- 
rary  happens  when  the  equatorial  rarefaction  is  the  great- 
:st.#  Thus  it  appears,  that  it  is  the  situation  of  the  lands, 
Lnd  their  effect  in  heating  and  rarefying  the  atmosphere, 
vhich  are  the  principal  causes  of  the  monsoons.  Still, 
lowever,  it  must  be  owned,  that  the  explanation  is  im- 
>erfect,  and  the  observations  we  possess  too  few  to  form 
i  theory.  In  the  commencement  of  tbe  monsoons,  to 
lse  the  seamen's  phrase,  they  creep  along  the  shore,  they 
hen  spread  into  the  ocean  :  at  first  they  are  feeble,  they 
fterwards  become  vigorous  ;  they  then  gradually  dimi- 
lish,  and  finally  come  to  a  change ;  and  this  twice  in  a 
rear,  agreeable  to  our  solar  progress. 

The  sun  is  the  undoubted  cause  of  the  sea  and  land 
>reezes,  which  are  wisely  appointed  by  Divine  Providence 
o  make  some  of  the  hotter  climates  habitable.  The  sea 
>reezes  in  the  West  Indies  begin  to  appear  about  nine 
),clock  in  the  morning,  in  a  fine  black  curl  upon  the 
vater,  approaching  the  shore  ;  it  increases  gradually  till 
loon,  and  dies  away  at  four  or  five  in  the  afternoon. 


Nicholson's  Philosophy,  vol.  ii.  p.  61  and  62, 


462  OF    TRADE-WINDS 

About  six  in  the  evening  it  changes  to  a  land  breeze, 
which  blows  from  the  land  to  the  sea,  and  lasts  till  eight 
in  the  morning.  There  is  an  interval  in  the  morning  and 
evening  between  the  changing  of  the  breezes,  when  the 
wind  is  stationary,  like  tfre  water  before  the  turning  of  the 
tide  ;  and  these  intervals  are  the  hottest  parts  of  the  day. 

The  breezes  are  thus  accounted  for :  when  the  sun  is 
up,  his  heat  takes  more  effect  on  the  land  than  on  the 
water,  so  that  the  heat  is  accumulated,  and  the  air  over 
the  land  is  rarefied  ;  and  as  it  mounts  upward,  the  colder 
air  from  the  sea  comes  in  to  keep  up  the  equilibrium.  In 
the  evening  the  dews  are  so  excessive,  and  the  cold  so 
sudden  on  the  land,  from  the  quick  descent  of  the  sun 
below  the  horizon,  that  the  water  in  the  night  is  warmer 
than  the  land  ;  and  the  air  of  the  sea,  being  then  most 
rarefied,  the  draught  of  air  is  contrary  to  what  it  was  in 
the  day. 

In  the  northern  temperate  zone  the  winds  are  variable. 
but  the  most  general  are  the  S.  VV.  and  W.  and  the  N.  E. 
and  E.  In  the  northern-temperate  and  frigid  zones,  th( 
winds  are  more  tempestuous  in  winter  than  in  summer.* 

"  In  our  climates,  a  tempest  is  but  rarely  known,  am; 
its  ravages  are  registered  as  an  uncommon  calamity;  but. 
in  the  countries  that  lie  between  the  tropics,  and  for  i 
good  space  beyond  them,  its  visits  are  frequent,  and  its 
effects  anticipated.  >  In  these  regions  the  winds  vary  theii 
terrors,  sometimes  involving  all  things  in  a  suffocating 
heat ;  sometimes  mixing  all  the  elements  of  fire,  air,  wa 
ter,  earth  together  ;  sometimes  with  a  momentary  swift 
ness  passing  over  the  face  of  the  country,  and  destroying 
all  things  in  their  passage  ;  and  sometimes  raising  wholt 
sandy  deserts  in  one  country,  to  deposit  them  in  another 
We  have,  therefore,  very  little  reason  to  envy  those  cli 
mates,  the  luxuriance  of  their  soil,  or  the  brightness  o 
their  skies.  Our  own  cloudy  atmosphere,  that  wraps  u 
round  in  obscurity,  though  it  fails  to.  gild  our  prospect: 
with  sunshine,  or  our  groves  with  fruitage,  nevertheles: 
answers  the  calls  of  industry ;  the  labourer  toils  in  th< 
certain  expectation  of  a  moderate  but  happy  return." 

*  Dalton".^  Meteorological  Observations,  p.  88. 


AND    MONSOONS.  463 

The  rains  in  the  West  Indies  are  by  no  means  the  things 
hey  are  with  us.  Our  heaviest  rains  are  but  dews  compa- 
-atively :  they  are  rather  floods  of  water,  poured  from 
he  clouds  with  a  prodigious  impetuosity ;  the  rivers  rise 
n  a  moment ;  new  rivers  and  lakes  are  formed  ;  and  in 
l  short  time  all  the  low  countries  are  under  water. 

It  is  in  the  rainy  season,  principally  in  the  month  of 
August,  that  they  are  assaulted  by  hurricanes,  which  de- 
stroy at  a  stroke  the  labours  of  many  years,  and  prostrate 
he  most  exalted  hopes  of  the  planter,  and  that  often  when 
le  thinks  himself  out  of  the  reach  of  fortune.  It  is  a  sud- 
len  and  violent  storm  of  wind,  rain,  thunder,  and  light- 
ling,  attended  with  a  furious  swelling  of  the  seas,  and 
iometimes  with  an  earthquake  ;  in  short,  with  every  cir- 
:umstance  which  the  elements  can  assemble  that  is  terri- 
ble and  destructive.  First  they  see,  as  a  prelude  to  the 
msuing  havock,  whole  fields  of  sugar  canes  whirled  into 
he  air,  and  scattered  over  the  face  of  the  country.  The 
strongest  trees  of  the  forest  are  torn  up  by  the  roots,  and 
iriven  about  like  stubble ;  their  wind-mills  are  swept 
iway  in  a  moment ;  their  works,  the  fixtures,  the  ponde- 
rous copper-boilers  and  stills  of  several  hundred  weight, 
ire  wrenched  from  the  ground  and  battered  to  pieces ; 
;heir  houses  are  no  protection ;  their  roofs  are  torn  off 
it  one  blast,  whilst  the  rain,  which  in  an  hour  rises  five 
feet,  rushes  in  upon  them  with  irresistible  violence. 

There  are  signs  by  which  the  Indians  of  these  islands 
:aught  our  planters  to  prognosticate  the  approach  of  a 
lurricane.  The  hurricane  comes  on  either  in  the  quarter 
Dr  at  the  full  or  change  of  the  moon.  If  it  come  on  at 
:he  full,  then  at  the  preceding  change  the  sky  is  troubled, 
:he  sun  more  red  than  usual ;  there  is  a  dead  calm  below, 
and  the  mountain  tops  are  free  from  those  mists  which 
usually  hover  about  them.  In  the  caverns  of  the  earth  and 
in  wells,  you  hear  a  hollow  rumbling  sound,  like  the  rush- 
ing of  a  great  wind.  At  night  the  stars  seem  much  lar- 
ger than  usual,  and  surrounded  with  a  sort  of  burs  ;  the 
north-west  sky  has  a  black  and  menacing  appearance;  the 
sea  emits  a  strong  smell,  and  rises  into  vast  waves  often 
without  any  wind.  The  wind  itself  now  forsakes  its  usual 
steady  easterly  stream,  and  shifts  about  to  the  west,  from 


46  i  OF    TRADE-WINDS,    &C 


whence  it  sometimes,  with  intermissions,  blows  violently 
and  irregularly  about  two  hours  at  a  time.  You  hav< 
the  same  signs  at  the  full  moon  ;  the  moon  herself  is  sur 
rounded  with  a  great  bur,  and  sometimes  the  sun  has  th< 
same  appearance. 

The  nature  of  the  soil  over  which  the  wind  blows  ha; 
a  great  effect  upon  the  quality  of  the  air  :  the  vast  sand] 
deserts  of  Africa  and  Arabia  give  a  burning  heat  and  blast 
ing  quality  to  the  air  that  passes  over  them.  These  horrh 
regions  lie  to  the  southward  and  eastward  of  the  Medi 
terranean  ;  and  hence  it  is  that  travellers,  who  have  hac 
the  opportunity  of  making  the  comparison,  tell  us,  tha 
the  air  of  the  West  India  islands  is  nothing  to  the  hot  suf 
focating  winds  which  blow  in  the  night  at  Minorca  ai 
Gibraltar,  for  these  latter  are  scarcely  supportable  by  tl 
human  frame.     At  Goree,  in  the  river  Senegal,  there 
an  easterly  wind  from  the  inland  parts,  with  which  thos 
who  are  suddenly  met  by  it  in  the  face  are  scorched  up  a 
by  a  blast  from  a  furnace. 

An  extraordinary  blasting  wind  is  felt  occasionally  a 
Falkland  Islands.  Happily  its  duration  is  short ;  it  seldon 
continues  above  twenty-four  hours.  .It  cuts  the  herbagi 
down  as  if  fires  had  been  made  under  them  ;  the  leave 
are  parched  up  and  crumble  into  dust ;  fowls  are  seize< 
with  cramps,  so  as  never  to  recover  ;  men  are  oppresset 
with  a  stopped  perspiration,  heaviness  at  the  breast,  am 
sore  throat,  but  recover  with  care. 

But,  beyond  all  others  in  its  dreadful  effects,  is  th 
samiel,  or  mortifying  wind,  of  the  deserts  near  Bagdad 
The  camels,  either  by  instinct  or  experience,  have  notic 
of  its  approach,  and  are  so  well  aware  of  it,  that  they  ar 
said  to  make  an  unusual  noise,  and  cover  up  their  nose 
in  the  sand.  To  escape  its  effects,  travellers  throw  them 
selves  as  close  as  possible  to  the  ground,  and  wait  till  i 
has  passed  by,  which  is  commonly  in  a  few  minutes.  A 
soon  as  they  who  have  life  dare  to  rise  again,  they  exa 
mine  how  it  fares  with  their  companions,  by  plucking  d 
their  arms  or  legs  ;  for,  if  they  are  destroyed  by  the  wind 
their  limbs  are  absolutely  mortified,  and  will  come  asun 
der.  It  is  said  of  this  wind,  that  when  it  happens  to  mee 
with  a  shower  of  rain  in  its  course,  and  blows  across  it,  it  i 


OF    THE    AURORA    BOREALIS.  465 

once  deprived  of  its  noxious  quality,  and  becomes  mild 
and  innocent.  It  is  also  said,  that  it  was  never  known 
to  pass  the  walls  of  a  city. 


OF    THE    AURORA  BOREALIS. 

No  person  has  paid  so  much  attention  to  this  subject 
as  Mr.  Dalton  ;  he  is  also  the  only  one  that  I  know  of 
who  has  given  a  clear  and  satisfactory  account  of  this 
phenomenon.  To  this  work  I  must  refer  you  ;  con- 
tenting myself  here  with  laying  before  you  his  account 
of  the  appearances  of  the  aurora  borealis,  without  en- 
tering  into  his  explanation  thereof. 

The  appearances  of  the  aurora  come  under  four  dif- 
ferent descriptions.  1.  A  horizontal  light,  like  the 
morning  aurora,  or  break  of  day.  2.  Fine  slender  lu- 
minous beams,  well  defined,  and  of  dense  light  These 
often  continue  a  quarter,  a  half,  or  a  whole  minute 
apparently  at  rest,  but  oftener  with  a  quick  lateral  mo- 
tion. 3.  Flashes  pointing  upward,  or  in  the  same  di- 
rection as  the  beams,  which  they  always  succeed.  These 
are  only  momentary,  and  have  no  lateral  motion  ;  but 
they  are  generally  repeated  many  times  in  a  minute. 
They  appear  much  broader,  more  diffuse,  and  of  a 
weaker  light  than  the  beams  :  they  grow  gradually 
fainter  till  they  disappear ;  and  sometimes  continue 
for  hours  flashing  at  intervals.  4.  Arcs,  nearly  in  the 
form  of  a  rainbow  ;  these,  when  complete,  go  quite 
across  the  heavens,  from  one  point  of  the  horizon  to 
the  opposite  point. 

When  an  aurora  happens,  these  appearances  seem  to 
succeed  each  other  in  the  following  order  :  ] .  the  faint 
rainbow-like  arcs ;  2.  the  beams ;  and  3.  the  flashes. 
As  for  the  northern  horizontal  light,  it  appears  to  con- 
sist of  an  abundance  of  flashes  or  beams  blended  to- 
gether by  the  situation  of  the  observer. 

The  beams  of  the  aurora  borealis  appear  at  all  place? 
to  be  arcs  of  great  circles  of  the  sphere,  with  the  eye  in 
the  centre  ;  and  these  arcs,  if  prolonged  upwards, 
would  all  meet  in  one  point. 

VOL.  IV.  so 


466  THE    SOURCES    OF    HEAT. 

The  rainbow-like  arcs  all  cross  the  magnetic  meridian 
at  right  angles.  When  two  or  more  appear  at  once, 
they  are  concentric,  and  tend  to  the  east  and  west :  also 
the  broad  arc  of  the  horizontal  light  tends  to  the  mag- 
netic east  and  west,  and  is  bisected  by  the  magnetic 
meridian  ;  and  when  the  aurora  extends  over  any  part 
of  the  hemisphere,  whether  great  or  small,  the  line  se- 
parating the  illuminated  part  of  the  hemisphere  from 
the  clear  part,  is  half  the  circumference  of  a  great  circle 
crossing  the  magnetic  meridian  at  right  angles,  and 
terminating  in  the  east  and  west :  moreover,  the  beams 
perpendicular  to  the  horizon  are  only  those  on  the  mag- 
netic meridian. 

That  point  in  the  heavens  to  which  the  beams  of  the 
aurora  appear  to  converge,  at  any  place,  is  the  same  as 
that  to  which  the  south  pole  of  the  dipping  needle 
points  at  that  place. 

The  beams  appear  to  rise  above  each  other  in  suc- 
cession ;  so  that  of  any  two  beams,  that  which  has  the 
higher  base,  has  also  the  higher  summit. 

Every  beam  appears  broadest  at  or  near  the  base, 
and  to  grow  narrower  as  it  ascends  ;  so  that  the  conti- 
nuation of  the  bounding  lines  would  meet  in  the  com- 
mon centre  to  which  the  beam  tends. 

The  height  of  the  rainbow- like  arcs  of  the  aurora  are 
estimated  by  Mr.  Dalton  to  be  above  the  earth's  surface 
-about  150  English  miles. 


OF    THE    SOURCES    OF    HEAT    AND    COLB.* 

If  the  changes  of  the  weather  depended  on  the  course 
of  the  year,  and  the  temperature  of  climates  were  go- 
verned by  their  situation  with  respect  to  the  sun,  that 
is,  by  their  latitude,  then  the  weather  might  be  reduced 
to  some  regular  theory.  But  this  is  so  far  from  being 
the  case,  that  the  latitude  of  a  place  cannot  be  consider- 


*  Kirwan%8  Estimate  of  the  Temperature  of  different  Latitudes. 
Jones's  Physiological  Disquisitions* 


THE    SOURCES    OF    HEAT.  46? 

ed  as  an  index  to  the  temperature  of  the  climate  :  for 
we  find  the  hottest  days  in  the  coldest  climates  ;  and 
the  coldest  weather,  and  even  perpetual  snow,  are  found 
in  countries  bordering  on  and  immediately  under  the 
equator  :  so  that  we  must  recur  to  some  other  causes 
besides  the  immediate  influence  of  the  solar  rays. 

1.  But  though  the  sun  is  not  the  only  cause,  its  pre- 
sence is  undoubtedly  the  principal  source  of  heat,  as 
well  as  light,  and  its  absence  the  primary  cause  of  cold. 
He  is  indeed  the  great  spirit  of  the  world  :  all  things 
revive  at  his  approach  ;  winter  and  frost  lie  behind 
him. 

2.  The  second  source  of  heat  is  the  earth.  Nobody 
has  yet  been  found  so  absurd  as  to  suppose  that  human 
perspiration  was  owing  to  the  air  that  surrounds  the 
skin  ;  it  originates  in  an  intertial  cause ;  it  is  occasion- 
ed by  a  heat  within,  not  the  air  without.  It  is  the  same 
with  respect  to  the  earth  ;  which,  by  imparting  its  heat 
to  the  atmosphere,  moderates  the  rigour  of  the  winter's 
cold.  Whether  we  suppose  that  this  heat  arises  from 
a  central  source,  or  that  the  globe  from  its  first  crea- 
tion was  endued  with  a  heat  sufficient  for  all  the  purpo- 
ses it  was  intended  to  answer  ;  yet  it  is  evident  that  it 
is  renewed  and  preserved  by  the  influence  of  the  sun, 
and  that  there  is  always  a  silent  and  imperceptible  heat 
proceeding  from  the  earth. 

M.  de  Luc  shows,  that  our  globe  has  a  provision  of 
fire  spread  through  its  whole  mass  ;  so  that,  wherever 
there  is  no  chemical  operation  to  disengage  or  to  ab- 
sorb it,  this  fire  maintains  the  same  degree  of  expansive 
force.  From  observation  we  also  find,  that  the  same 
degree  of  heat  reigns  in  all  subterraneous  places,  ex- 
cept in  mines  where  there  is  reason  to  suspect  some 
chemical  operation.  With  respect  to  those  parts  of 
the  globe  which  are  nearest  the  surface,  the  fire  passes 
therefrom  into  the  air,  when  its  expansive  force  exceeds 
that  of  the  fire  in  the  air,  and  mice  versa.  Thus  a  cer- 
tain equilibrium  is  preserved  near  the  surface,  though 
subject  to  certain  vicissitudes. 

The  solar  rays  exercise  two  distinct  functions  ;  in 
the  one  acting  as  fire,  in  the  other  increasing  the  expan- 


468  THE    SOURCES    OF    COLD. 

sive  force  of  the  existing  fire.  Various  combinations 
of  fire  are  continually  forming,  as  well  upon  the  surface 
of  the  globe  as  in  the  atmosphere  ;  combinations  which 
are  afterwards  under  other  circumstances  destroyed. 
These  compositions  and  decompositions  occasion  the 
greater  part  of  terrestrial  phenomena. 

3.  The  next  great  source  of  heat  is  the  condensation 
of  vapour.  Vapour  contains  a  quantity  of  fire  :  it  is  this 
fire  which  causes  it  to  assume,  and  supports  it  in  an 
aerial  expanded  state  ;  when  condensed  into  a  liquid 
form,  it  lets  go  this  fire,  which  warms  the  surrounu 
atmosphere :  hence  the  sultriness  frequently  experi- 
enced before  rain. 


OF    THE    SOURCES    OF    COED. 

1.  As  the  earth  is  one  of  the  principal  sources  of  heat 
in  the  atmosphere  that  surrounds  it,  so  is  distance  from 
the  earth  a  source  of  cold  ;  the  greatest  cold  prevailing 
in  the  highest  regions  of  the  atmosphere  :  for,  where 
the  re-action  is  wanting  by  a  superficial  pressure,  but 
little  effect  can  be  received  from  the  rays  of  the  sun ; 
and  it  is  further  proved  by  experiments  with  a  burning 
glass,  that  a  clear  unclouded  air  receives  no  heat  from 
these  rays.  Hence,  when  we  ascend  to  a  lighter  air,  at 
a  distance  from  the  surface,  the  heat  is  not  sufficient  to 
melt  the  snow ;  and  we  find  the  highest  mountains, 
even  under  the  equinoctial,  perpetually  covered  there- 
with ?  thus,  the  mean  height  of  the  lower  term  of  con- 
gelation in  winter,  in  this  latitude,  may  be  considered  in 
general  to  be  at  6260  feet  from  the  surface,  and  the 
mean  height  of  the  upper  term  at  1125  feet.  We  can- 
not in  this  lecture  consider  any  of  the  minute  ex- 
ceptions. 

Sir  William  Young  gives  a  remarkable  instance  of  the 
effect  of  hills  in  arresting  vapours  and  producing  rain, 
while  the  exhalations  from  the  trees  on  its  surface  cool 
and  temper  the  air ;  observing,  that  the  smooth  polished 
parbadoes  and  our  Leeward  Islands  are  parched  up, 
whilst  the  towering  and  rugged  Dominica,  St.  Vincent, 


THE   SOURCES    OF    COLD.  469 

Grenada,  and  Tobago,  enjoy  incessant  rains  and  deli- 
cious verdure. 

It  is  generally  agreed,  that  the  clearing  away  of  wood 
in  time  lessens  the  vapours,  and  consequently  the  rain 
of  a  country.  Several  fine  parishes  in  Jamaica,  which 
used  to  produce  large  crops  of  sugar  canes,  and  were 
Dnce  the  richest  spots  in  the  island,  are  now  dry  for 
nine  months  in  the  year,  and  are  turned  into  cattle- 
pens,  through  the  clearing  away  of  the  neighbouring 
woods. 

Water  is  very  plentiful  in  those  countries  where 
woods  and  forests  abound,  and  the  purest  springs  are 
generally  found  beneath  the  friendly  shelter  of  a  grove. 

The  natural  history  of  every  country  shows,  that  in 
proportion  as  the  woodlands  are  cleared,  the  water 
courses  diminish. 

In  America,  unfortunately  for  the  inhabitants,  this 
truth  is  too  well  known ;  for,  since  the  woods  in  the 
vicinity  of  their  towns  have  been  cut  down,  many  long 
established  mill  races  have  become  dry,  and  others  have 
been  reduced  so  low,  as  to  cause  very  great  interrup- 
tions to  the  miller,  who  must  wait  a  considerable  time 
for  the  dams  to  fill  between  every  few  hours  work. 

Hence  we  may  learn  the  important  necessity  of  pre- 
serving the  trees,  from  beneath  whose  humid  shades  a 
water  spring  discharges  its  streams  ;  and  hence,  too, 
we  may  learn,  that  the  smallest  springs  may  be  improv- 
ed by  planting  around  them  a  grove  of  trees,  particu- 
larly the  oak,  so  highly  valued  by  the  Greeks,  the 
Romans,  and  our  ancient  Druids ;  who,  considering 
the  health  of  man  and  the  fertility  of  the  soil  to  be  ab- 
solutely dependent  upon  plenteous  streams  of  water, 
consecrated  their  groves  to  preserve  their  springs. 

2.  The  next  great  source  of  cold  is  evaporation. 
The  same  cause  which  makes  the  condensation  of  va- 
pour a  source  of  heat,  makes  evaporation  productive 
of  cold ;  as  it  absorbs  the  fire  in  the  latter  instance, 
which  it  gives  out  in  the  former  :  it  is  this  which  gives 
the  particles  of  vapour  their  aerial  form.  When  fire 
passes  from  fluids  which  it  has  heated,  its  course  is  up- 


470  OF    EVAPORATION. 

wards,  and  it  always  carries  with  it  a  thin  stratum  of  the 
fluid  in  the  form  of  vapour  :  thus  evaporation  not  only 
tempers  the  heat  occasioned  by  the  sun's  rays,  but  i^ 
one  great  source  of  cold. 


OF    EVAPORATION. 

Of  evaporation  it  may  be  observed,  1.  That  in  our 
climates  the  quantity  of  it  is  four  times  greater  from  the 
21st  of  March  to  the  21st  of  September,  than  it  is  from 
the  21st  of  September  to  the  21st  of  March. 

2.  That  it  is  greater  in  proportion  as  the  difference 
in  temperature  between  the  air  and  evaporating  surface 
is  greater  ;  though,  if  the  air  be  15  degrees  colder  than 
the  evaporating  surface,  there  is  no  evaporation,  but  a 
deposit  of  moisture  from  the  air. 

3.  The  degree  of  cold  produced  by  evaporation,  is 
always  much  greater  when  the  air  is  warmer  than  the 
evaporating  surface,  than  that  which  is  produced  when 
this  surface  is  warmer  than  the  air.  Hence  warm  winds, 
as  the  Serocco  and  Harmatan  are  more  desiccative  than 
cold  winds. 

4.  Evaporation  is  more  copious  when  the  air  is  less 
loaded  with  vapours,  and  is  consequently  powerfully 
promoted  by  cold  winds  flowing  into  warmer  countries. 

5.  That  it  is  greatly  increased  by  a  current  of  air  or 
wind  flowing  over  the  evaporating  surface ;  not  only 
because  the  evaporating  surface  is  thereby  increased, 
but  also  because  the  vapour  is  thereby  removed,  and 
prevented  from  attaining  its  maximum  :  hence  it  is  ge- 
nerally remarked,  that  calm  days  are  the  hottest. 

6.  Tracts  of  land  covered  with  trees  or  vegetables 
emit  more  vapour  than  the  same  space  covered  with 
water  :  on  this  principle  it  is,  that  the  air  about  a  wood 
or  forest  is  made  colder  by  the  evaporation  from  trees 
and  shrubs,  while  the  plants  themselves  are  kept  in  a 
more  moderate  heat,  and  secured  from  the  burning  heat 
of  the  sun  by  the  vapour  perspired  from  the  leaves. 
Thus,  we  find  the  shade  of  vegetables  more  effectual  to 


OF    ANNUAL    TEMPERATURE.  471 

cool  us,  as  well  as  more  agreeable,  than  that  from  rocks 
and  buildings. 

7.  The  heat  and  cold  of  different  countries  are  trans- 
mitted from  one  country  to  another  by  the  medium  of 
winds. 

OF    ANNUAL    TEMPERATURE. 

Within  ten  degrees  of  the  pole,  there  is  very  little  dif- 
ference in  the  annual  temperature,  nor  is  there  much 
within  ten  degrees  of  the  equator. 

The  temperature  of  different  years  differs  very  little 
near  the  equator,  but  they  differ  more  and  more  as  their 
latitudes  approach  the  pole. 

It  scarce  ever  freezes,  unless  in  very  elevated  situa- 
tions, in  latitudes  under  35° ;  and  it  scarce  ever  hails  in 
latitudes  higher  than  60°. 

Between  the  latitudes  of  35°  and  60°,  in  places  adja- 
cent to  the  sea,  it  generally  thaws  when  the  sun's  altitude 
is  40°,  and  seldom  begins  to  freeze  until  the  sun's  meri- 
dian altitude  is  below  40°. 

The  greatest  cold  in  all  latitudes  in  our  hemisphere  is 
generally  about  half  an  hour  before  sun-rise :  the  greatest 
heat  in  all  latitudes  between  60°  and  45°  is  found  to  be 
about  half  past  two  o'clock  in  the  afternoon ;  between 
latitudes  45°  and  35°,  at  two  o'clock  ;  between  latitudes 
35°  and  25°,  at  half-past  one ;  and  between  latitude  25° 
and  the  equator,  at  one  o'clock. 

The  month  of  January  is  the  coldest  in  every  latitude, 
July  is  the  warmest  month  in  all  latitudes  above  48°  ;  but 
in  lower  latitudes,  August  is  generally  the  warmest. 

December  and  January  differ  but  little,  and  there  is 
no  great  difference  between  June  and  July.  In  latitudes 
above  30°,  the  months  of  August,  September,  October, 
and  November,  differ  more  from  each  other  than  those 
of  February,  March,  April,  and  May ;  in  latitudes  under 
30°  the  difference  is  not  so  great.  The  temperature  of 
April  approaches  more  every  where  to  the  mean  annual 
temperature  than  that  of  any  other  month  :  whence  we 
may  infer,  that  the  effects  of  natural  causes,  operating 
over  a  large  extent,  do  not  arrive  at  their  maximum  un- 


472  OF    ANNUAL    TEMPERATURE. 

til  the  causes  begin  to  diminish  ;  but  that  after  these  ef- 
fects have  arrived  at  their  maximum,  the  decrements  are 
more  rapid  than  the  increments  originally  were,  during 
their  progress  to  that  maximum. 

The  differences  between  the  hottest  and  the  coldest 
months,  within  twenty  degrees  of  the  equator,  are  in- 
considerable, except  in  some  peculiar  situations ;  but 
they  increase  in  proportion  as  we  recede  from  the  equa- 
tor. 

In  the  highest  latitudes,  we  often  meet  with  a  heat  of 
75°  or  80° ;  and  particularly  in  the  latitudes  of  59°  and 
60°  the  heat  of  July  is  frequently  greater  than  in  latitude 
51°. 

At  the  time  of  the  equinoxes,  when  the  sun  passes 
from  one  hemisphere  into  the  other,  there  is  generally 
some  disturbance  in  the  weather ;  the  winds  are  then 
mostly  higher :  at  the  vernal  equinox,  they  are  for  the 
greater  part  easterly,  cold,  dry,  and  searching.  The  sol- 
stitial point  of  the  summer  is  more  apt  to  be  distinguish- 
ed by  violent  rains,  and  what  w.e  call  a  midsummer  flood. 
The  winter  being  less  rainy  than  the  summer,  nothing 
particular  happens  at  the  winter  solstice,  except  that  the 
frost  sets  in  more  severely,  with  some  continuance  of 
snow,  which  lies  long  upon  the  ground. 

The  temperature  of  a  climate  depends  on  many  cir- 
cumstances, particularly  on  the  disposition  of  the  land ; 
as  its  elevation,  its  exposure  to  the  winds,  and  the  course 
of  the  mountains  that  are  found  in  it.  Thus  the  writer  of 
Anson's  voyage  informs  us,  that  while  they  coasted  near 
the  land  of  South  America,  which  has  those  vast  ridges 
of  mountains,  the  Andes  and  Cordillieras,  the  air  was 
rendered  temperate  by  the  wind  that  blew  over  them ; 
but  when  they  had  passed  beyond  this  tract  of  land,  and 
sailed  by  the  isthmus  of  Darien,  where  the  country  is 
flatter,  the  air  became  insupportably  close  and  sultry. 

All  countries  lying  to  the  windward  of  high  moun- 
tains, or  extensive  forests,  are  warmer  than  those  to  the 
leeward  in  the  same  latitude. 

The  vicinity  to  the  sea  is  another  circumstance  which 
affects  the  temperature  of  a  climate  ;  as  it  moderates  the 
heats  from  the  lancj>  and  brings  the  atmosphere  down  to 


OF    ANNUAL    TEMPERATURE.  473 

a  standard  best  fitted  to  the  human  constitution.  This 
is  probably  the  reason  why  there  is  so  great  a  proportion 
of  sea  on  the  terraqueous  globe,  and  particularly  why  it 
is  so  largely  distributed  about  the  middle  region  of  the 
^arrh.  In  our  hemisphere,  countries  that  lie  southward 
rf  any  sea,  are  warmer  than  those  that  have  that  sea  to 
the  south  of  them  ;  because  the  winds  that  should  cool 
them  in  winter  are  mitigated  by  passing  over  the  sea ; 
whereas,  those  that  are  northward  of  the  sea,  are  cooled 
in  summer  by  the  breezes  from  it.  A  northern  or  sou- 
thern bearing  of  the  sea  renders  a  country  warmer  than 
an  eastern  or  western  bearing. 

Islands  participate  more  of  temperature  arising  from 
the  sea,  and  are  therefore  warmer  than  continents.  Most 
large  islands  have  their  greatest  extent  from  north  to 
south.  With  us,  the  southern  parts  are  proportionably 
colder  than  the  northern.  A  ridge  of  mountains  generally 
traverses  islands  in  the  direction  of  their  length. 

The  soil  of  large  tracts  of  land  has  its  share  in  influ- 
encing the  temperature  of  the  weather :  thus,  stones  or 
sand  heat  and  cool  more  readily,  and  to  a  greater  degree, 
than  the  earth  or  vegetable  mould ;  hence  the  violent 
heats  of  the  most  sandy  deserts  of  Arabia  and  Africa,  and 
the  burning  heat  and  blasting  qualities  of  the  wind  that 
passes  over  them  ;  hence  also  the  intense  cold  of  Terra 
del  Fuego,  and  other  stony  countries  in  cold  latitudes. 

Living  vegetables  have  a  considerable  effect  in  altering 
climates,  and  affecting  the  weather.  Wooded  countries 
are  much  colder  than  those  that  are  open  and  cultivated ; 
thus,  part  of  Guiana  has  only  been  cleared  from  wood 
since  the  beginning  of  this  century,  and  the  heat  in  that 
part  is  already  excessive  ;  whereas,  in  the  wooded  parts, 
the  inhabitants  are  obliged  to  light  a  fire  every  night. 

Every  habitable  latitude  enjoys  a  heat  of  60  degrees  at 
least,  for  two  months ;  which  heat  seems  necessary  for 
the  growth  and  maturity  of  corn.  The  quickness  of  ve- 
getation in  the  higher  latitudes  proceeds  from  the  dura- 
tion of  the  sun  above  the  horizon.  Rain  is  little  wanted,  as 
the  earth  is  sufficiently  moistened  by  the  liquifaction  of 
the  snow  that  covers  it  during  the  winter.  In  all  this,  we 

VOL.  IV.  3  *> 


474  OF    ATMOSPHERICAL    ELECTRICITY. 

cannot  sufficiently  admire  the  wise  disposition  of  Provi- 
dence. 

It  is  owing  to  the  same  provident  hand,  that  the  globe 
of  the  earth  is  intersected  with  seas  and  mountains,  in  a 
manner  that  on  its  first  appearance  seems  altogether  irre- 
gular and  fortuitous,  presenting  to  the  eye  of  ignorance 
the  view  of  an  immense  ruin :  but,  when  the  effects  of 
these  seeming  irregularities  on  the  face  of  the  globe  are 
carefully  inspected,  they  are  found  most  beneficial,  and 
even  necessary  to  the  welfare  of  its  inhabitants ;  for,  to 
say  nothing  of  the  advantages  of  trade  and  commerce, 
which  could  not  exist  without  these  seas,  we  have  seen 
that  it  is  by  their  vicinity  that  the  cold  of  the  higher  lati- 
tudes is  moderated,  and  the  heat  of  the  lower.  It  is  by 
the  want  of  seas,  that  the  interior  parts  of  Asia,  as  Sibe- 
ria and  Great  Tartary,  as  well  as  those  of  Africa,  are 
rendered  almost  uninhabitable ;  a  circumstance  which 
furnishes  a  strong  agreement  against  the  opinion  of  those, 
who  think  these  countries  were  the  original  habitations 
of  man.  In  the  same  manner,  mountains  are  necessary, 
not  only  as  the  reservoirs  of  rivers,  but  as  a  defence  against 
the  violence  of  heat  in  the  warm  latitudes.  Without  the 
Alps,  Pyrenees,  Apennine,  the  mountains  of  Dauphine 
Auvergne,  &c.  Italy,  Spain,  and  France  would  be  de 
prived  of  the  mild  temperature  they  at  present  enjoy 
without  the  Balgate  hills,  or  Indian  Apennine,  India 
would  have  been  a  desert :  hence  Jamaica,  St.  Domingo, 
Sumatra,  and  most  other  intertropical  islands,  are  fur- 
nished with  mountains,  from  which  the  breezes  proceed 
that  refresh  them. 


OF    ATMOSPHERICAL    ELECTRICITY. 

So  little  is  known  with  any  certainty  concerning  atmos- 
pherical electricity,  that  I  shall  detain  you  but  a  short 
time  with  what  I  have  to  say  thereon.  If  every  solution 
of  continuity,  every  expansion  and  contraction  of  material 
substances,  are  sources  of  electrical  appearances,  we  need 
not  be  surprized  at  finding  it  in  great  abundance  among 
the  clouds :  in  this  view  of  the  subject,  the  perpetual  os- 


OF    ATMOSPHERICAL    ELECTRICITY.  475 

dilations  of  the  air  must  be  also  a  means  of  rendering  it 
sensible  to  us.  Mr.  Bennet's*  electrometer,  which  I  have 
already  described,  is  the  best  and  readiest  instrument  for 
observing  the  changes  in  the  electricity  of  the  atmos- 
phere. 

The  following  positions  have  been  deduced  from  some 
observations  on  the  electrical  state  of  the  atmosphere. 
1st.  That  in  the  spring,  when  plants  begin  to  grow,  we 
are  told  that  temporary  electrical  clouds  begin  to  appear, 
and  pour  forth  electric  rain.  2.  That  the  electricity  of  the 
clouds  and  of  the  rain  increases,  till  that  part  of  autumn, 
when  the  last  fruits  are  gathered.  It  is  hence  supposed 
to  actuate  and  animate  vegetation,  and  to  give  to  rain 
that  power  which  renders  it  more  propitious  to  vegeta- 
bles than  any  other  kind  of  watering. 

Aerial  electricity  varies  according  to  the  situation  ;  it 
is  generally  strongest  in  elevated  and  insulated  situations; 
not  to  be  observed  under  trees,  in  streets,  in  houses,  or 
any  inclosed  places  ;  though  it  is  sometimes  to  be  found 
pretty  strong  on  quays  and  bridges.  It  is  also  owing  not  so 
much  to  the  absolute  height  of  the  places,  as  their  situa- 
tion ;  thus  a  projecting  angle  of  a  high  hill  will  often 
exhibit  a  stronger  electricity  than  the  plain  at  the  top  of 
the  hill,  as  there  are  fewer  points  in  the  former  to  de- 
prive the  air  of  its  electricity. 

The  intensity  of  the  atmospheric  electricity  is  varied 
by  a  great  many  circumstances,  some  of  which  may  be 
accounted  for,  others  cannot.  When  the  weather  is  not 
serene,  it  is  impossible  to  assign  any  rule  for  their  varia- 
tion, as  no  regular  correspondence  can  then  be  perceived 
with  the  different  hours  of  the  day,  nor  with  the  various 
modifications  of  the  air.  The  reason  is  evident ;  when 
contrary  and  variable  winds  reign  at  different  heights, 
when  clouds  are  rolling  over  clouds,  these  winds  and 
clouds,  which  we  cannot  perceive  by  any  exterior  sign, 
influence  however  the  strata  of  air  in  which  we  make  our 
experiments,  and  produce  those  changes  of  which  we  only 


f  See  the  Rev.  Mr.  Bennei's  New  Experiments  on  Electricity,  Derby, 


476  OF    ATMOSPHERICAL    ELECTRICITY. 

see  the  result,  without  being  able  to  assign  either  the 
cause  or  its  relation.  Thus,  in  stormy  weather,  we  see 
the  electricity  strong,  then  null,  and  in  a  moment  after 
arise  to  its  former  force  ;  one  instant,  vitreous  ;  the  next, 
resinous ;  without  being  able  to  assign  any  reason  for 
these  changes.  M.  de  Saussure  says,  that  he  has  seen  these 
changes  succeed  with  such  rapidity,  that  he  had  not  time 
to  note  them  down. 

When  rain  falls  without  a  storm,  these  changes  are  not 
so  sudden ;  they  are,  however,  very  irregular,  particu- 
larly with  respect  to  the  intensity  of  force ;  the  quality 
thereof  is  more  constant.  Rain  or  snow  almost  uniformly 
gives  vitreous  electricity. 

The  state  of  the  air,  in  which  the  electricity  is  strong- 
est, is  foggy  weather ;  this  is  always  accompanied  with 
electricity,  except  when  the  fog  is  going  to  resolve  into 
rain. 

The  most  interesting  observations,  and  those  which 
throw  the  greatest  light  upon  the  various  modifications 
of  electricity  in  our  atmosphere,  are  those  that  are  made 
in  serene  weather.  In  winter,  during  which  most  of  M. 
de  Saussure9s  observations  were  made,  and  in  serene  wea- 
ther, the  electricity  was  generally  weakest  in  an  evening, 
when  the  dew  had  fallen,  until  the  moment  of  the  sun's 
rising ;  its  intensity  afterwards  augmented  by  degrees, 
sometimes  sooner,  and  sometimes  later ;  but  generally 
before  noon  it  attained  a  certain  maximum,  from  whence 
it  again  declined,  till  the  fall  of  the  dew,  when  it  would 
be  sometimes  stronger  than  it  had  been  during  the  whole 
day ;  after  which  it  would  again  gradually  diminish  du- 
ring the  whole  night ;  but  is  never  quite  destroyed,  if  the 
weather  be  perfectly  serene. 

Atmospherical  electricity  seems  therefore,  like  the  sea, 
to  be  subject  to  a  flux  and  reflux,  which  cause  it  to  in- 
crease and  diminish  -twice  in  24  hours.  The  moments  of 
its  greatest  force  are  some  hours  after  the  rising  and  set- 
ting of  the  sun  ;  those,  when  it  is  weakest,  precede  the 
rising  and  setting  thereof. 

The  electricity  of  serene  weather  is  much  weaker  in 
summer  than  in  winter  ;  this  renders  it  more  difficult  to 

erve  these  gradations  in  summer  than  in  winter ;  be- 


SIGNS    OF    THE    WEATHER*  4/7 

sides  a  variety  of  accidental  causes,  which  at  the  same 
time  render  them  more  uncertain.  In  general,  in  summer, 
when  the  ground  has  been  dry  for  some  days,  and  the  air  is 
dry  also,  the  electricity  generally  increases  from  the  rising 
of  the  sun  till  three  or  four  in  the  afternoon,  when  it  is 
strongest ;  it  then  diminishes  till  the  dew  begins  to  fall, 
which  again  re-animates  it ;  though  after  this  it  declines, 
and  is  almost  extinguished  during  the  night. 

But  the  serene  days  that  succeed  rainy  weather  in  sum- 
mer, generally  exhibit  the  same  diurnal  periods  or  states 
of  electricity,  that  are  to  be  observed  in  winter. 


ON    PROGNOSTIC    SIGNS    OF    THE    WEATHER. 

There  is  no  part  of  meteorology  which  interests  man- 
kind so  much,  as  the  predictions  it  furnishes  of  the  change 
of  weather.  The  theory  of  it  only  engages  the  attention 
by  animating  us  with  the  hopes  of  thereby  bringing  the 
knowledge  of  these  predictions  to  perfection. 

And  the  far  greater  part  of  those  who  purchase  mete- 
orological instruments,  buy  them,  not  so  much  to  know 
the  actual  state  of  the  elements,  as  to  foresee  the  changes 
thereof.  This  science  is,  however,  very  imperfect ;  for  k 
is  but  of  late  years  that  we  began  to  make  observations 
on  the  changes  of  the  weather  ;  but  that  its  progress  has 
been  rapid  and  successful  may  be  seen  in  the  works  of 
De  Luc,  De  Sans sure ,  Jones, '  Marshall,  and  Kirwan. 
But  these  observations  will  be  still  more  valuable  to  pos- 
terity ;  for  we  can  scarce  expect  them  in  sufficient  num- 
ber in  our  own  age  to  deduce  from  them  a  general  and 
perfect  theory. 

To  attain  this  end  it  will  be  necessary  to  multiply  ob- 
servations on  as  great  a  number  of  signs  as  possible  ;  for 
it  is  only  by  their  combination  and  concurrence  that  we 
can  expect  to  remove  the  uncertainty  inseparable  from 
each  in  itself.  Thus  the  barometer  is  not  always  a  certain 
sign  ;  the  same  may  be  said  of  the  thermometer,  the  hy- 
grometer, and  the  action  of  winds.  But  if  they  all  concur 
together,  there  is  but  little  chance  of  being  deceived  ;  and 
there  would  be  still  less,  if  to  these  were  joined  other 


478  PROGNOSTIC    SIGNS 

signs,  which  are  easy  to  observe,  and  which,  by  their 
combination  would  render  our  prediction  certain. 

No  sign,  nor  any  instrument  of  observation,  should 
therefore  be  neglected,  either  from  a  love  of  ideal  per- 
fection, or  fears  of  inaccuracy.  Thus,  though  the  hy. 
grometer  be  at  present  a  very  imperfect  instrument, 
yet  one  certain  sign  has  already  been  obtained  from  its 
indications,  and  more  may  be  reasonably  expected. 
Even  the  words  very  dry,  very  moist.,  moderately  dry,  mo- 
derately moist,  though  of  vague  determination,  may 
throw  much  light  on  the  state  of  the  atmosphere. 

It  is  necessary  that  the  observer  should  enter  into  a 
precise  detail  of  the  various  states  of  the  sky  and  the 
clouds.  What  can  we  learn  from  the  words  covered, 
and  cloudy,  or  half  covered  sky,  &c.  ?  Nothing  ;  since  it 
is  well  known,  that  a  covered  sky,  in  one  case,  is  almost 
as  certain  an  indication  of  fine  weather,  as  in  another  it 
is  an  indubitable  presage  of  rain.  The  accurate  ob- 
server piques  himself  on  a  thermometer,  with  which  he 
can  observe  within  a  degree,  and  a  barometer  that  he 
can  depend  upon  to  less  than  the  hundredth  of  an  inch ; 
but  is  silent  on  the  transparency  of  the  air,  on  dews,  on 
the  elevation,  the  form,  the  sign,  the  disposition,  the 
colour,  and  the  density  of  the  clouds  ;  things  that  may 
be  observed  with  ease,  and  described  without  trouble ; 
being  attended  with  no  other  inconvenience  than  that 
of  extending  the  size  of  our  meteorological  tables. 

There  is  a  phenomenon  which  has  not  been  sufficiently 
attended  to,  namely,  the  undulating  motion  of  the  fir- 
mament, or  that  diurnal  tumult  in  the  air,  which  is  kept 
up  by  the  heat  of  the  sun.  What  the  sun  raises  from 
the  earth  by  the  heat  of  the  day,  is  sustained  in  the  at- 
mosphere by  its  heat,  and  the  agitation,  or  expansive 
undulation  of  the  air.  This  motion  is  often  visible  to 
the  naked  eye,  but  in  the  field  of  a  powerful  telescope 
it  is  very  conspicuous  ;  all  objects  appear  in  violent  agi- 
tation, and  the  line  of  the  sensible  horizon,  which 
ought  to  be  clear  and  well  defined,  is  waved  like  a  field 
of  corn  in  the  wind,  or  the  surface  of  the  sea  in  a 
storm.  So  long  as  this  agitation  continues,  the  vapours 
stay  in  the  air;  but  when  it  subsides,  and  the  sun  ()• 


OF    THE    WEATHER.  479 

parts,  they  are  condensed,  and  fall  down  to  the  earth  in 
the  night  as  dew. 

In  the  present  state  of  this  part  of  science,  when  we 
are  unacquainted  with  so  many  phenomena,  and  still 
more  ignorant  of  their  causes,  general  rules  will  often 
be  found  to  fail,  and  particular  ones  will,  without  much 
circumspection,  prove  to  be  a  source  of  error.  Amongst 
the  variety  of  means  for  predicting  the  changes  of  the 
weather,  the  barometer  is  undoubtedly  one  of  the  best ; 
and  is  in  this,  as  well  as  many  other  respects,  one  of 
the  greatest  acquisitions  to  natural  philosophy. 

The  usual  ranges  of  the  mercurial  column  in  this  la- 
titude are  comprized  between  28  and  31  inches,  of 
which  the  middle,  or  29*,  is  considered  as  the  variable : 
I  think  it  should  be  placed  somewhat  higher.  Near  the 
pole,  the  variations  of  the  barometer  are  much  greater. 


OF    PROGNOSTICS    BY    THE    BAROMETER. 

Ever  since  the  barometer  has  been  invented,  philoso- 
phers have  endeavoured  to  account  for  the  variations  in 
the  height  of  a  local  barometer,  but  hitherto  in  vain. 
M.  de  Luc,  in  the  first  volume  of  his  Recherches  sur 
les  Modifications  de  1' Atmosphere,  has  given  a  critical 
and  very  interesting  account  of  the  various  physical 
opinions  that  have  been  invented  for  this  purpose  by 
Pascal,  Beak,  Wallis,  Garden,  Halley,  &c.  &c.  and 
shown  that  they  are  all  imperfect,  and  inadequate  to  the 
solution  of  the  phenomena.  He  then  proposes  one  of 
his  own  ;  which  with  that  candour  that  ever  distin- 
guishes a  lover  of  the  truth  he  has  since  abandoned.  To 
give  a  particular  account  of  the  various  hypothesis 
would  occupy  a  volume,  and  that  to  little  purpose.  As 
I  know  of  none  that  can  be  depended  on,  I  shall  content 
myself  with  only  relating  the  bare  phenomena.  The 
two  great  sources  of  error  on  this  subject  have  been, 
1st.  The  difficulty  of  observing  the  whole  of  the  ap- 
pearances ;  and  2dly,  The  facility  with  which  the  mind 
embraces  and  supports  a  favourite  hypothesis. 


480         dalton's  general  observations. 

There  is  one  striking  phenomenon  in  the  variations 
of  the  barometer,  which  should  be  particularly  attended 
to  in  every  theory,  because  it  is  as  great  as  it  is  certain 
and  invariable  ;  namely,  that  the  variations  diminish  in 
proportion  as  you  approach  the  equator,  and  augment 
as  you  advance  towards  the  poles.  The  countries,  how- 
ever, that  are  situated  about  the  equator,  are  subject  to 
the  changes  of  the  weather,  though  it  is  more  constant 
there  than  in  the  temperate  climates  :  there  are  changes 
there  of  humidity  and  dryness,  rains  and  fair  weather, 
storms  and  tempests,  &c.  much  more  violent  than  with 
us ;  and  yet  all  these  take  place  without  any  way  af- 
fecting the  barometer. 


MR.  DALTON's    GENERAL    RULES   AND  OBSERVATIONS 
FOR    JUDGING    OF    THE    WEATHER. 

2.  The  barometer  is  highest  of  all  during  a  long 
frost,  and  generally  rises  with  a  N.E.  wind  ;  It  is  lowest 
of  all  during  a  thaw  following  a  long  frost,  and  is  often 
brought  down  by  a  S.W.  wind. 

2.  When  nearest  the  high  extreme  for  the  season 
of  the  year,  there  is  very  little  probability  of  immediate 
rain. 

3.  When  the  barometer  is  low  for  the  season,  there 
is  seldom  a  great  weight  of  rain,  though  a  fair  day  in 
such  a  case  is  rare.  The  general  tenor  of  the  weather 
at  such  times  is,  short,  heavy,  and  sudden  showers, 
with  squalls  of  wind  from  theS.W.,  the  W.  or  N.W. 

4.  In  summer,  after  a  long  continuance  of  fair  wea- 
ther, with  the  barometer  high,  it  often  falls  gradu- 
ally, and  for  one,  two,  or  more  days,  before  there  is 
much  appearance  of  rain.  If  the  fall  be  sudden  and 
great  for  the  season,  it  will  probably  be  followed  by 
thunder. 

5.  When  the  appearances  of  the  sky  are  very  promis- 
ing for  fair  weather,  and  the  barometer  at  the  same 
time  low,  it  may  be  depended  upon  that  the  appear- 
ances will  not  remain  such  long.  On  these  occasions 
the  face  of  the  sky  changes  very  suddenly. 


FURTHER  INDICATIONS  FROM  THE  BAROMETER.      481 

6.  Very  dark  and  dense  clouds  pass  over,  when  the 
barometer  is  high,  without  rain ;  but  when  the  baro- 
meter is  low,  it  sometimes  rains  almost  without  any  ap- 
pearance of  clouds. 

7.  All  appearances  being  the  same,  the  higher  the 
barometer  is,  the  greater  is  the  probability  of  fair 
weather. 

8.  Thunder  is  generally  preceded  by  hot  weather, 
and  followed  by  cold  and  showery  weather. 

9.  A  sudden  and  extreme  change  of  the  temperature 
of  the  atmosphere,  either  from  heat  to  cold,  or  cold  to 
heat,  is  generally  followed  by  rain  within  24  hours. 

10.  In  winter,  or  during  a  frost,  if  it  begin  to  snow, 
the  temperature  of  the  air  generally  rises  to  32°,  and 
continues  there  while  the  snow  falls ;  after  which,  if 
the  weather  clear  up,  expect  a  severe  cold. 

1 1 .  The  aurora  borealis  is  a  prognostic  of  fair  weather. 


FURTHER    INDICATIONS    OF    THE    WEATHER    BY    THE 
BAROMETER. 

In  general,  when  the  barometer  falls,  there  is  rain  ; 
but  when  the  mercury  rises,  it  is  a  sign  of  fair  weather. 

If  the  mercury  fall  in  a  frost,  we  may  expect  snow, 
or  a  thaw  ;  but  if  it  rise  in  winter,  with  a  north  or  east 
wind,  it  generally  portends  a  frost. 

If  the  mercury  sink  slowly  and  gradually,  we  may 
expect  that  the  rain  will  be  of  some  continuance  ;  and 
if  the  rise  be  gradual,  we  may  judge  that  the  fine 
weather  will.be  lasting.  If  it  fluctuate  much,  rising 
and  falling  suddenly,  the  weather  is  unsettled  and 
changeable:  if  it  fall  very  low,  there  will  be  much 
rain ;  but  if  its  falls  be  low  and  sudden,  a  high  wind 
generally  ensues :  when  exceeding  low,  storms  and 
tempestuous  weather  may  be  expected  ;  but  if  an  extra- 
ordinary fail  happen,  without  any  remarkable  change 
near  at  hand,  it  is  probable,  that  there  is  a  storm  at  a 
distance. 

The  descent  of  the  barometer  is  not,  however,  always 
an  indication  of  rain,  for  it  will  often  fall  for  wind  ;  nor 

VOL.  IV.  ,.        3Q 


482  FURTHER    INDICATIONS 

is  its  rise  a  certain  sign  of  fair  weather,  particularly  if 
the  wind  be  northerly  or  easterly.  If  the  fine  weather 
be  lasting,  with  a  westerly  wind,  the  mercury  generally 
rests  a  little  above  changeable,  but  somewhat  below 
thirty  inches. 

In  the  summer  months  the  barometer  does  not  vary 
so  much  as  in  winter  ;  the  greatest  variations  are  in  the 
first  two,  and  the  last  two  months  of  the  year,  but  par- 
ticularly in  the  first  and  last.  A  northeast  wind  gen- 
erally makes  the  barometer  in  this  country  rise,  and  it 
is  generally  lowest  with  a  south-westerly  wind. 

If  the  mercury  continue  to  fall  while  it  rains,  it  will 
be  likely  to  rain  the  next  day :  when  the  mercury  is 
pretty  high,  and  has  fallen  to  foretel  rain,  and  yet  rises 
before  the  rain  falls,  it  is  an  indication  that  there  will  be 
but  little.  In  fair  weather,  when  the  mercury  has  con- 
tinued high  and  rising,  if  it  fall  about  noon,  and  rise 
again  towards  the  evening,  a  single  shower  may  be  ex- 
pected on  the  evening  or  noon  of  the  next  day,  and 
then  fair  weather.  When  the  mercury  rises  gradually 
about  half  a  tenth,  and  continues  to  do  so  for  many 
days,  the  fair  weather  may  be  expected  to  continue  for 
some  time,  unless  wind  intervenes,  particularly  from 
the  S.  W.  by  S. 


FROM    THE    THERMOMETER. 

In  winter,  if  the  cold  diminish  suddenly,  it  in  ge- 
neral portends  rain ;  in  summer,  a  sudden  augment- 
ation of  heat  is  also  a  forerunner  of  rain. 


FROM    THE    BAROMETER    AND    THERMOMETER. 

If  the  air  in  foggy  weather  becomes  hotter  by  the 
action  of  the  sun  alone,  the  fog  generally  dissipates  and 
the  air  remains  serene  :  but  if  the  barometer  fall,  and 
the  change  of  temperature  be  from  a  south  or  south- 
west wind,  the  fog  rises  and  forms  itself  into  clouds, 
and  its  ascension  is  generally  a  sign  of  rain. 


[   483  J 


FROM  THE  BAROMETER,  HYGROMETER,  WIND,  AND 
STATE  OF  THE  SKY. 

The  barometer  being  high  and  stationary,  the  natu- 
ral and  factitious  hygrometers  indicating  dry  air,  the 
canopy  of  the  sky  lofty,  and  the  wind  north-easterly, 
are  the  surest  signs  of  settled  fair  weather ;  while  a  light 
and  moist  atmosphere,  the  canopy  of  the  sky  low,  and  a 
south-west  wind,  certainly  portend  a  wet  season. 


FROM    CLOUDS. 

When  the  clouds  are  formed  like  fleeces  deep  and 
dense  towards  the  middle,  and  very  white  at  the  edges, 
with  a  bright  blue  sky  about  them,  they  generally  soon 
fall  in  hail,  snow,  or  in  hasty  showers  of  rain.  In  the 
north  of  England,  such  clouds  are  called  woolpacks. 

There  is  no  sign  of  rain  more  certain  than  two  dif- 
ferent currents  of  clouds,  especially  if  the  undermost 
fly  fast  before  the  wind ;  when  this  happens  in  sum- 
mer, there  is  seldom  wind  at  the  time,  and  thunder  ge- 
nerally follows.  In  winter  the  light  vapour,  or  scud  as 
the  sailors  call  it,  often  comes  rapidly  against  the  wind, 
and  a  gale  is  soon  after  to  be  expected. 

The  transparency  of  the  air  is  to  the  inhabitants  of 
the  Alps  one  of  the  most  certain  signs  of  rain ;  when 
the  distant  objects  appear  distinct  and  well  defined, 
when  the  sky  is  of  a  deep  blue,  they  consider  rain  as 
near  at  hand,  though  no  other  signs  appear.  I  have 
been  informed  by  a  gentleman,  to  whom  I  am  under 
obligations  for  other  observations,  that  this  sign,  from 
the  transparency  of  the  air,  is  by  no  means  local,  but 
is  often  observed  in  England ;  that  in  such  a  state  of 
the  air,  the  sailors  say  the  land,  or  other  object,  looms 
near,  and  expect  bad  weather. 

When  the  sky,  in  a  rainy  season,  is  tinged  with  a 
sea-green  colour,  particularly  near  the  horizon,  when  it 
ought  to  be  blue,  the  rain  will  continue  and  increase.  If  it 


484  SUPERIORITY    OF    THE    NORTHERN 

be  of  a  deep  dead  blue,  it  will  be  showery  :  this  is  more 
particularly  found  to  hold  true  near  the  sea  coast. 

Clouds  of  a  similar  appearance  produce  thunder  in 
summer,  and  snow  in  winter ;  such  clouds  are  broken, 
and  irregularly  shaped,  heaped  one  on  another,  and 
from  their  uncommon  density  project  towards  the  earth. 
After  a  thunder  storm,  when  it  has  been  of  considerable 
duration,  the  wind  generally,  if  not  always,  veers  to  the 
quarter  from  whence  the  first  clap  proceeded. 

A  close  sultry  day,  the  current  of  air  scarcely  per- 
ceptible,  is  often  succeeded  by  a  change  in  the  wind. 

The  wind  shifting  from  point  to  point  round  the 
compass,  generally  denotes  rain.  If,  after  a  continued 
rain  from  a  muddy  sky,  the  horizon  appear  lighter  in 
any  quarter,  expect  the  wind  from  that  quarter. 


OF  THE  SUPERIORITY  OF  THE  NORTHERN  HEMIS- 
PHERE OVER  THE  SOUTHERN,  FROM  THE  REV. 
MR.  JONES*  S  PHYSIOLOGICAL  DISQUISITIONS. 

The  superiority  of  the  northern  hemisphere  of  the 
world,  above  the  southern,  is  very  manifest.  It  has 
more  land,  more  sun,  more  heat,  more  light,  more  arts, 
more  sense,  more  learning,  more  truth,  more  religion. 
The  land  of  the  southern  hemisphere,  that  is,  the  land 
which  lies  on  the  other  side  of  the  equinoctial  line,  does 
not  amount  to  one  fourth  part  of  what  is  found  on  the 
north  side. 

The  sun,  by  reason  of  the  eccentricity  of  the  earth's 
orbit,  and  the  situation  of  the  aphelion,  makes  our 
summer  eight  days  longer  than  the  summer  of  the  other 
hemisphere  ;  which,  in  the  space  of  four  thousand  years 
(for  so  long  it  is  since  any  universal  change  has  taken 
place  in  the  earth),  amounts  to  upwards  of  eighty-seven 
years ;  and  so  much  more  sun  has  this  hemisphere  en- 
joyed than  the  other.  What  effects  may  have  been 
arising  gradually  in  all  that  time,  we  cannot  ascertain ; 
but  such  a  cause  cannot  have  been  without  its  effect : 
and  I  think  it  is  allowed,  that  the  temperature  of  the 
earth  and  atmosphere,  in  the  highest  latitudes  of  the 


OVER  THE    SOUTHERN    HEMISPHERE.  485 

north,  is  much  more   mild  and  moderate  than  in  the 
correspondent  latitudes  of  the  south.     The  dreary  face 
of  Statenland,  with  the  weather-beaten  Cape  of  South 
America,  a  climate  so  severe  as  scarcely  to  admit  of 
any  human  inhabitants,  is  no  nearer  to  the  pole  than 
th_j  northern  counties  of  England  :  but  the  difference 
in  the  atmosphere,  and  in  the  aspect  of  the  earth,  is 
almost  incredible  ;  and  this  is  the  more  remarkable,  be- 
cause there  is  no  mountainous  country  betwixt  that  and 
the  pole  to  account  for  the  icy  blasts  that  prevail  there. 
But  it  is  also  further  observable,  that  the  northern 
hemisphere  is  better  provided  for  by  night  as  well  as 
by  day.     The  stars  of  superior  magnitudes  are  much 
more  numerous  on  this  side  the  equinoctial  than  on  the 
other :  we  have  nine  stars  of  the  first  magnitude,  and 
they  but  four  ;  and  the  stars  of  the  Great  Bear,  so  con- 
spicuous in  this  hemisphere,   having  nothing  to  equal 
them  about  the  other  pole.     When  the  sun  is  remote 
from  us  in  the  winter,  our  longest  nights  are  illuminat- 
ed by  the  principal  stars  of  the  firmament ;  when  the 
sun  enters   Capricorn,   there  comes  to  the  meridian, 
about  midnight,  the  whole  constellation  of  Orion,  the 
brightest  in  the  heavens,  containing  two  stars  of  the 
first  magnitude,   four  of  the  second,  and  many  others 
of  inferior  sizes ;  and  upon  the  meridian,  or  near  it, 
there  are  four  more  stars  of  the  first  magnitude,  Capel- 
la,  Sirius,  Procyon,  and  Aldebaran.     No  other  portion 
of  the  heavens  affords  half  so  much  illumination  ;  and 
it  is  exactly  accommodated  to  our  midnight,  when  the 
nights  are  longest  and  darkest.     If  the  mid-winter  of 
the  southern  hemisphere  be  compared,  the  inferiority 
of  the  nocturnal  illumination  is  wonderful. 

Though  it  will  carry  us  a  little  beyond  the  bounds 
of  physics,  the  parallel  is  so  glaring  between  the  natu- 
ral and  intellectual  superiority  of  this  part  of  the  world, 
that  your  time  will  not  be  lost  while  we  reflect  upon  it. 
Here  the  arts  of  war  and  peace  have  always  flourished  ; 
as  if  this  part  of  the  globe  had  been  allotted  to  a  supe- 
rior race  of  beings.  Asia  and  Europe,  from  the  re- 
remotest  times,  have  been  the  seats  of  science,  elo- 
quence, and  military  power  ;  compared  with  which,  the 


486  SUPERIORITY    OF    THE    NORTHERN,  &C. 

southern  regions  have  ever  been,  as  we  now  find  them, 
beggarly  and  barbarous ;  possessed  by  people  stupid 
and  insensible,  illiterate,  and  incapable  of  learning. 
Where  are  the  poets,  the  historians,  the  orators,  the 
philosophers  of  the  southern  world  ?  We  may  as  well 
search  for  the  sciences  among  the  beasts  of  the  wilder- 
ness. 

All  the  inventions,  by  which  mankind  have  done  ho- 
nour to  themselves  in  every  age,  have  been  confined  to 
this  side  of  the  world.  Here  the  mathematical  sciences 
have  flourished  ;  printing  has  been  found  out ;  gun- 
powder and  fire-arms  invented  ;  navigation  perfected ; 
magnetism  and  electricity  cultivated  to  the  astonish- 
ment of  the  wisest ;  and  philosophy  extended  by  ex- 
perimental inquiries  of  every  kind.  There  would  be 
no  end,  if  we  were  to  trace  this  comparison  through  eve- 
ry improvement ;  for  here  we  have  every  thing  that 
can  adorn  human  life,  and  there  they  have  nothing. 

But  the  difference  is  most  conspicuous,  when  we 
compare  the  north  and  south  in  point  of  religion  ;  to 
which,  indeed,  that  pre-eminence  is  owing  on  our  side, 
which  has  extended  to  every  branch  of  social  civiliza- 
tion and  intellectual  improvement.  It  it  notorious  at 
this  day,  that  arts  and  learning  flourish  to  the  highest 
degree,  in  those  countries  only  that  are  enlightened  by 
Christianity,  and  no  where  so  much  as  in  this  kingdom, 
where  that  religion  is  established  in  its  purest  form. 
May  it  long  continue  !  and  may  we  know  our  own  fe- 
licity in  the  enjoyment  of  it !  for  religion  is  undoubt- 
edly the  sun  that  gives  light  to  the  mind ;  the  vital 
spirit  that  animates  the  human  understanding  to  its 
highest  achievements  ;  though  many  have  been  in- 
debted to  it,  without  being  sensible  of  their  obligation, 
or  without  confessing  it ;  and  others  have  turned  against 
it  that  light  which  they  borrowed  from  itself. 

The  northern  hemisphere  then,  whatever  preference 
it  may  have  in  a  physical  capacity,  has  been  much  more 
honoured  by  the  superior  advantages  of  learning  and 
religion  :  here  knowledge  first  began  to  be  diffused, 
and  the  world  itself  was  first  inhabited,  in  the  finest 
climates  of  the  earth,  which  are  about  the  latitudes  36 


CONCLUSION.  s  487 

degrees,  &c.  north  :  here  the  church  was  first  settled ; 
and  the  Hebrew  nation,  rising  by  degrees  till  the  reign 
of  Solomon,  formed  a  wise,  wealthy,  and  splendid  king- 
dom, long  before  the  powers  of  Greece  and  Rome  were 
heard  of :  here  the  light  of  Christianity  was  afterwards 
manifested,  and  with  it  the-lights  of  learning  have  been 
extended  to  the  parts  where  they  were  never  known  be- 
fore, till  both  of  them  reach  to  the  utmost  boundaries 
of  the  west,  in  the  once  unknown  regions  of  the  At- 
lantic world. 


CONCLUSION. 

I  have  now  finished  my  course  of  lectures,  and  have 
given  you  a  general  view  of  the  principal  phenomena  in 
nature ;  nor  have  I  been  inattentive  to  the  discoveries 
made  therein  by  man.  I  have  endeavoured  to  point  out 
the  abuse  that  may  be  made  of  physical  inquiries,  and 
to  guard  you  against  the  errors  by  which  they  may  be 
perverted  and  rendered  a  prop  to  support  the  weak  fa- 
bric of  infidelity  and  falsehood.  From  these  lectures 
it  evidently  appears,  "  1.  That  man  is  composed  of  two 
substances,  of  which  one  perceives  without  being  per- 
ceived by  the  senses  ;  and  the  other  is  perceived  with- 
out having  any  perception  in  itself.  2.  That  man,  in 
his  present  state,  can  perceive  nothing  more  of  the  uni- 
verse than  what  is  transmitted  to  him  by  his  organs, 
whose  faculties  are  very  limited.  3.  That  there  are  evi- 
dently effects  perceptible  by  man,  which  are  occasioned 
by  beings  that  he  cannot  perceive.  4.  That  man  deprived 
only  of  one  sense,  sight,  would  have  been  ignorant  of 
the  greater  part  of  what  he  knows  of  the  universe, 
namely,  of  entire  classes  of  beings,  and  of  the  relations 
of  these  beings  to  each  other,  and  to  those  with  which 
he  is  acquainted.  5,  and  lastly.  By  every  rule  of  ana- 
logy, and  from  many  phenomena,  it  is  highly  probable, 
that,  there  exist  may  classes  of  beings,  related  to  each 
other,  and  to  man,  which  he  cannot  in  his  present  state 
perceive."* 

*  De  Luc ,  Lettres  Physiques  et  Morales,  torn.  v.  p.  11,  and  689. 


488  CONCLUSION. 

The  spiritual  powers  of  man  are  roused  into  action 
by  the  medium  of  the  senses.  His  understanding  ex- 
pands itself  by  the  perceptions  the  senses  transmit ;  so 
that,  notwithstanding  the  extent  of  his  powers,  he  can 
make  no  progress  in  matters  higher  than  sense,  unless 
he  take  the  creation  for  his  lesson,  and  the  Omniscient 
Creator  for  his  Preceptor.  It  is  therefore  weak  and 
perverse  in  him,  without  the  very  elements  of  know- 
ledge in  his  head,  to  desert  such  a  wise  and  kind  in- 
structor, and  then  set  up  for  an  independent  discoverer. 
Put  the  philosopher  to  the  trial,  who  pretends  to  know  so 
much  of  a  Deity  without  allowing  him  to  discover  him- 
self and  explain  his  own  works,  and  you  will  soon  see 
the  wise  man  confounded  by  his  own  wisdom.  If  this 
wanted  proof,  I  need  only  mention  the  writings  of 
Helvetius,  Voltaire,  Diderot,  De  la  Metrie,  and  the  whole 
school  of  Condorcet. 

In  contradiction  to  these  men,  I  have  endeavoured 
to  show  that  philosophy  is  illustrated,  and  just  views  of 
nature  are  exhibited  by  the  sacred  writings.  What  in- 
deed can  we  think  of  those  who  would  have  us  believe 
they  credit  the  scriptures,  while  they  take  upon  them 
to  correct  its  stile,  as  not  philosophically  just  ?  who 
would  have  us  believe,  that  he  who  holds  all  nature  in 
his  hand,  does  not  know  how  to  accommodate  his  doc- 
trines to  the  capacities  of  the  vulgar,  without  speaking 
with  philosophical  impropriety  of  his  own  works  ?  Will 
they,  indeed,  teach  him  to  speak,  who  gave  a  mouth  to 
man,  whose  word  was  sufficient  to  cause  the  mighty  sun 
to  shine,  and  daily  diffuse  his  treasures  of  light  around 
the  heavens,  irradiating  the  shifting  hemispheres  of  the 
revolving  earth,  and  at  whose  command  it  is  surround- 
ed by  the  liquid  air  ?  Shall  the  writings  of  men  have 
excellencies  in  our  eyes,  and  his  have  no  beauty,  who 
hath  meted  out  the  heavens,  who  knoweth  the  ba- 
lancing of  the  clouds,  and  by  whose  knowledge  the 
deeps  are  broken  up  ? 

Both  his  word  and  his  works  prove,  that  he  has 
employed  and  displayed  infinite  wisdom,  power,  and 
goodness,  in  the  creation  of  this  universe ;  that  he  has 
with  stupendous  artifice  stored   our  globe  with  every 


CONCLUSION.  489 

thing  necessary,  not  only  for  the  support,  but  for  the 
felicity  of  man :  all  his  works  are  stamped  with  the 
characters  of  the  infinite  perfections,  and  overflowing 
goodness  of  the  Author.  He  has  given  to  man,  and 
to  him  alone,  a  capacity  to  be  entertained  with  the  mag- 
nificence,  the  beauty,  the  harmony,  and  the  order  of 
the  universe  ;  and  has  so  moulded  his  heart  and  spirit, 
as  to  make  pleasure  attendant  on  admiration,  and  love 
and  gratitude  the  necessary  companions  of  the  senses  of 
favours  received. 

Let  us  then  praise  the  God  of  heaven,  from  whom 
we  have  received  so  much — whose  mercy  is  extended 
over  all. 

Let  every  thing  that  hath  breath  praise  him  ;  and  let 
man,  the  priest  of  the  creation,  offer  up  a  sacrifice  of 
thanksgiving  unto  the  Most  High. 


APPENDIX  TO  LECTURE  LIL 
BY  THE  E.  EDITOR. 

CONTAINING    A    FURTHER    DESCRIPTION    OF    METE- 
OROLOGICAL    INSTRUMENTS  J    WITH    FIGURES. 


I  HE  barometer,  as  already  described  by  our  Au- 
thor, is  called  the  chamber  barometer.  When  the  in- 
strument is  constructed  to  be  used  at  sea,  on  board  a 
ship,  it  is  called  the  marine  barometer,  and  which  is 
made  somewhat  different  from  the  chamber  one,  in  or- 
der to  prevent  the  violent  concussions  of  the  mercury 
on  the  top  of  the  tube,  and  the  unsettled  state  of  its  al- 
titude, caused  by  the  motion  of  the  ship.  There  have 
vol.  iv.  J  3R 


490  THE    BAROMETER. 

been  various  contrivances  to  obviate  these ;  but  the  best 
appears  to  me,  to  consist  in  drawing  about  two  feet  of  the 
lower  portion  of  the  barometer  tube  to  a  fine  aperture, 
almost  capillary :  the  resistance  so  occasioned  to  the  mo- 
tion of  the  mercury  in  the  tube,  is  found  sufficient  to  re- 
tard and  destroy  a  violent  motion,  and  to  produce  a  just 
altitude  of  the  mercury.  It  requires  a  little  longer  time 
for  the  settling  of  the  mercury  to  its  true  altitude,  but 
this  is  of  no  consequence  to  the  observation. 

The  frame  of  the  instrument  is  suspended  on  gimbals 
near  to  the  centre  of  gravity,  and  occasionally  to  be 
screwed  either  to  the  ceiling  or  side  of  a  cabin ;  and 
from  these  positions  the  instrument  has  been  found  to 
answer  sufficiently  well  all  the  purposes  for  which  it  is 
wanted  at  sea.  Fortunate  is  it  for  the  mariner,  when  by 
the  alterations  of  the  altitude  of  the  mercury  he  can  fore- 
tel  the  approach  of  a  storm,  or  tempestuous  state  of  the 
atmosphere. 

The  instrument  should  be  accompanied  with  a  thermo 
meter. 


A  BAROMETER  TO  MEASURE  THE  HEIGHTS  OF  MOUN- 
TAINS, DEPTHS  OF  VALLIES,  HEIGHTS  OF  BALLOONS, 
&C.  &C. 

The  barometer  has  been  found  to  be  the  most  conve- 
nient and  accurate  instrument  that  can  be  used  for  these 
purposes.  By  experiments  made  by  M.  de  Saussure,  de 
Luc,  and  Sir  George  Shuchburgh,  it  appears  that  heights 
and  depths  have  been  ascertained  to  a  few  feet  in  several 
thousands.  The  instrument  requires  to  be  made  with  the 
utmost  accuracy,  and  great  diligence  and  attention  paid 
to  the  adjustments,  &c.  during  the  observation.  The  tube 
of  this  sort  of  barometer  has  its  lower  extremity  drawn 
out  to  a  small  aperture  ;  a  floating  index  applied,  so  as 
to  be  depended  upon  to  at  least  the  500th  part  of  an 
inch,  as  a  gage  point ;  the  frame  made  very  light  either 
of  wood  or  a  brass  tube ;  the  scale  of  inches  extended 
downwards  to  about  17  or  18  inches,  and  a  portable 
mahogany  tripod  having  folding  legs  with  gimbals,  made 


DE    LUC'S    HYGROMETER,  491 

to  support  it  when  in  use,  or  serve  as  a  case  for  the  in- 
strument  when  not  in  use. 

To  measure  heights,  &c.  in  the  most  accurate  manner, 
the  observer  must  be  provided  with  two  barometers,  or 
in  case  of  an  accident,  with  a  third  :  the  nonius  or  slid- 
ing plate  to  the  scale  of  inches  should  subdivide  it  into 
the  500th  part  at  least.  There  should  be  a  thermometer 
attached  to  each  instrument,  and  two  detached  corre- 
spondent ones  for  the  pocket.  The  manner  of  making 
observations  and  computing  from  these  instruments,  the 
reader  will  see  in  Sir  George  Shuchburgb1  s  account  in  the 
Philosophical  transactions,  vol.  67  and  68. 

Geometrical  measurement  with  the  assistance  of  good 
angular  instruments  is  the  best  method,  when  a  good 
base  is  afforded ;  but,  as  few  countries  afford  a  suitable 
base,  or  favourable  circumstances  in  the  figure  or  situa- 
tion of  the  mountains  to  be  measured,  the  barometer  is 
the  instrument  most  frequently  adopted. 


DE    LUC'S    HYGROMETER. 

M.  de  Luc's  hygrometer,  made  of  a  fine  slip  of  whale- 
bone, is  the  most  approved  instrument  of  the  kind,  and 
in  the  most  general  use.  It  has  been  found  by  him  of 
greater  expansibility  than  any  other  substance,  such  a 
slip,  lengthening  about  one-eighth  of  itself  from  extreme 
dryness  to  extreme  moisture ;  it  is  a  substance  easy  to  be 
cut  into  slips ;  and  they  have  been  made  so  fine,  as  in  a 
length  of  six  or  eight  inches  to  weigh  only  one-tenth  of 
a  grain  ;  on  this  account  it  is  the  most  suitable  substance 
for  a  common  hygrometer. 

Fig.  1  of  the  following  page  shows  its  form  as  now 
made  for  common  use ;  it  is  made  of  various  dimen- 
sions, but  the  figure  is  about  one-half  the  size  of  those 
generally  made.  The  frame-work  is  of  brass,  lightly 
made,  and  can  easily  be  understood  by  the  figure,  with- 
out a  detailed  description  here.  The  slip  of  whalebone  is. 
represented  by  a  £,  and  at  its  end,  <z,  is  shown  a  sort  of 
pincers  made  of  flattened  bent  wire,  tapering  in  the 
part  that  holds  the  slip,  and  pressed  by  a  sliding  ring. 


[     492     ] 


Fig.  I 


Fig.  2. 


FlgA 


The  end,  £,  is  fixed  to  a  moveable  bar,  c9  which  is  mov 
ed  by  a  screw  for  adjusting  first  the  index  on  the  dial 
ring.  The  end,  a,  of  this  slip  is  hooked  to  a  thin  brass 
wire,  to  the  other  end  of  which  is  also  hooked  a  very  thin 
lamina  that  has  at  that  end  pincers  similar  to  those  of  the 
slip,  and  which  is  fixed  by  the  other  end  to  the  axis  of  a 
pin  in  a  proper  hole.  The  spring,  d,  by  which  the  slip  is 
stretched,  is  made  of  silver  gilt  wire ;  it  acts  on  the  slip 
as  a  weight  of  about  twelve  grains.  The  central  pulley  or 


DE   LUC'S    HYGROMETER.  493 

^xis  has  very  small  pivots,  the  shoulders  of  which  are 
prevented  from  coming  against  the  frame  by  their  ends 
being  confined,  though  freely,  between  the  flat  bearings 
of  two  screws,  the  fronts  of  which  appear  in  the  figure. 
The  dial-ring  at  top  is  divided  into  100  equal  parts,  show- 
ing the  expansion  of  the  slip  from  extreme  dryness,  mark- 
ed D,  to  extreme  moisture,  marked  M. 

A  principal  nicety  in  constructing  the  instrument  is  in 
so  proportioning  the  diameter  of  the  axis  or  pulley,  that 
this  whole  expansion  may  exactly  commensurate  the  cir- 
cumference of  the  ring,  as  pointed  out  by  the  index. 
The  form  of  the  instrument  as  now  described  adapts  it 
chiefly  for  use  within  doors.  For  exposure  at  the  outside 
of  a  window,  the  slip  of  whalebone  is  generally  inclosed 
within  a  half  tube  of  brass  perforated  with  two  vertical 
rows  of  large  holes,  and  a  whole  tube  is  made  to  go  over 
this  half  one,  which  has  also  two  vertical  rows  of  holes 
at  such  a  distance  from  one  another,  that  when  one  of 
them  corresponds  to  one  of  the  rows  of  the  half  tube, 
the  other  is  in  front.  The  rows  of  holes  of  the  tube  are 
to  be  turned  towards  the  room  to  prevent  the  rain  from 
falling  on  the  slip,  and  the  dial  with  the  axis  and  index 
being  included  in  a  box  with  a  glass  in  front,  no  rain  can 
get  into  the  instrument.  It  must  also  be  fixed  in  a  place 
not  much  exposed  to  the  sun,  or  be  screened  from  it 
without  preventing  the  circulation  of  the  air. 

In  the  preparation  of  the  slip  of  whalebone,  the  points 
of  extreme  dryness  and  extreme  moisture  are  to  be  care- 
fully ascertained.  The  former  M.  de  Luc  directs  to  be 
had  by  means  of  large  pieces  of  quick-lime,  taken  from 
the  kiln  and  suffered  only  to  lose  the  red  heat,  put  into  a 
lime-vessel  j  and  the  latter,  simply  by  immersion  in  wa- 
ter. 

The  best  lime-vessel  M.  de  Luc  constructed  and  con- 
trived, is  as  follows :  it  consists  of  two  tin  vessels,  the  first 
of  which  and  the  most  used  is  16J  inches  high,  15|  inch- 
es wide,  and  5  inches  deep.  The  front  of  this  vessel  is  a 
plate  of  glass,  and  the  back  a  tin  plate  slider,  which  be- 
ing taken  off,  leaves  that  side  of  the  vessel  quite  open. 
The  second  vessel  has  the  same  dimensions  as  the  first, 
but  its  back  is  soldered,  and  its  front  is  of  woven  brass 


494 

wire.  This  vessel  may  be  applied  to  the  back  of  the  for- 
mer, in  such  a  manner  as  to  make  of  both  one  single 
vessel ;  which,' when  the  slider  of  the  fore  part  is  taken 
off,  is  only  divided  by  a  vertical  partition  of  the  woven 
brass  wire.  The  use  of  that  second  vessel  is  to  produce 
extreme  dryness  in  the  other,  for  which  purpose  it  is  fill- 
ed with  large  pieces  of  quick-lime  taken  from  the  kiln. 
When  that  vessel  is  not  used,  it  is  kept  in  a  tin  box  which 
it  fills  entirely ;  and  when  it  is  in,  as  well  as  while  it  is 
out  for  use,  that  box  is  kept  shut  with  putty,  by  which 
means  the  lime  may  serve  many  times,  in  the  following 
manner  :  when  I  want  to  produce  extreme  dryness  in  the 
first  vessel,  says  M.  de  Luc,  I  apply  it  to  the  second,  fast- 
ened with  hooks  ;  I  then  pull  out  the  slider  of  the  first, 
and  stop  with  putty  the  chinks  between  them.  When  that 
first  operation  is  completed,  I  put  again  the  slider  to  the 
fore  vessel,  and  take  off  the  other.  In  this  last  operation, 
some  moisture  might  be  introduced  through  the  chinks 
of  the  slider  before  they  are  again  stopped  with  putty,  es- 
pecially as  the  destruction  of  the  air  in  the  vessel  has  made 
room  for  more  air  to  come  in  ;  but  I  prevent  it,  by  making- 
first  the  apparatus  sensibly  warmer  than  it  was  when  I  put 
on  the  lime-vessel ;  by  which  means,  in  the  little  time  em- 
ployed for  the  operation,  the  motion  of  the  air  is  from 
the  inside  to  the  outside,  which  prevents  all  access  of 
moisture  in  the  vessel. 

On  the  top  of  this  vessel  may  be  made  square  open- 
ings with  close  shutters,  and  withinside  just  underneath 
a  wire  fastened  across  with  several  hooks,  upon  which 
are  to  be  suspended  hygrometers  to  be  adjusted  to  ex- 
treme dryness.  A  few  hours  is  the  time  generally  neces- 
sary, but  of  this  the  practitioner  can  very  readily  judge. 

This  vessel  was  constructed  by  M.  de  Luc  to  assist  in 
his  experiments  on  the  comparative  changes  of  weight 
and  dimensions  of  some  hygroscopic  substances,  but  it 
is  equally  useful  for  the  purpose  of  adjusting  the  hygro- 
meters. 

For  a  description  of  another  previous  vessel  for  this 
purpose,  as  well  as  a  valuable  paper  on  the  subject  of  hy- 
grometry  by  M.  de  Luc,  see  the  Philosophical  Transac- 
tions for  1791. 


C    49<s    ] 


six's  improved  thermometer. 

The  late  ingenious  Mr.  Six  constructed  a  thermome- 
ter that  was  a  self-register  of  the  extreme  degrees  of  heat 
and  cold  during  the  observer's  absence.  It  is  properly  a 
spirit  of  wine  thermometer,  with  mercury  connected  to 
support  two  indices  acting  upon  it  in  two  different  tubes. 
Each  index  acts  by  a  spring  within  the  tubes,  so  that  be- 
ing pressed  up  to  particular  divisions  by  the  extremes  of 
heat  and  cold  acting  on  the  spiiit,  they  remain  there  for 
the  observer's  inspection  ;  the  indices  being  chiefly  made 
of  inclosed  steel,  a  small  artificial  magnet  applied  to  them, 
will  bring  them  down  to  the  surface  of  the  mercury  for 
a  fresh  observation. 

This  thermometer  is  somewhat  difficult  to  be  construct- 
ed from  the  impracticability  of  obtaining  equable  bores 
of  the  glass  tubes  for  the  action  of  the  indices,  as  a  small 
irregularity  will  generally  obstruct  their  rise  by  the  ac- 
tion of  the  mercury.  It  requires  a  considerable  draught 
of  tubes  to  obtain  a  few  sufficiently  true,  and  then  they 
are  subject  to  be  broken  in  the  formation.  See  Mr.  Six's 
Treatise  on  the  Thermometer,  8vo.  1794. 


RAIN-GAGE. 

Fig.  2  is  a  representation  of  the  rain-gage  as  now  gene- 
rally adopted,  and  already  described  by  our  Author  at 
page  430.  The  funnel  at  top  may  be  either  square  or 
round,  but  the  former  perhaps  is  more  convenient.  The 
proportion  of  the  area  of  the  top  of  the  funnel  to  the  cylin- 
der, in  which  the  rain  descends,  is  as  9  to  1  ;  the  diame- 
ter of  the  former  being  12  inches  and  the  latter  4  inches ; 
the  scale  of  the  floating  index  is  therefore  in  its  divisions 
into  inches  extended  9  times,  and  9  inches  is  divided  into 
100  parts,  from  which  the  fall  of  rain  to  the  *# oth  of  an 
inch  can  be  readily  estimated.  A  small  pipe  near  the  bot- 
tom of  the  cylinder,  stopped  with  a  cork,  is  usually  ap- 
plied to  discharge  the  cylinder  of  the  water  when  quite 
filled  by  the  rain. 


[     496     ] 


WIND-GAGE, 


A  wind-gage,  or  instrument  to  measure  the  force  of 
the  wind  upon  any  given  surface,  is  an  article  of  a  very 
useful  nature,  and  a  perfect  sort  of  one  appears  yet  to  be 
wanting  to  complete  the  arrangement  of  meteorological 
instruments.  A  gage  invented  by  Dr.  Lind  appears  to 
be  the  best  and  most  convenient  yet  contrived  ;  Jig.  4,  is 
a  representation  of  the  instrument.  It  consists  of  two  glass 
tubes,  A B,  CD,  of  five  or  six  inches  in  length,  with 
equable  bores  of  about  four-tenths  of  an  inch  in  diameter; 
they  are  joined  together  by  a  narrow  tube  a  £,  drawn  out 
of  the  larger  ones,  having  a  bore  of  about  one- tenth  of  an 
inch  diameter.  On  the  top  of  A  B  is  fixed  a  brass  tube 
bent  outwards  with  its  mouth  open  towards  F.  On  the 
other  leg,  C  D,  is  cemented  a  cover,  with  a  round  hole, 
G,  in  the  upper  part  of  it,  two-tenths  of  an  inch  in  diame- 
ter ;  this  cover  and  tube  are  connected  together  by  a  slip 
of  brass,  and  serves  to  hold  the  scale  and  strengthen  the 
whole  instrument.  To  the  same  tube  is  soldered  a  piece 
of  brass  e9  with  a  hole  to  receive  the  brass  spindle  K  L ; 
and  at  /  there  is  another  piece  of  brass,  surrounding  and 
steadying  the  glass  tubes. 

There  is  a  shoulder  upon  the  spindle  at  /,  upon  which 
it  rests  and  turns,  and  a  nut  at  /,  to  prevent  its  being  blown 
off  the  spindle  by  the  wind.  The  instrument  is  turned 
round  upon  the  spindle  by  the  wind,  so  as  always  to  pre- 
sent the  mouth  of  the  tube  towards  it.  The  lower  end  of 
the  spindle  is  formed  to  a  screw,  by  which  it  may  be  fast- 
ened to  a  post  or  other  proper  place.  It  has  a  hole  at 
L,  to  admit  a  small  lever  for  screwing  it  by  with  more 
readiness. 

A  thin  brass  plate,  k,  is  fixed  on  the  tube  above  the 
round  hole  G,  to  prevent  the  admission  of  rain.  There 
is  also  a  bent  tube  A  B,  Jig.  3,  to  be  put  occasionally  on 
the  mouth  of  the  tube  F,  to  prevent  the  admission  of  rain 
into  the  gage,  when  left  exposed. 

The  force  or  momentum  of  the  wind  may  be  ascer- 
tained with  the  assistance  of  the  instrument,  by  filling  the 
tubes  half  full  of  water,  and  sliding  the  scale  a  little  up 


WIND-GAGE.  489 

or  down  till  its  0,  when  the  instrument  is  in  a  perpendi- 
cular position,  be  in  a  line  with  the  surface  of  the  water 
in  both  tubes.  The  instrument  being  thus  adjusted,  hold 
it  up  perpendicularly,  and  turning  the  mouth  of  the  tube 
towards  the  wind,  observe  how  much  the  water  is  de- 
pressed by  it  in  the  one  leg,  and  raised  in  the  other.  The 
sum  of  the  two  is  the  height  of  a  column  of  water,  which 
the  wind  is  capable  of  sustaining  at  that  time ;  and  every 
body  that  is  opposed  to  that  wind  will  be  pressed  upon 
by  a  force  equal  to  the  weight  of  a  column  of  water, 
having  its  base  equal  to  the  altitude  of  the  column  of 
water  sustained  by  the  wind  in  the  wind-gage.  Hence 
the  force  of  the  wind  upon  any  body,  where  the  surface 
opposed  to  it  is  known,  may  be  easily  found,  and  a  ready 
comparison  may  be  made  betwixt  the  strength  of  one 
gale  of  wind  and  that  of  another. 

The  force  of  the  wind  may  be  likewise  measured  with 
this  instrument,  by  filling  it  till  the  water  runs  out  at  the 
hole,  G.  For  if  it  be  then  held  up  to  the  wind,  a  quan- 
tity of  water  will  be  blown  out,  and  if  both  legs  of  the 
instrument  be  of  the  same  bore,  the  height  of  the  co- 
lumn sustained  will  be  equal  to  double  the  quantity  of 
water  in  either  leg,  or  the  sum  of  what  is  wanting  in  both 
legs.  But  if  the  legs  be  of  unequal  bores,  neither  of 
them  will  give  the  true  height  of  the  column  of  water, 
which  the  wind  sustained  ;  but  the  true  height  must  be 
found  by  a  formula  given  by  Dr.  Lind  in  the  Philos. 
Trans,  vol.  vii.  or  the  Encyclopaedia  Britannica,  vol. 
Ixv.  page  524,  edition  1797- 


Vol.  IV. 


APPENDIX. 

A  BRIEF  OUTLINE  OF 
PHYSICS ; 

OR, 

Natural  ^fnfostopfip : 

IN  THE  FORM  OF  A  COLLEGIATE  EXAMINATION. 
BY  THE  AMERICAN  EDITOR. 


Professor.  W  HAT  do  you  understand  by  physics,  or  natu- 
ral philosophy  ? 

Student.  It  is  that  science  which,  in  its  most  general  accep- 
tation, treats  of  all  the  various  phenomena  of  nature  in  the  ma- 
terial world :  But,  in  its  more  common  acceptation,  it  is  limited 
to  those  phenomena  which  relate  to  sensible  motion. 

P.  What  is  matter  ? 

S.  Whatever  is  perceivable  by  any  of  the  external  senses. 

P.  How  is  a  competent  knowledge  of  the  general  laws  of  na- 
ture to  be  acquired  ? 

S.  By  adequate  experiments,  and  careful  observations  and 
deductions. 

P.  What  rules  are  we  to  observe  in  deducing  general  laws, 
or  conclusions,  from  particular  phenomena  ? 

S.  1.  That  no  more  causes  of  natural  things  are  to  be  ad- 
mitted than  are  true,  and  sufficient  to  explain  the  phenomena. 

2.  That  effects  of  the  same  kind  are  to  be  considered  as  pro- 
duced by  the  same  cause. 

3.  That  those  qualities  whose  virtues  or  energies  can  neither 
be  increased  nor  diminished,  and  which  are  found  in  all  bodies 
on  which  experiments  can  be  made,  ought  to  be  admitted  as 
qualities  of  all  bodies  in  general. 


492  APPENDIX. 

4.  That  in  experimental  philosophy,  propositions  carefully  de- 
duced from  phenomena,  are,  notwithstanding  contrary  hypotheses, 
to  be  deemed  true,  until  other  phenomena  occur  by  which  they 
may  be  corrected,  or  rendered  liable  to  exceptions. 

P.  What  are  the  general  properties  of  matter? 

5.  1.  Extension  or  magnitude,  and  consequently  figurability. 

2.  Impenetrability,  or  solidity. 

3.  Vis  inertia,  or  resistance  to  change  of  state,  whether  of 
motion  or  rest. 

4.  Capacity  of  motion,  or  of  rest. 

5.  Attraction,  or  tendency  of  its  parts  toward  mutual  approach. 

6.  Repulsion. 

P.  What  is  extension  ? 

S.  That  property  of  matter  by  which  it  occupies  some  part 
of  space,  having  the  dimensions  of  length,  breadth,  and  thick- 
ness. 

P.  Enumerate  some  of  the  inferences  deducible  from  matter 
being  an  extended  substance. 

S.  1.  In  theory,  at  least,  matter  is  infinitely  divisible  ;  since 
every  particle,  however  small,  may  be  conceived  to  be  divided 
into  halves,  and  those  again  into  halves,  ad  infinitum :  and  in 
many  of  the  operations  both  of  nature  and  art,  it  is  actually  di- 
vided into  particles  inconceivably  small : — as  in  gold-leaf,  fifty- 
square  inches  of  which  will  weigh  only  a  single  grain  ; — in  the 
particles  of  water  reduced  to  the  form  of  steam  or  vapour; — in 
the  effluvia  from  oderate  bodies  ;  but  above  all,  in  the  particles 
of  light  from  luminous  bodies. 

2.  Any  finite  quantity  of  matter,  however  small,  being  divisi- 
ble into  an  infinite  number  of  particles,  may  occupy  any  finite 
space,  however  great,  leaving  no  pore,  or  interstice,  unoccu- 
pied, greater  than  any  given  magnitude,  however  small. 
P.  What  do  you  understand  by  impenetrability  ? 
S.  That  property  of  matter  by  which  one  body,  or  particle  ol 
matter,  prevents  all  others  from  occupying  the  same  part  of 
space  with  itself,  at  the  same  time. 

P.  If,  for  instance,  I  thrust  a  knife  into  an  apple,  is  not  this 
a  penetration  of  matter? 

S.  The  particles  of  which  any  aggregate  body  is  composed, 
may  indeed  be  separated,  by  an  intervening  body,  as  in  this  in- 
stance, but  this  is  not,  in  the  philosophical  sense  of  the  term, 
considered  as  a  penetration  of  matter,  or  of  the  particles  them- 
selves. 
P.  What  do  you  understand  by  vis  inertia  ? 
S.  That  property  of  matter  by  which  it  resists  any  change  of 
state,  whether  of  motion  or  rest ;  or  from  one  degree  of  motion 
to  another. 

P.   What  is  the  vis  inertia  of  a  body  proportional  to  ;  or  what 
is  the  measure  of  its  resistance  to  a  change  of  state  ? 

S.  The  measure  of  this   resistance  is  the  force  or  power  ne- 
cessary to  produce  the  given  change. 

P.  Explain  the  terms,  force,  power,  momentum,  and  quantity 
of  motion. 


APPENDIX.  '  493 


S.  These  terms,  which  are  all  of  the  same  import,  may  be 
considered  either  in  relation  to  their  instantaneous  effect,  or  to 
their  aggregate  effect  in  a  given  time.  In  relation  to  the  in- 
stantaneous effect  of  force  or  power,  (though,  strictly  speaking,  no 
such  case  can  ever  occur)  or  its  effect  produced  in  an  indefi- 
nitely small  portion  of  time,  it  is  in  the  compound  ratio  of  the 
weight  and  velocity  of  the  moving  body  or  power.  In  relation 
to  its  aggregate  effect,  it  is  in  the  compound  ratio  of  the  weight, 
mean  velocity,  and  time  of  action  ;  or  of  the  weight  and  square 
of  the  velocity — the  velocity  being  as  the  time. 


Laws  of  Motion, 

P.  What  are  the  general  laws  of  motion,  or  those  upon  which 
all  the  phenomena  of  moving  bodies  may  be  explained  ? 

S.  1.  That  every  body  will  continue  in  a  state  of  rest,  or  uni- 
form rectilineal  motion,  unless  compelled  to  change  its  state, 
by-some  external  force  impressed. 

2.  That  all  change  of  motion  is  proportional  to,  and  in  the 
direction  of,  the  impelling  force. 

3.  That  action  and  re-action  between  two  bodies,  are  always 
equal,  and  in  contrary  directions. 

P.  What  will  be  the  effect  when  a  body  is  put  in  motion  by 
a  simple  instantaneous  impulse,  one  acting  only  during  an  in- 
definitely small  portion  of  time  ? 

S.  It  will  acquire  an  equable  velocity,  or  will  move  through 
equal  spaces  in  equal  times,  and  in  the  direction  of  the  impel- 
ling force. 

P.  What  will  be  the  effect  when  a  body  is  put  in  motion  by 
two  simple  joint  impulses  ? 

S.  It  will  move  in  the  diagonal  of  a  parallelogram,  the  sides 
of  which  are  proportional  to,  and  in  the  direction  of,  the  impel- 
ling forces  respectively. 

P.  How  would  you  find  the  joint  effect  of  three  or  more 
forces  ? 

S.  By  first  finding  the  joint  effect  of  any  two  of  them,  then 
of  that  and  a  third,  &c.  Thus,  any  two  or  more  forces  may  be 
combined  into  one  equivalent  force ;  and,  on  the  contrary,  any 
force  may  be  resolved  into  two  or  more,  one  of  which  may  be 
in  any  given  proportion  and  direction. 


Of  the  Impact  of  Bodies* 

P.  How  are  bodies  considered  in  relation  to  the  effect  of 
impact  ? 

S.  Either  as  elastic,  non-elastic,  or  hard  :  though,  perhaps,  n» 
body  in  nature  is  perfect  in  any  of  these  respects. 

P.  What  is  an  elastic  body? 


494  APPENDIX. 

S.  One  that  has  an  inherent  power  of  restoring  its  figure,  or 
position  of  parts,  when  altered  by  any  external  force,  as  soon  as 
this  force  ceases  to  act. 

P.  What,  a  non-elastic  body  ? 

S.  One  that  has  no  inherent  power  of  restitution,  but  will  re- 
tain any  figure  or  position  of  parts,  produced  by  external  pres- 
sure, after  this  ceases  to  act. 

P.  What,  a  hard  body  ? 

S.  One  that  can  suffer  no  change  of  figure  by  an  external 
force,  without  being  broken  to  pieces. 

P.  Upon  what  general  principle  or  law  may  all  the  effects  of 
bodies,  striking  each  other,  be  explained  ? 

S.  That  the  sum  of  the  momenta,  or  forces,  reckoned  towards 
the  same  point,  or  in  the  same  direction,  will  be  the  same  after 
the  stroke  as  before  it  ;  for,  from  the  3d  general  law  of  motion, 
that  action  and  re-action  are  equal  and  contrary,  one  of  the  bo- 
dies will  gain  exactly  as  much  motion  by  the  stroke  as  the  other 
will  lose. 

P.  Apply  this  principle  to  the  case  of  the  impact  of  non-elastic 
bodies. 

S.  When  one  non-elastic  body  strikes  another,  they  will  both 
move  on  with  a  common  velocity,  since  they  have  no  power  of 
separating.  If,  therefore,  A  and  B  represent  the  weight  of  two 
such  bodies,  a  and  b,  their  respective  velocities  before  the  stroke, 
and  v  their  common  velocity  after  the  stroke  ;  then  A  a+B  b  will 
represent  the  sum  of  their  momenta  before  the  stroke;  and 
A  v-f-B  v  that  of  their  momenta  after  the  stroke ;— 'hence  we 
have  the  following  general  equation  or  theorem,  which  may  be 
applied  to  the  solution  of  all  questions  respecting  the  impact  of 
non-elastic  bodies,  viz. 

Aa-fBb 

Aa+Bb  =  (A+B)xv;  0r,  v= . 

A+B 

P.  Apply  the  same  general  principle  to  the  case  of  the  impact 
of  elastic  bodies. 

S.  When  one  elastic  body  sbikes  another,  the  motion  (to- 
wards the  same  point)  lost  by  the  first  body,  and  consequently 
gained  by  the  second,  will  be  exactly  double  of  what  it  would  be 
in  the  case  of  non-elastic  bodies  : — This  arises  from  the  nature  of 
perfect  elasticity,  where  the  power  or  re-action  of  the  bodies  to 
recover  their  original  figure  will  be  exactly  equal  to  the  power 
by  which  that  figure  was  changed.  Hence,  if  A  and  B  represent 
the  weights  of  two  elastic  bodies,  a  and  b  their  respective  velo- 
cities before  the  stroke,  and  a'  b'  their  respective  velocities  after 
A  a+B  b 

the  stroke;  then =  the  common  velocity  of  A  and  B 

A+B 
after  impact,  if  non-elastic.    This  velocity,  multiplied  by  A  and 
B,  will  give  the  momenta  of  A  and  B  respectively  ;  the  first  sub- 

AB 

tracted  from  A  a  will  leave x(a — b)  the  momentum  lost  by 

A+B 


APPENDIX. 


495 


A,  and  consequently  gained  by  B,  if  non-elastic ;  and,  therefore, 
the  double  of  this  will  be  the  gain  and  loss  of  momenta  when 
elastic.  This  quantity,  therefore,  subtracted  from  the  momen- 
tum of  A,  and  added  to  that  of  B,  before  the  stroke,  will  give 
the  momenta  of  A  and  B  respectively  after  the  stroke  ;  and  these 
momenta  divided  by  the  respective  weights  of  the  bodies,  A  and 

B,  will  give   their   respective  velocities  after  the    stroke,    viz. 

2  B  2  A 

a X(a — b)=the  velocity  of  A  ;  andbH X(a — b)= 

A+B  A-fB 

the  velocity  ofB;  and  these  are  general  theorems  applicable  to 
the  solution  of  all  questions  relative  to  the  impact  of  elastic 
bodies. 

P.  When  the  body  B  moves  in  a  contrary  direction  to  that  of 
A  before  the  stroke,  or  when  B  is  at  rest,  how  will  the  general 
theorems  be  affected  ? 

S.  When  B  moves  in  a  contrary  direction  to  that  of  A  the 
sign  of  b  must  be  changed  to  its  contrary,  and  when  B  is  striken 
at  rest,  then  b  will  =  0. 

Of  Central  Forces . 

P.  What  do  you  understand  by  central  forces  ? 

S.  Those,  by  the  joint  influence  of  which,  one  or  more  bodies 
may  move  round  a  centre. 

P.  How  many  forces  may  be  adequate  to  this  end,  and  how 
must  they  act  ? 

S.  Two  forces,  properly  circumstanced,  may  produce  this  ef- 
fect ;  namely,  a  projectile,  or  centrifugal  force,  acting  on  the 
body  for  an  instant  only  ;  and  a  centripetal  force  acting  towards 
the  centre  with  a  continued  influence. 

P.  What  effect  would  the  projectile  force,  alone,  produce  on 
the  body  ? 

S.  It  would  produce  a  rectilineal  motion,  with  an  equable  velo- 
city proportional  to,  and  in  the  direction  of,  the  projectile  force. 

P.  What,  the  centripetal  force  alone  ? 

S.  It  would  produce  a  rectilineal  motion  towards  the  centre, 
with  an  accelerated  velocity. 

P.  Explain  the  joint  effect  of  these  two  forces. 


496  APPENDIX. 

S.  Let  A  be  a  body  moving  with  an  equable  velocity  in  the 
circular  orbit  ACE;  and  let  A  B  be  an  indefinitely  small  arch 
of  the  circle  ;  then,  if  A  B,  one  side  of  the  parallelogram  AC 
represent  the  centrifugal  force,  B  C=A  D  will  represent  the  cen- 
tripetal force.  But,  from  similar  triangles,  AE  :  AC  ::  AC:  AD. 
""That  is,  the  centripetal  force  is  as  the  square  of  the  arch  AC 
directly,  and  the  diameter  of  the  orbit  AE  (or  its  radius  A  c) 
inversely  :  and,  from  this  general  proportion,  by  substituting  the 
various  terms  relative  to  this  subject,  may  be  deduced  all  Lhjt 
other  necessary  proportions  or  theorems. 

P.  Please  to  deduce  a  few  of  the  most  general. 
S.  Let  a=  the  arch  of  a  circular  orbit  described  by  a  bouy 
in  any  given  time. 

d  =  the  radius,  or  distance  of  the  orbit  from  the  centre 

of  motion, 
v  =  the  velocity. 

p  —  the  periodical  time  of  one  revolution, 
n  =  the  number  of  revolutions  in  a  given  time, 
f  =  centripetal  force. 
aa 
Then,  as  above  f :    .  But  a  :  v 


d    '  .'  p 

d  I 

.».     f  :   _.  but  p  :  — 

pp  n 

.♦.     f:       dnn. 

P.  In  the  solar  system,  it  has  been  observed,  that  the  squares 
ef  the  periodical  times  of  the  several  planets  revolving  round 
the  sun,  are  as  the  cubes  of  their  mean^distances  ;  what  proper, 
lion  then  has  the  centripetal  force  to  the  distance  ? 

S.  By  substituting   ddd  in  place  of  pp  in  the  proportion  f : 

d  I 

we  shall  have  f : ;  that  is,  the  centripetal  force  is  in- 

pp  dd 

versely  as  the  square  of  the  distance. 

P.  Mention  some  of  the  laws  of  motion  of  bodies  moving  in 
an  elliptical  orbit. 

S.  In  this  case,  the  line  connecting  the  body  and  the  centre 
of  motion,  Will,  as  in  the  equable  motion  in  a  circular  orbit,  pass 
over  equal  areas  in  equal  times  ;  and,  if  the  centripetal  force 
be  inversely  as,  the  square  of  the  distance,  the  centre  of  raoii'm 
will  be  in  one  of  the  foci  of  the  ellipsis. 


C     497     ] 


Of  Attraction* 

P.  What  do  you  understand  by  attraction  ? 

S.  Some  unknown  cause,  by  the  continued  operation  of  which 
bodies,  or  the  particles  or  elements  of  bodies,  tend  mutually  to 
approach  each  other,  or  adhere  together. 

P.  How  many  known  species  of  attraction  are  there  ? 

S.  There  may  be  reckoned  four  species  of  attraction,  viz.  the 
attraction  of  cohesion,  that  of  gravitation,  that  of  magnetism, 
and  that  of  electricity.  To  these  may  be  added,  chemical  at- 
traction or  affinity,  and  the  attraction  of  galvanism  :  though  the 
former  may  probably  be  referred  to  the  attraction  of  cohesion, 
and  the  latter  to  that  of  electricity. 


Of  the  Attraction  of  Cohesion*, 

P.  What  do  you  understand  by  the  attraction  of  cohesion  ? 

S.  That  species  of  attraction  which  exists  between  the  parti- 
cles of  the  same  aggregate  body  ;  and,  therefore,  frequently 
termed  the  attraction  of  aggregation.  It  acts  only  when  the  par- 
ticles are  in  contact,  or  rather  at  an  insensible  distance. 

P.  What  effects  Will  the  different  degrees  of  this  attraction 
have  on  the  texture  of  the  body  ? 

S.  The  body  will  be  hard,  soft,  or  fluid,  according  as  this  at- 
traction is  strong,  moderate,  or  weak. 

P.  Is  not  this  attraction  sometimes  observable  between  differ- 
ent aggregate  bodies  ? 

S.  It  does  frequently  take  place  between  the  particles  of  different 
bodies,  by  mere  justa-position,  as  between  two  pieces  of  polished 
marble,  glass,  metal,  or  the  like ;  which  will  adhere  together 
with  more  or  less  force,  according  to  circumstances. 

P.  On  what  circumstances  does  the  force  of  this  species  of 
attraction  depend  ? 

S.  Chiefly  on  the  quantity  of  touching  surface,  and  the  near- 
ness of  their  approach  :  though  between  different  bodies  there 
seems  to  be  also  a  kind  of  affinity  or  specific  attraction,  even 
where  no  commixture,  or  solution  takes  place ;  and  which  is 
stronger  between  some,  and  weaker  between  others,  independ- 
ently of  the  quantity  of  touching  surface.  Thus  the  surface  of 
polished  glass  attracts  water  more  powerfully  than  it  does  spirits 
of  wine,  oil,  or  any  other  fluid. 

P.  Mention  some  of  the  phenomena  of  nature  or  art  which 
may  be  explained  on  the  principles  of  cohesive  attraction. 

S.  1.  The  rising  of  water  or  other  fluid  in  a  capillary  glass 
tube  above  the  surface  of  the  fluid  into  which  it  is  immersed. 
This  is  produced  by  the  attraction  "of  a  ring  of  particles  in  the 
tube,  in  contact  with  the  surface  of  the  fluid :  for  the  parts  of 
the  tube  below  the  surface  ©f  the  suspended  column,  attracting 

VOL.  IV.  3T 


498  APPENDIX. 

equally  both  upwards  and  downwards,  can  contribute  nothing, 
(except  by  mere  friction)  towards  its  suspension. 

2.  All  the  phenomena  of  absorption,  as  the  rising  of  water, 
Sec.  in  any  spongy  substance,  as  loaf-sugar,  sand,  cloth  &c. 

3.  The  strong  adhesion  of  polished  surfaces,  as  of  glass,  me- 
tal, Sec,  particularly  the  coating  of  mercury  on  a  looking-glass— 
with  many  others. 

P.  In  what  proportion  are  the  heights  to  which  a  fluid  will  rise 
in  different  capillary  tubes  to  the  size  of  these  tubes? 

S.  The  heights  will  be  inversely  as  their  diameters.  For, 
the  heights  would  be  directly  as  the  diameters,  (or  circumfer- 
ences) on  account  of  the  quantity  of  attracting  particles  in  con- 
tact with  the  surface  of  the  fluid,  and  inversely  as  the  squares  of 
the  diameters,  (or  areas)  on  account  of  the  weight  of  the  co- 
lumn suspended:  that  is,  inversely  as  the  diameters. 

P.  May  not  the  ascent  of  sap  in  trees  and  other  vegetables, 
be  accounted  for  on  the  principles  of  this  capillary  attraction? 

S.  There  is  no  doubt  that  this  attraction  is  the  means  which 
nature  uses  in  producing  this  phenomenon,  though  it  is  not  of 
itself  adequate  to  this  end.  It  can  only  be  rendered  effectual  by 
the  inexplicable  operation  of  a  principle  of  vegetable  life. 

P.  Are  there  any  evidences  of  repulsion  between  the  particles 
of  matter  ? 

S.  There  are  sundry  phenomena  which  cannot  well  be  ex- 
plained on  any  other  principle.  Thus  air  or  any  other  aeriform 
fluid  when  compressed  by  external  force,  will  immediately  ex- 
pand, as  soon  as  the  compressing  force  ceases  to  act,  and  re- 
sume its  former  volume.  The  re-action  of  springs,  and  other 
elastic  bodies,  as  well  as  many  of  the  phenomena  of  electricity, 
magnetism,  and  chemistry  cannot  be  satisfactorily  accounted 
for  on  any  other  principle. 


Of  the  Attraction  of  Gravitation. 

P.  What  do  you  understand  by  the  attraction  of  gravitation  ? 

S.  That  species  of  attraction  by  which  all  the  bodies  in  the 
solar  system,  and  probably  in  the  universe,  have  a  mutual  ten- 
dency to  approach  each  other. 

P.  What  are  the  general  laws  of  this  species  of  attraction  ? 

S.   1.  It  is  common  to  all  bodies,  and  mutual  between  them. 

2.  It  is  proportional  to  the  quantity  of  matter  in  the  bodies. 

3.  It  is  continually  exerted  in  all  directions,  and  in  right 
lines,  from  the  centre  of  the  attracting  body. 

4.  Its  energy  or  force  decreases  as  the  square  of  the  distance 
increases. 

P.  Describe  and  explain  the  phenomena  of  falling  bodies. 

5.  A  body  near  the  surface  of  the  earth,  if  suffered  to  fall 
freely  from  a  state  of  rest,  and  without  considering  the  resist- 
ance of  the  medium  through  which  it  falls,  would  descend  with 


APPENDIX.  499 

an  equably-accelerated  velocity  ;  the  velocity  acquired  being  as 
the  time  of  descent ;  the  spaces  descended  through  in  equal 
successive  intervals  of  time,  as  the  odd  numbers  1,  3,  5,  7,  Sec. 
and  consequently  the  whole  spaces  descended  through  from  the 
commencement  of  motion  as  the  squares  of  the  times  of  descent, 
or  as  the  squares  of  the  last  acquired  velocity,  or  as  the  squares 
of  the  mean  velocity,  which  is  half  the  last  acquired  velocity. 

P.  Through  what  space  would  a  heavy  body  descend  from  a 
state  of  rest  near  the  surface  of  the  earth,  in  the  first  second  of 
time,  and  what  velocity  would  it  acquire  ? 

S.  It  has  been  found  by  experiment,  that  a  body,  in  such  cir- 
cumstances, would  descend  16  feet  1  inch  in  the  first  second 
of  time,  and  consequently  acquire  a  velocity  of  32  feet  2  inches 
per  second. 

P.  In  what  ratio  would  the  weights  of  a  body  be,  on  the 
surface  of  two  different  spheres  of  equal  densities,  but  of  unequal 
magnitudes  ? 

S.  On  one  account,  the  weights  would  be  directly  as  the  quan- 
tities of  attracting  matter,  or  as  the  magnitudes  of  the  spheres, 
or  cubes  of  their  diameters  :  but,  on  another  account,  inversely 
as  the  squares  of  their  distances  from  the  centres,  or  cf  their 
diameters ;  hence  the  weights  of  the  body  on  different  spheres 
would  be  directly  as  thtir  diameters. 

P.  If  a  body  were  placed  within  a  hollow  sphere  or  shell  of 
attracting  matter,  and  acted  upon  by  no  other  body,  how  hence 
it  be  affected  ? 

S.  It  would  be  equally  attracted  in  every  direction,  and  would 
remain  at  rest  in  any  position  in  which  it  should  be  placed. 

P.  In  what  ratio  would  the  weight  or  gravity  of  a  body 
decrease  from  the  surface  of  an  attracting  sphere  downwards? 

S.  Directly  as  its  distance  from  the  centre. 

P.  In  what  ratio  would  its  gravity  decrease  from  the  sur- 
face upwards  ? 

S.  Inversely  as  the  square  of  its  distance  from  the  centre. 


Of  Projectiles. 

P.  What  is  meant  by  a  projectile  r 

S.  A  body  thrown  in  any  direction  by  an  impetus,  or  impel- 
ling force ;  as  by  the  spring  of  a  bow,  the  explosion  of  gun- 
powder, or  the  like,  and  acted  upon,  at  the  same  time,  by 
gravity. 

P.  Explain  the  effects  of  these  two  forces. 

S.  By  the  projectile  force  alone,  considered  as  acting  only  for 
an  instant,  or  indefinitely  small  portion  of  time,  and  without 
considering  the  resistance  of  the  medium,  the  body  would  move 
with  an  equable  velocity,  and  in  the  direction  of  the  projectile 
force :  and  by  the  continued  action  of  gravity)  it  would  be  drawn 


500 


APPENDIX. 


downwards  from  the  line  of  direction,  with  an  accelerated  velo» 
city. 

P.  What  path  would  the  projectile'describe,  by  the  joint  in- 
fluence of  these  two  forces  ? 

S.  In  any  distance  not  exceeding  a  few  miles,  near  the  sur- 
face of  the  earth,  where  the  action  of  gravity  may  be  considered 
as  equal  through  every  part  of  the  projectile's  path,  and  exerted 
in  parallel  right  lines,  the  projectile,  moving  in  an  unresisting 
medium,  would  describe  a  parabolic  curve. 

P.  In  this  case,  what  relation  would  there  be  between  the 
time  of  flight,  the  horizontal  randum  or  range,  the  greatest 
height  of  the  projectile — and  the  angle  of  elevation  ? 

S.  The  time  of  flight  would  be  as  the  sine  of  the  angle  of 
elevation — the  horizontal  randum  as  the  sine,  and  the  greatest 
height  as  the  versed  sine,  of  double  the  angle  of  elevation. 

P.  Please  to  demonstrate  these  proportions  by  a  figure. 


S.  Let  A  M  represent  the  force  or  velocity  of  a  body  pro- 
jected in  the  direction  A  M.  This  force  is  resolvable  into  A  L, 
perpendicular,  and  LM,  parallel,  to  the  horizontal  plane  AC. 
The  heights  to  which  the  projectile  would  rise  with  the  forces  AL, 
and  A  M  respectively,  would,  by  the  laws  of  falling  bodies,  be  as 
D  A  L  to  □  A  M :  that  is,  from  the  properties  of  similar  tri- 
angles, as  ALto  AG.  Therefore,  since  AG  may  represent  the 
height  with  velocity  AM,  AL  (  =  DB)  will  represent  the  height 
with  velocity  AL.  Hence  DB,  the  greatest  height  of  the  pro- 
jectile, will  be  proportional  to  AL,  the  versed-sine  of  (  <  ANM) 
double  the  angle  of  elevation  CAM.  Again,  since  the  vertical 
force  or  velocity  of  the  projectile  is  to  its  horizontal  force  or 
velocity,  as  AL  to  LM  ;  and  since  AL  is  passed  through  with 
a  continually-decreasing  velocity  terminating  in  a  state  of  rest, 
the  mean  velocity  will  be  double  the  initiant  velocity,  or  that 
with  which  LM  is  uniformly  described.  It  follows  then,  that 
while  a  body  would  ascend  to  L,  it  would  with  the  same  initiant 
velocity  pass  through  a  horizontal  space  =2LM=AD=|  AC, 
the  horizontal  randum.    Hence,  the  time  of  flight  will  be  as 


APPENDIX.  501 

AM,  or  \  AM  that  is,  as  the  sine  of  the  angle  of  elevation  ;  and 
the  horizontal  randum  will  be  as  LM,  the  sine  of  double  the 
angle  of  elevation. 

P.  Mention  a  few  of  the  practical  inferences,  deducible  from 
these  general  proportions. 

S.  1.  An  elevation  of  45  degrees  above  the  horizon  will  give 
the  greatest  horizontal  randum  :  since  the  sine  of  double  45°, 
or  go0,  =  radius,  the  greatest  possible  sine. 

2.  If  the  angular  distance  between  any  plane  and  a  vertical 
line  be  bisected,  it  will  give  the  angle  of  elevation  from  which  u 
projectile  will  be  thrown  to  the  greatest  distance  on  that  plane. 

3.  Any  two  elevations  equally  above  and  below  45  degrees, 
will  give  equal  horizontal  randums  :  for  the  doubles  of  any  two 
such  angles  being  supplements  of  each  other,  will  have  equal 
sines. 

4.  At  an  elevation  of  45  degrees,  the  greatest  height  of  the 
projectile  will  equal  half  the  vertical  height  to  which  it  would 
be  thrown  by  the  same  impetus  ;  and  the  horizontal  randum  will 
equal  double  that  height. 

5.  Supposing  a  heavy  body  to  descend  perpendicularly  from 
a  state  of  rest  through  a  space  of  16  feet  in  one  second  of  time, 
—then, 

Eight  times  the  square  root  of  half  the  greatest  horizontal 
randum  in  feet,  will  be  the  velocity  in  feet  per  second. 

One  fourth  the  square  root  of  the  greatest  horizontal  randum, 
or  half  the  square  root  of  the  height  in  feet,  will  be  the  time  of 
flight  in  seconds. 

P.  Can  the  theory  of  projectiles  in  vacuo,  be  applied  with  any 
great  advantage  to  practical  gunnery? 

S.  The  resistance  of  the  air,  especially  to  bodies  moving  with 
great  velocity,  is  so  very  considerable,  that  but  little  advantage 
can  be  derived  from  this  theory ;  and,  therefore,  conclusions 
drawn  from  actual  experiments  are  chiefly  to  be  relied  on. 


Of  the  Centre  of  Gravity, 

P.  What  is  meant  by  the  centre  of  gravity  ? 

S.  It  is  that  point  in  a  body  by  which  if  it  were  suspended, 
the  body  would  rest  in  any  position  indifferently. 

P.  From  what  general  principles  may  the  centre  of  gravity  of 
,a  body  be  investigated  ? 

S.  The  momentum  of  any  body  in  moving  round  a  fixed  cen- 
tre of  motion,  will  be  in  the  compound  ratio  of  the  weight  of 
the  body,  and  its  distance  from  that  centre.  Now,  it  is  obvious, 
that  when  the  sum  of  the  momenta  of  all  the  particles  of  a  body, 
on  one  side  of  any  plane  passing  through  the  centre  of  motion, 
is  equal  to  the  sum  of  the  momenta  of  all  the  particles  on  -the 
other  side  of  that  plane,  these  momenta  will  be  in  exact  equili- 
toio,  and  this  centre  of  motion  will  be  the  centre  of  gravity. 


APPENDIX. 

P.  How  would  you  find  the  common  centre  of  gravity  between 
any  two  given  bodies  ? 

S.  By  the  following  proportion,  viz.  as  the  weight  of  both  bo- 
dies is  to  their  distance  from  each  other,  so  is  the  weight  of  one 
of  them  to  the  distance  of  the  centre  of  gravity  from  the  other. 

P.  How,  between  three  or  more? 

S.  By  first  finding  the  common  centre  of  gravity  between  any 
two  of  them,  and  then  between  these  two,  (both  considered  as 
placed  in  this  centre)  and  a  third,  &c. 

P.  How  would  you  find  this  centre  in  a  plane  triangle  ? 

S.  By  first  bisecting  any  two  sides  of  the  triangle,  and  from 
the  points  of  bisection  drawing  lines  to  the  opposite  angles ; 
for  then,  the  intersection  of  these  two  lines,  which  would  be  at 
the  distance  of  -J-  of  each  from  the  base,  would  be  the  centre  of 
gravity. 

P.  Where  is  the  centre  of  gravity  in  a  hollow  cone  or  pyra- 
mid? 

S.  It  is  in  the  axis,  at  -J  of  its  length  from  the  base. 

P.  Where,  in  a  solid  cone  or  pyramid  ? 

S.  It  is  in  the  axis,  at  \  of  its  length  from  the  base. 

P.  How  would  you  find  the  centre  of  gravity  of  any  given 
body,  mechanically  ? 

S.  By  suspending  it  successively  from  two  different  points, 
as  far  from  the  centre  as  convenient,  and  then  the  centre  of  gra- 
vity will  be  in  the  common  section  of  two  vertical  planes  passing 
through  these  points  of  suspension.  For  the  centre  of  suspen- 
sion, the  centre  of  gravity,  and  the  centre  of  the  earth,  will, 
when  the  body  is  at  rest,  be  always  in  the  same  right  line. 

P.  In  what  case  would  a  body  set  on  a  plane,  stand,  and  in 
what  case  would  it  fall  or  tumble  down  ? 

S.  When  the  centre  of  gravity  of  the  body  is  supported,  that 
is,  when  a  line  perpendicular  to  the  horizon  and  passing  through 
the  centre  of  gravity  falls  within  the  base,  the  body  will  stand ; 
or,  if  the  plane  be  inclined  to  the  horizon,  and  the  friction  do 
not  prevent  it,  the  body  will  slide  down  the  plane  :  but  when  the 
vertical  line  falls  without  the  base,  the  body  will  roll  or  tumble 
down. 


Of  the  Mechanic  Powers. 

i?.  What  is  a  mechanic  pewer? 

S.  Any  machine  or  contrivance,  by  the  aid  of  which,  a  greater 
weight  or  power  may  be  raised  or  overcome  by  a  less.  Or, 
more  generally,  by  which  the  velocity  or  direction  of  the  weight 
may  be  changed. 

P.  How  many  simple  mechanic  powers  are  there  ? 

S.  They  are  usually  reckoned  six  in  number,  viz.  the  lever, 
the  axis  and  wheel,  the  pulley  and  tackle,  the  inclined  plane, 
the  wedge,  and  the  screw ;  for  to  one  or  other  of  these,  simple 


APPENDIX.  503 

or  combined,  may  most  or  all  machines  for  raising  weights, 
overcoming  resistance,  or  producing  motion,  be  reduced. 

P.  On  what  general  principles  may  all  the  mechanic  powers, 
in  their  various  applications,  be  explained  ? 

S.  The  momentum,  force,  or  power,  of  a  body,  acting  in  any 
direction,  is  proportional  to  the  weight  of  the  body  multiplied 
into  its  velocity  in  that  direction  ;  and  when  the  body  moves  in 
the  arch  of  a  circle,  the  velocity  will  be  proportional  to  the  radius 
of  the  circle,  or  distance  of  the  body  from  the  centre  of  motion. 
Now,  when  the  momenta  of  two  bodies  or  weights  are  equal, 
and  in  opposite  directions,  they  will  be  in  equilibrio. 


Of  the  Lever* 

P.  What  is  a  lever  ? 

S.  A  lever,  in  a  mathematical  sense,  may  be  considered  as  a 
moveable,  inflexible,  line,  acted  upon  by  two  forces,  the  power 
and  the  weight,  applied  to  different  parts  thereof,  with  the 
re-action  of  a  third  point  called  the  fulcrum,  or  centre  of  motion. 

P.  How  are  levers  usually  distinguished  ? 

S.  Into  three  kinds  or  forms,  according  to  the  position  of  the 
power,  weight,  and  fulcrum  ;  as, 

1st.  Where  the  fulcrum  is  between  the  power  and  the  weight. 

2d.  Where  the  weight  is  between  the  fulcrum  and  the  power. 

3d.  Where  the  power  is  between  the  fulcrum  anci  the  weight. 

P.  How  may  the  general  principle  be  applied  in  rinding  the 
ratio  betweea  the  power  and  the  weight,  in  the  lever? 

S.  The  power  will  be  to  the  weight  in  the  reciprocal  ratio  of 
their  distances  from  the  fulcrum. 

P.  To  which  of  the  mechanic  powers  may  the  common  scale- 
beam,  and  steel-yard  be  referred? 

S.  To  a  lever  of  the  1st  form:  in  the  common  scale-beam, 
the  fulcrum  being  equally  distant  from  the  power  and  the  weight, 
but  in  the  steel-yard,  the  distance  of  the/*«z,  or  power,  from  the 
fulcpum,  will  be  to  the  distance  of  the  weight  from  the  fulcrum, 
in  the  reciprocal  ratio  of  the  power  and  the  weight. 

P.  What  are  the  properties  chiefly  to  be  attended  to  in  the 
construction  of  a  good  scale-beam  ? 

S.  1.  It  should  be  of  such  form  as  to  be  as  inflexible,  and  to 
move  with  as  little  friction,  as  possible. 

2.  The  fulcrum,  or  centre  of  motion,  antl  the  two  centres  of 
suspension,  viz.  that  of  the  power  and  that  of  the  weight,  should 
be  all  in  the  same  right  line ;  for  then,  and  not  otherwise,  the 
ratio  between  the  momentum  of  the  power  and  that  of  the 
weight  will  be  the  same  in  all  positions  of  the  beam. 

3.  The  centre  of  gravity  of  the  beam  should  be  but  a  very  lit- 
tle below  the  centre  of  motion :  for  then  the  least  difference  of 
weight  in  the  opposite  scales  will  produce  a  sensible  preponde- 
rance. 


504  APPENDIX. 


P.  How  may  a  false  beam,  or  one  whose  arms  arc  of  different 
lengths,  be  detected,  and  the  quantity  of  error  ascertained? 

S.  By  changing  the  contents  of  the  different  scales:  for  then 
the  weight  that  was  placed  at  the  shorter  end,  and  in  apparent 
equilibrio  with  the  other,  will  preponderate  when  placed  at  the 
longer  end,  and  the  true  weight  of  the  article  will  be  found  by 
taking  a  geometrical  mean  between  its  apparent  weights  at  the 
different  ends  of  the  beam. 


Of  the  Axis  and  Wheel* 

P.  When  will  there  be  an  equilibrium  in  the  axis  and  wheel  ? 

S.  When  the  power,  acting  from  the  circumference  of  the 
wheel,  is  to  the  weight,  acting  from  the  circumference  of  the 
axis,  as  the  radius  of  the  axis  is  to  that  of  the  wheel. 

P.  How  may  the  power  of  the  axis  and  wheel  be  advanta- 
geously increased  ? 

S.  By  making  one  part  of  the  axis  thicker  than  the  other, 
with  a  rope  winding  round  the  thicker  part,  while  it  is  unwound 
from  the  smaller  part,  and  the  weight  attached  to  a  pulley  sup- 
ported on  the  bend  of  the  rope  ;  for  then  the  power  will  be  to 
the  weight  as  the  difference  between  the  diameters  of  the  thick- 
er and  smaller  parts  of  the  axis,  is  to  the  diameter  of  the  wheel. 


Of  the  Pulley  and  Tackle. 

P.  How  would  you  compute  the  power  of  any  tackle  or  sys- 
tem of  pullies  ? 

S.  By  considering,  from  the  number  and  arrangement  of  the 
pullies,  what  ratio  there  would  be  between  the  velocity  of  the 
power  and  that  of  the  weight,  if  both  put  in  motion ;  for  this 
ratio  will  be  reciprocally  as  the  power  to  the  weight,  when  in 
equilibrio. 

P.  How  may  this  ratio  be  expressed,  when  a  single  rope  only 
is  used,  and  one  fixed  block? 

S.  In  this  case,  the  power  will  be  to  the  weight  as  unity  to 
the  number  of  ropes  that  support  the  lower  block,  to  which  the 
weight  is  attached. 

P.  How,  when  each  pulley  has  its  own  rope,  made  fast  at  one 
end  to  a  fixture,  and  at  the  other  end  to  another  block  ? 

S.  In  this  case,  the  power  will  be  to  the  weight  as  unity  to 
the  last  term  of  a  geometrical  series,  whose  first  term  is  1 ,  com- 
mon ratio  2,  and  number  of  terms,  the  number  of  pullies  in  the 
tackle  :  or  the  effect  will  be  doubled  with  the  addition  of  every 
pulley. 


[     505     ] 


Of  the  Inclined  Plane. 

P.  When  a  body  is  to  be  drawn  up  an  inclined  plane,  by  a 
descending  weight  attached  to  a  rope,  passing  over  a  pulley  at 
the  vertex  of  the  plane,  and  fastened  to  the  weight,  what  ratio 
will  there  be  between  the  power  and  the  Aveight  when  in  equili- 
brio;  or,  when  the  weight  is  just  supported  on  the  plane  by  the 
power  suspended  from  the  end  of  the  rope? 

S.  The  power  will  be  to  the  weight  as  the  height  of  the  plane 
to  its  length  :  for,  while  the  power,  if  put  in  motion,  would  de- 
scend through  a  space  equal  to  the  length  of  the  plane,  the 
weight  would  ascend  to  the  top  of  the  plane  ;  consequently,  the 
velocities  of  the  power  and  of  the  weight,  in  opposite  directions^ 
would  be  as  the  length  of  the  plane  to  its  height. 


Of  the  Wedge. 

P.  How  would  you  compute  the  power  of  the  wedge  ? 

S.  The  wedge  may  be  considered  as  two  inclined  planes 
placed  together.  And  since,  in  the  inclined  plane,  the  power  is 
to  the  weight,  as  the  height  of  the  plane  to  its  length  ;  it  follows, 
that  in  the  wedge,  or  double  inclined  plane,  the  power  will  be  to 
the  weight  (or  resistance  to  be  overcome,  or  obstacle  to  be  re- 
moved) as  the  base  of  the  wedge  to  the  sum  of  the  lengths  of 
its  sloping  sides. 

P.  How  is  the  power  usually  applied  to  the  wedge  ? 

S.  By  the  stroke  of  a  hammer,  mallet,  or  the  like,  and  is  to 
be  estimated  by  the  weight  of  the  hammer,  Sec  multiplied  by 
the  square  of  the  velocity  with  which  it  strikes. 


Of  the  Screw. 

P.  How  would  you  estimate  the  power  of  the  screw  ? 

S.  In  the  screw,  the  power  will  perform  one  revolution  while 
the  screw  progresses  only  one  thread:  Hence,  the  power  will 
be  to  the  weight,  when  in  equilibrio,  as  the  distance  between 
two  threads  of  the  screw,  to  the  circumference  of  the  circle  de- 
scribed by  the  power. 

P.  In  what  manner  may  the  power  of  the  screw  be  advanta- 
geously increased  I 

S.  By  having  the  threads  cut  coarser  on  one  part  of  the  screw 
than  on  the  other,  each  working  in  a  separate  box ;  for,  in  this 
construction,  the  power  will  be  increased  in  proportion  as  the 
difference  between  the  threads  of  the  coarser  and  finer  screws 
is  decreased. 

VOL.  IV.  3  U 


C     506     ] 

Of  Friction, 

P.  What  is  friction  ? 

S.  It  is  the  resistance  to  motion  in  bodies  rubbing  against 
one  another,  arising  chiefly  from  the  asperity  of  the  rubbing 
surfaces. 

P.   How  may  the  quantity  and  laws  of  friction  be  determined  ? 

S.  Only  from  experiments. 

P.  What  are  the  results  of  the  best  experiments  on  this  sub- 
ject? 

S.  1.  That  friction,  in  hard  bodies,  acts  as  a  uniformly-re- 
tarding force,  similar  to  that  of  gravity. 

2.  That  it  increases  with  the  weight  of  the  moving  body,  but 
in  a  less  ratio. 

3.  That,  consequently,  the  quantity  of  friction  will  increase 
with  the  quantity  of  rubbing  surface,  though  not  in  the  same 
ratio. 

P.  In  calculating  the  effects  of.  machines,  what  allowance 
should  be  made  for  friction  ? 

S.  In  large  compound  machines,  it  is  usual  to  deduct  about 
one  third  on  the  account  of  friction ;  though  this  will  be  very 
different  in  different  machines,  both  on  account  of  the  rubbing 
materials,  and  on  many  other  accounts. 

Of  the  Motion  of  Bodies  on  Inclined  Planes* 

P.  Explain  and  demonstrate  the  laws  of  the  motion  of  bodies 
on  inclined  planes. 


S.  Let  AC  represent  an  inclined  plane,  AB  parallel,  and  BC 
perpendicular  to  the  horizon,  and  BD  perpendicular  to  AC 
Then  if  CB  represent  the  absolute  weight  or  gravity  of  a  body 
at  C,  by  the  resolution  of  forces,  DB  will  represent  its  relative 


APPENDIX.  50? 

weight  on  the  plane  AC,  and  CD  the  gravity  or  force  with 
which  it  will  be  urged  down  the  plane.  While,  therefore,  the 
body  would  descend  freely  by  its  absolute  gravity  from  C  to  B, 
it  would  slide  down  the  plane  from  C  to  D*  But  the  ultimate, 
or  the  mean  velocities  are  as  the  spaces  passed  through  in  equal 
times  ;  that  is,  the  velocity  in  descending  freely  by  the  influence 
of  gravity  is  to  the  velocity  in  sliding  down  the  inclined  plane,  as 
CB  to  CD;  or,  by  similar  triangles,  as  CA  the  length  of  the 
plane,  to  CB  its  height.  It  follows,  from  the  above,  that  the 
times  in  which  a  body  would  slide  down  all  chords  of  a  given 
circle,  beginning  or  terminating  in  either  extremity  of  the  verti- 
cal diameter,  will  be  equal  to  each  other  ;  the  times  of  descent 
down  different  inclined  planes  CA,  CA  of  the  same  height  are 
directly  as  their  lengths  ;  and  the  ultimate,  or  least  acquired 
velocities,  equal  to  each  other. 

Of  Pendulums, 

P.  What  is  a  pendulum  ? 

S.  A  body  suspended  by,  and  oscillating  from,  a  certain  point 
(generally  the  upper  extremity)  called  the  centre  of  suspension, 
or  centre  of  motion. 

P.  Explain  the  general  doctrine  of  pendulums. 

S.  If  the  bob,  or  lower  extremity  of  the  pendulum,  vibrate 
through  small  arches,  which  consequently  nearly  coincide  with 
their  chords,  the  times  of  vibration  (as  the  times  of  descent  of  a 
heavy  body  down  different  chords  of  a  circle)  will  be  all  nearly 
equal  to  each  other,  and  proportional  to  the  square-root  of  the 
diameter  (or  the  radius)  of  the  circle  ;  that  is,  the  length  of  the 
pendulum  ;  and,  consequently,  the  lengths  of  different  pendu- 
lums will  be  to  one  another  as  the  squares  of  their  respective 
times  of  vibration. 

P.  What  is  meant  by  the  length  of  a  pendulum  ? 

S.  The  distance  from  the  centre  of  suspensipn  to  a  point  called 
the  centre  of  oscillation,  which,  in  the  bob-pendulum,  is  a  little 
above  the  centre  of  gravity  of  the  bob  ;  but  in  a  right  line, 
it  is  at  two  thirds,  and  in  a  small  cylinder  or  prism,  at  very 
nearly  two  thirds  of  its  length  from  the  centre  of  motion,  or 
upper  extremity.  A  second-pendulum  of  this  kind?  as  recom- 
mended by  Mr.  Jefferson,  when  secretary  of  state,  would,  per- 
haps, be  the  best  universal  standard  of  measure  that  nature  affords. 

P.  What  must  the  length  of  a  pendulum  be  to  vibrate  se- 
conds ? 

S.  It  will  be  somewhat  different  in  different  latitudes :  for, 
both  on  account  of  the  spheroidal  figure  of  the  earth,  and  its  diur- 
nal rotation,  the  gravity,  and  consequently  the  length  of  the 
pendulum,  will  be  least  at  the  equator,  and  gradually  increase 
to  the  poles.  In  latitude  45°,  the  second-pendulum  has  been 
found,  by  experiment,  to  be  3<J.l4912  inches. 

F.  What  proportion  is  there  between  the  length  of  a  pendu- 
lum, and  the  space  through  which  a  heavy  body  would  fall  from, 
a  state  of  rest,  in  the  time  of  one  vibration  ? 


50$  APPENDIX. 

S.  It  is  demonstrable,  that  the  time  of  one  entire  vibration,  in 
a  very  sm:ill  arch,  is  to  the  time  of  a  heavy  body's  descending 
through  half  the  length  of  the  pendulum,  as  the  circumference  of 
a  circle  to  its  diameter.  Or,  which  is  the  same  thing,  the  con- 
stant number  4.93482,  multiplied  by  the  length  of  the  pendu- 
lum, will  give  the  space  through  which  a  body  will  descend  in  the 
time  of  one  vibration. 

P.  What  are  the  chief  sources  of  irregularity  in  the  going  of 
a  clock  ? 

S.  The  chief  sources  of  irregularity  are,  1st,  the  change  of 
temperature ;  for,  heat  expanding  the  pendulum  will  cause  the 
clock  to  go  slower,  and  cold  contracting  it  will  cause  the  clock 
to  go  quicker.  2d.  Imperfection  in  the  wheel-work  ;  for,  when 
the  friction  in  one  part  of  the  revolution  is  less  than  in  another 
part,  the  maintaining  power,  meeting  with  different  degrees  of 
resistance,  will  cause  the  arch  of  vibration  of  the  pendulum  to  be 
greater  (and  thus  the  clock  to  go  slower)  at  one  time  than  at  an- 
other. 

P.  How  may  these  irregularities  be  remedied  ? 

S.  The  irregularity  arising  from  the  change  of  temperature 
may  be  remedied  by  making  a  compound  pendulum-rod,  called 
the  gridiron-pendulum  ;  consisting  of  alternate  bars  of  brass  and 
iron,  which,  being  differently  affected  by  heat  and  cold,  may  be  so 
arranged  and  combined,  as  to  counteract  the  effects  of  the 
change  of  temperature,  and  cause  the  pendulum  always  to 
main  of  the  same  length. 

The  best  remedy  .against  an  error  arising  from  a  change  in 
the  angle  of  vibration,  is  to  make  this  angle  as  small  as  possible; 
for  the  vibrations  in  very  small,  yet  unequal  arches  of  a  circle, 
though  not  perfectly,  are  yet  very  nearly,  isochronous.  Increasin; 
the  weight  of  the  bob,  will  also  contribute  to  the  same  end. 


Of  Hydrostatics, 

P.  What  do  you  understand  by  hydrostatics  ? 

S.  That  science  which  treats  of  fluids,  chiefly  liquids,  witl 
respect  to  their  weight  and  prtssure. 

P.  What  is  a  fluid  ? 

S.  A  body  that  readily  yields  to  any  partial  or  unequal  pres- 
sure, and  which,  as  soon  as  this  pressure  is  removed  will  imme- 
diately resume  its  former  position  of  parts. 

P.  How  many  general  kinds  of  fluids  are  there? 

S.  Two,  viz.  elastic  or  compressible,  as  air;  and  non-elastic 
or  incompressible,  as  water.  Fluids  of  the  latter  kijid  are  also 
frequently  termed  liquids. 

P.  Explain  the  chief  properties  of  non-elastic  fluids. 

S.  1.  All  the  parts  of  an  incompressible  fluid,  while  of  the 
same  temperature,  will  be  of  the  same  density. 

2.  The  pressure  of  fluids  on  equal  surfaces  will  be  as  their 
perpendicular  heights. 


APPENDIX.  sas 

3.  The  pressure  of  a  fluid  at  equal  depths  below  the  surface, 
■will  be  equal  in  all  directions.  This  arises  from  the  essential 
property  of  a  fluid,  viz.  that  it  readily  yields  to  any  partial  or  un- 
equal pressure:  it  will,  therefore,  never  be  quiescent  till  all  its 
parts  are  acted  upon,  and  consequently  re-act,  with  equal  forces. 

Of  Specific  Gravities, 

P.  What  do  you  mean  by  the  specific  gravity  of  a  body  ? 

S.  The  ratio  of  its  weight  or  density  tj  that  of  an  equal  bulk 
of  some  other  body,  as  pure  water,  which  is  usually  taken  as  the 
standard  of  comparison. 

P.  Explain  the  general  principles  from  which  rules  may  be 
deduced  for  finding  the  specific  gravities  of  bodies. 

S.  1.  When  a  body  immersed  in  water,  or  any  other  incom- 
pressible fluid,  is  of  the  same  specific  gravity,  then  it  will  rest 
in  any  part  of  it  indifferently. 

2.  When  the  specific  gravity  of  the  body  is  greater  than  that 
of  the  fluid  in  which  it  is  immersed,  it  will  sink,  and  rest  on  the 
bottom  with  a  pressure  equal  to  the  excess  of  its  weight  above 

.  that  of  an  equal  bulk  of  the  fluid. 

3.  W^hen  the  specific  gravity  of  the  body  is  less  than  that  of 
the  fluid  in  which  it  is  immersed,  it  will  ascend,  and  so  much  of 
it  will  emerge  above  the  surface  as  will  be  equal  to  the  difference 
between  the  weight  of  the  body  and  that  of  an  equal  bulk  of  the 

■  fluid.  Hence,  a  floating  body  will  displace  its  own  weight  of  the 
fluid  in  which  it  floats,  and  a  sinking  body  will  loose  in  weight 
that  of  its  own  bulk  of  the  fluid  in  which  it  sinks. 

P.  From  these  general  principles,  what  practical  rules  may  be 
deduced,  for  finding  specific  gravities  ? 

S.  1.  WThen  the  body  is  a  solid  heavier  tlfan  water — First  find 
its  weight  in  air,  which  may  be  considered  as  its  absolute  weight, 
and  afterwards  (suspending  it  by  a  hair  or  thread  of  silk)  find 
its  weight  in  pure  water;  then  say,  as  the  weight  lost  in  water: 
unity ::  the  weight  in  air:  the  specific  gravity  of  the  body; — 
water  being  considered  as  the  unit  or  standard  of  comparison. 

2.  When  the  body  is  lighter  than  water — First  attach  to  it 
some  body  sufficient  to  sink  it  in  water,  and  of  tins  compound, 
as  well  as  of  the  heavier  body,  find  the  weight  both  in  air  and  in 
water ;  then  from  the  loss  of  weight  in  water  sustained  by  the 
compound,  subtract  that  sustained  by  the  heavier  body,  and  the 
remainder  will  be  the  weight  of  water  equal  in  bulk  to  the  lighter 
body  ;  hence  say,  as  this  remainder:  unity  : :  the  weight  of  the 
lighter  body  in  air:  its  specific  gravity. 

3.  To  find  the  specific  gravity  of  a  fluid — First  take  any  solid 
body,  as  a  piece  of  glass,  that  will  sink  both  in  water  and  in  the 
given  fluid  ,  and  having  weighed  it  in  air,  weigh  it  afterwards  in 
both  these  fluids: — then,  as  the  loss  it  sustains  in  water:  unity  : : 
the  loss  it  sustains  in  the  given  fluid:  specific  gravity  of  that 
fluid. 


510  APPENDIX. 

4.  To  find  the  specific  gravity  of  a  body  soluble  in  water- 
First  find  its  specific  gravity  with  respect  to  some  fluid  in  which 
it  is  insoluble,  and  then  the  specific  gravity  of  this  fluid  to  water; 
and  the  product  of  these  two  will  be  the  specific  gravity  required. 

P.  Having  the  specific  gravity  of  a  compound,  and  that  of  its 
several  ingredients,  how  may  the  proportion  of  these  ingredients 
be  found  ? 

5.  By  the  rule  of  allegation  alternate,  as  in  common  arithme- 
tic ;  thus,  link  the  specific  gravities  of  the  several  ingredients, 
two  and  two,  one  greater  than  the  specific  gravity  of  the  com- 
pound with  one  less ;  then  take  the  differences  between  each  of 
these  and  that  of  the  compound,  and  place  them  alternately,  or 
to  these  with  which  they  are  respectively  linked,  and  these  dif- 
ferences will  express  the  proportions  of  the  several  ingredients  in 
the  compound. 

P.  Will  the  specific  gravity  of  a  compound  be  a  true  arithme- 
tical mean  between  those  of  the  several  ingredients  ? 

S.  In  many  cases  it  will  not,  being  sometimes  greater  and 
sometimes  less,  according  as  the  bulk  of  the  compound  is  less  or 
greater  than  that  of  the  several  ingredients  ;  but  the  difference 
is  generally  inconsiderable. 

Of  Pneumatics, 

P.  What  is  understood  by  pneumatics  ? 

S.  That  science  which  treats  of  air,  and  other  elastic  andco 
pressible  fluids. 

P.  In  what  ratio  will  the  density,  elasticity,  or  re-action  of  ai 
be  to  the  compressing  force  applied  ? 

S.  In  the  direct  l^tio  of  the  compressing  force  ;  and  this  with- 
out any  known  limit;  air  still  remaining  an  elastic  fluid  under 
the  greatest  possible  pressure. 

P.  In  what  ratio  will  the  density  of  the  atmosphere  decrease 
upwards  ? 

S.  With  the  decrease  in  the  pressure  of  the  superincumbent 
parts:  namely,  in  the  geometrical  ratio  of  the  heights,  taken  in 
arithmetical  progression  :  or,  as  the  natural  numbers  to  their  lo- 
garithms. 

P.  What  is  the  height  of  the  earth's  atmosphere? 

S.  The  height  of  the  atmosphere  where  its  density  is  suflB 
cient  to  reflect  the  rays  of  light,  is,  from  the  phenomena  of  twi 
light,  found  not  much  to  exceed  40  miles. 

P.  What  is  the  specific  gravity  of  air  ;  and  what,  the  whole 
weight  of  a  column  of  the  atmosphere,  of  any  given  base  ? 

S.  Near  the  earth's  surface,  the  specific  gravity  of  air,  at 
mean  temperature,  is  to  that  of  water  as  1  to  863  nearly:  and 
its  weight  or  pressure  about  15  pounds  on  every  superficial  inch. 
But  as  this  pressure,  like  that  of  all  other  fluids,  acts  equally  in 
all  directions,  it  will  scarcely  be  sensible,  unless  where  partially 
applied. 


APPENDIX.  511 

P.  How  is  the  weight  of  the  atmosphere  ascertained? 

S.  By  an  instrument  called  a  barometer;  the  most  simple,  and 
perhaps  the  best  form  of  which  is — a  long  glass  tube,  close  at 
one  end,  and  open  at  the  other  :  this  tube  being  filled  with  mer- 
cury, from  which  the  air  is  to  be  expelled  by  boiling  it  in  the 
tube,  is  closed  at  the  open  end  by  the  finger;  then  inverted,  and 
set  vertically  in  a  bason  of  the  same  fluid.  The  finger  being  now 
withdrawn,  the  mercury  will  subside,  and,  leaving  a  vacuum  in 
the  upper  part  of  the  tube,  a  column  of  it  will  be  supported  by 
the  pressure  of  the  atmosphere  acting  on  the  lower  end  of  the 
tube,  equal  in  weight  to  an  entire  column  of  the  atmosphere  of 
the  same  base  with  that  of  the  mercurial  column. 

P.  What  will  be  the  height  of  this  mercurial  column,  sup- 
ported by  the  pressure  of  the  atmosphere  ? 

S.  At  the  surface  of  the  earth,  not  far  above  the  level  of  the 
sea,  it  is  found  to  vary  from  about  28  to  SI  inches ;  29  1-2  being 
the  mean  height. 

P.  Suppose  water  were  substituted  in  place  of  the  mercury ; 
what  would  the  average  height  of  the  column  then  be  ? 

S.  It  would  be  inversely  as  its  specific  gravity ;  that  is  about 
33  feet. 

P.  To  what  use  may  the  barometer  be  applied,  besides  that  of 
indicating  the  variations  in  the  weight  of  the  atmosphere? 

S.  It  will  also  serve  to  measure  the  height  of  mountains,  or 
other  elevations. 

P.  How  is  this  to  be  done? 

S.  It  has  been  found  by  experiment,  that,  in  a  mean  state  of 
the  atmosphere,  the  column  of  mercury  will  fall  one  inch  in  as- 
cending about  900  feet  from  the  common  level  of  the  surface  of 
the  earth,  from  which  the  following  rule  has  been  deduced,  viz. 
Find  the  height  of  the  mercury  at  the  same  time  both  at  the  bot- 
tom and  top  of  the  given  elevation,  then  take  the  difference  of 
the  logarithms  of  these  two  heights  of  the  mercury  in  inches* 
and  the  first  four  figures,  following  the  decimal  point,  will  be  the 
height  of  the  elevation  in  fathoms,  and  the  remaining  figures  will 
be  decimal  parts.  When  great  accuracy,  however,  is  required,  a 
small  correction  must  be  applied  on  account  of  the  different  tem- 
peratures at  the  two  stations. 

P.  How  may  the  temperature  of  the  air,  or  of  any  other  me- 
dium, be  ascertained? 

S.  By  an  instrument  called  a  thermometer* 

P.  Give  a  description  of  this  instrument. 

S.  The  most  common,  and  perhaps  the  best  kind  of  thermo- 
meter, is  a  glass  tube  of  a  small  bore,  with  a  hollow  ball  or  bulb 
at  one  end.  This  bulb,  with  part  of  the  tube,  is  filled  with  mer- 
cury, or  spirits  of  wine  ;  and  the  other  end  of  the  tube  (the  air 
being  previously  expelled  by  heat)  hermetically  sealed.  A  change 
of  temperature  will  then  be  indicated  by  the  rising  or  falling  of 
the  fluid  in  the  tube,  occasioned  by  its  expansion  or  contraction 
by  heat  or  cold. 


«B  APPENDIX. 

P.  How  arc  the  different  degrees  of  temperature  marked  on 
the  scale  ? 

S.  There  are  two  fixed  or  permanent  points  of  temperature, 
from  which  all  the  others  are  found.  One  is  that  at  which  water 
begins  to  freeze,  or  ice  to  thaw  ;  and  the  other  that  at  which  wa- 
ter boils,  under  a  mean  pressure  of  the  atmosphere.  The  first 
is  marked  0,  and  the  last  80°,  on  Raumeur's  scale  ;  but  on  Fahr- 
enheit's scale,  the  first  is  marked  32°,  and  the  last  212°  ;  the  0 
being  the  temperature  of  a  mixture  of  snow  and  salt. 

P.  How  may  the  state  of  the  air  with  respect  to  moisture  be 
ascertained  ? 

S.  By  an  instrument  called  a  hygrometer. 

P.  Describe  this  instrument. 

S.  There  are  various  instruments  of  this  kind — A  very  sensi- 
ble and  accurate  one  may  be  made  as  follows — Fasten  together, 
by  glue  or  otherwise,  two  very  thin  strips  of  wood,  about  a  foot 
long  and  half  an  inch  broad,  the  grain  of  the  one  being  at  right 
angles  to  that  of  the  other ;  and  let  the  upper  end  be  made  fast 
to  any  plane  board  or  wall,  the  lower  end  being  at  liberty  to  move. 
Now,  as  moisture  has  little  or  no  effect  on  the  length  of  a  piece 
of  wood,  but  a  very  sensible  one  on  its  breadth,  especially  when 
thin,  it  follows,  that  on  the  increase  of  moisture  in  the  air,  this 
hygrometer  will  be  bent  into  a  curve,  convex  on  the  side  of  the 
transverse  fibres,  and  vice  versa,  and  the  degrees  may  be  marked 
on  a  curve,  drawn  on  the  wall  or  board,  described  by  the  lower 
end  of  the  hygrometer.  If  whalebone  be  used  instead  of  wood, 
the  instrument  will  be  still  more  sensible,  as  this  substance  is 
more  affected  by  moisture  than  any  kind  of  wood. 

P.  How  may  the  points  of  extreme  dryness  and  moisture  be 
ascertained  ? 

S.  The  point  of  extreme  dryness  may  be  found  by  enclosing 
the  instrument  in  a  vessel  containing  a  quantity  of  quick-lime 
fresh  from  the  kiln;  and  the  point  of  extreme  moisture,  by  ex- 
posing it  to  the  steam  of  boiling  water. 


Of  Sound. 

P.  What  is  sound  ? 

S.  It  is  the  sensation  produced  by  the  vibrations  of  the  parts 
of  a  sounding  body,  and  communicated  through  the  air,  or  other 
elastic  medium,  to  the  organs  of  hearing. 

P.  What  are  the  general  principles  upon  which  the  various 
phenomena  of  sound  may  be  explained  ? 

S.  1.  The  propagation  of  sound  is  probably  occasioned  by  a 
kind  of  vibratory  motion  communicated  from  that  of  the  sound- 
ing body,  to  the  air  or  other  elastic  medium. 

2.  Soui  d  is  propagated  in  all  directions  from  the  sounding 
body,  with  a  uniform  velocity  :  Through  air,  this  velocity  is 
about  1142  feet  per  second;  through  other  media,  it  is  probably 
greater  or  less  according  to  their  respective  densities. 


APPENDIX.  513 

3.  Sound  is  most  distinctly  heard  when  the  ear  is  in  the  same 
medium  with  the  sounding  body. 

4.  Sound  is  reflected  by  solid  bodies  of  all  kinds,  especially 
those  of  a  firm  texture  and  smooth  or  polished  surface  ;  and 
thus  echoes  or  reverberated  sounds  are  produced. 

5.  Sound,  like  light,  is  reflected  from  rough  or  irregular  sur- 
faces in  all  directions;  but  from  smooth  surfaces  it  is  most  co- 
piously reflected  in  an  angle  from  the  reflecting  surface  equal 
to  the  angle  of  incidence. 

P.  What  is  the  particular  characteristic  of  musical  sounds  ? 

S.  Musical  sounds  are  those,  (chijfly  produced  by  the  impulse 
or  vibration  of  strings,  bells,  or  portions  of  air,  as  in  wind-in- 
struments,) which,  either  in  succession  or  coincidence,  commu- 
nicate an  agreeable  sensation  to  the  ear. 

P.  What  is  the  distinction  between  melody  and  harmony? 

S.  Melody  is  an  agreeable  succession,  and  harmony  an  agreea- 
ble coincidence  of  musical  sounds. 

P.  On  what  circumstances  do  the  greater  or  less  degrees  of 
melody  and  harmony  depend  ? 

S.  Probably  on  the  more  or  less  frequent  coincidence  in  the 
vibrations  of  the  the  sounding  bodies.  Thus,  when  the  vibra- 
tions of  two  musical  strings,  for  instance,  are  made  in  equal 
times,  and  therefore  coincide  at  every  vibration,  the  melody  or 
harmony  is  the  most  perfect,  and  called  a  unison.  When  the 
times  of  vibration  are  as  2  to  1,  it  is  called  an  octave,  when  as 
3  to  2,  it  is  called  a  fifth;  and  these  three  are  termed  perfect 
concords.  But  5  to  4,  4  to  3,  and  5  to  3,  are  termed  imperfect 
concord;  all  the  rest  being  discords ;  and  in  the  more  or  less  ju- 
dicious arrangement  of  these,  consists  the  greater  or  less  per- 
fection of  any  piece  of  music. 

P.  On  what  circumstances  does  the  quickness  or  slowness  in 
the  vibration  of  a  musical  string  depend  ? 

S.  On  its  length,  thickness  and  tension  ;  the  times  of  a  vi- 
bration, in  different  strings,  being  directly  as  their  length  and 
thickness,  and  inversely  as  the  square-root  of  their  tending  force. 

P.  What  is  the  distinction  between  tone  and  time  in  music  ? 

S.  Tone  has  a  respect  to  the  degree  of  acuteness,  or  pitch  of 
the  note  or  sound,  and  depends  on  the  quickness  or  slowness  of 
vibration  in  the  sounding  body.  Time  has  a  respect  to  the 
longer  or  shorter  continuance  of  the  musical  tone. 

P.  In  what  manner  are  these  expressed  or  designated  in  the 
notation  of  music  ? 

S.  Tone  is  usually  expressed  by  the  first  seven  letters  of  the 
alphabet,  and  time  by  the  terms,  breve,  semi-breve,  minim, 
crotchet,  quaver,  semi-quaver  and  semi-demi-quaver ;  each 
of  these  expressing  half  the  time  of  the  one  immediately  pre- 
ceding it. 

P.  How  is  music  expressed  to  the  eye  ? 

S.  By  certain  characters  expressive  of  the  time,  placed  on  a 
stave  of  five  parallel  lines ;  the  lines  or  spaces  on  which  the 

YOU  IV,  3  X 


514  APPENDIX. 

characters  are  placed,  called  A,  B,  C,  &c.  expressing  the  tone 
or  pitch. 


Of  Hydraulics, 


P.  What  does  hydraulics  treat  of? 

S.  It  treats  of  the  motion  of  water  in  pipes,  or  of  running  wa- 
ter in  general. 

P.  What  are  the  general  laws  upon  which  the  phenomena  of 
running  water  may  be  explained  ? 

S.  Nearly  the  same  with  those  which  are  observable  in  the 
motion  of  other  heavy  bodies. 

1.  Abstractly  from  the  effects  of  friction,  and  all  other  causes  of 
obstruction,  the  motion  of  water  down  any  conduit  or  channel, 
would  correspond  to  the  descent  of  a  heavy  solid  body  down  an 
inclined  plane :  that  is,  the  velocity  would  continually  increase 
with  the  time  of  descent,  and  be  as  the  square-root  of  the  head, 
or  perpendicular  height  of  the  fountain  from  which  it  flowed. 

2.  Hence,  the  velocity  of  water  issuing  from  an  orifice  in  a 
vessel,  or  communicating  with  any  reservoir,  will  be  the  same 
as  that  which  a  heavy  body  would  acquire  in  falling  freely 
through  the  perpendicular  height  of  the  surface  of  the  water 
above  the  orifice  ;  and,  consequently,  the  discharge,  (or  velocity) 
under  different  heads,  will  be  as  the  square-roots  of  these  heads 
respectively. 

3.  Water  spouting  from  an  orifice  in  the  side  of  a  vessel,  will 
obey  the  laws  of  projectiles,  and,  consequently,  describe  a  para- 
bolic curve.  If  the  orifice  be  equally  distant  from  the  surface 
of  the  water  and  the  horizontal  plane  on  which  the  vessel  stands, 
it  will,  like  a  projectile  from  an  angle  of  45  degrees,  spout  to  the 
greatest  horizontal  distance  ;  and  from  any  two  orifices  equally 
above  and  below  this  middle  point,  the  water"  will  spout  to  equal 
distances. 

4.  But,  in  fact,  running  water  from  its  friction  along  the  sides 
and  bottom  of  the  conduit  or  channel,  the  angular  deviations  in 
its  course,  its  tenacity,  or  want  of  perfect  fluidity,  and  other 
causes,  which,  by  the  aid  of  experiment  only,  can  be  subjected 
to  calculation,  will  be  retarded  in  its  motion;  and  when  the  re- 
tarding forces  become  equal  to  the  accelerating  force  from  gra- 
vity, the  motion  will  become  uniform,  and  the  running  water  is 
then  said  to  be  in  train. 

P.  What  will  the  quantity  of  water  discharged  from  different 
orifices  be  proportional  to  ? 

5.  It  will  be  in  the  compound  ratio  of  the  time,  area  of  the 
orifice,  and  velocity,  or  square-root  of  the  head. 

P.  Will  water  running  along  a  horizontal  pipe  under  a  given 
head,  be  affected  by  the  length  of  the  pipe  ? 

S.  It  will  be  retarded  by  the  re-action  of  the  water  in  the 
pipe  ;  and  the  obstruction  or  retardation  of  the  velocity  or  dis- 


APPENDIX.  5  IS 

charge,  will  be  nearly  in  proportion  to  the  square-root  of  the 
length  of  the  pipe. 

P.  In  a  river,  or  stream  of  running  water,  what  will  the  ge- 
neral phenomena  be  ? 

S.  From  the  friction  along  the  sides  and  bottom  of  the  chan- 
nel, the  velocity  will  be  greatest  at  the  surface  in  the  middle  of 
the  stream,  and  slowest  at  the  bottom  and  sides  ;  the  surface, 
from  shore  to  shore,  being  concave. 

Of  Hydraulic  Machines. 

P.  What  are  the  hydraulic  machines  in  most  common  use  ? 

S.  The  pump,  the  fire-engine,  and  the  syphon. 

P.  How  many  kinds  of  pumps  are  there  ? 

S.  They  may  generally  be  reduced  to  two  kinds;  lifting- 
pumps,  and  forcing-pumps. 

P.  Give  a  short  description  of  the  construction  and  operation 
of  the  common  lifiing-pump. 

S.  This  pump  consists  of  a  hollow  cylinder,  the  lower  er.d  of 
which  is  immersed  under  the  surface  of  the  stagnant  water  to 
be  raised.  In  this  cylinder  is  fixed  a  box  with  a  valve  opening 
upwards,  at  any  convenient  height,  not  more  than  about  30 
feet  above  the  surface  of  the  water ;  and  to  the  extremity  of  a 
pump-rod  is  fastened  another  box  (or  piston)  fitted  to,  and  move- 
able in,  the  cylinder  or  pump-barrel,  with  its  valve  also  opening 
upwards.  Now,  supposing  the  pump  to  be  full  of  water,  and  the 
pump-rod  thrust  down — then,  on  raising  the  piston,  the  water 
above  it  will  be  lifted  up  and  a  quantity  will  run  out  equal  to  the 
capacity  of  that  part  of  the  cylinder  through  which  the  piston 
ascended  ;  the  valve  in  the  lower  or  fixed  box  will  be  opened  by 
the  pressure  of  the  atmosphere  on  the  surface  of  the  stagnant 
water,  and  the  water  will  ascend  in  the  pump  and  keep  close  to 
the  upper  piston.  On  the  descending  stroke  of  the  piston,  the 
valve  in  the  lower  box  will  be  closed,  and  the  water,  being  thus 
prevented  from  returning,  will  open  the  valve  in  the  upper  box, 
and  suffer  it  to  descend ;  and  thus,  by  alternately  raising  and 
lowering  the  pump-rod  with  its  piston,  the  water  will  be  dis- 
charged. 

P.  What  are  the  construction  and  operation  of  a  forcing-pump  ? 

S.  In  this  the  lower  box  is  the  same  with  that  in  the  common 
lifting-pump,  but  the  box  at  the  end  of  the  pump-rod  has  no 
valve,  and  is  hence  called  a  plunger.  On  the  descent,  therefore, 
of  the  plunger,  the  valve  in  the  lower  box  being  closed,  the  wa- 
ter will  be  forced  into  a  lateral  tube,  and  may  thus  be  raised  to 
any  height  proportioned  to  the  force  with  which  the  plunger  is 
made  to  descend.  An  air-vessel  is  most  frequently  attached  to 
the  lateral  tube,  into  which  the  water  is  forced,  and  the  air  in 
the  upper  part  of  the  vessel  being  thus  condensed,  re-acts  by  its 
elasticity  on  the  water  in  the  air-vessel,  and  impels  it  through  a 
tube  opening  into  the  water  in  the  air-vessel,  with  a  continued 


516  APPENDIX. 

motion.     The   engines  for  extinguishing  fire  are  on  this  con- 
struction. 

P.  Describe  and  explain  the  construction  and  operation  of 
the  syphon. 

S.  The  syphon  is  a  bent  tube  generally  made  in  the  form  of 
an  inverted  U  (f»)  with  one  leg  longer  than  the  other.  It  is 
chiefly  used  for  drawing  off  liquor  from  one  vessel  into  another: 
for  this  purpose  the  shorter  end  of  the  syphon  is  introduced  in- 
to the  liquor  through  the  bung-hole  or  over  the  edge  of  tne  ves- 
sel from  which  it  is  to  be  drawn,  and  the  tube  being  filled  by 
suction,  or  in  any  other  convenient  way,  the  liquor  will  imme- 
diately begin  to  run,  and  will  continue  to  issue  from  the  longer 
end,  as  long  as  this  is  below  the  surface  of  the  liquor  in  the 
vessel. 

P.  How  do  you  account  for  this  operation  of  the  syphon  ? 

S.  The  column  of  liquor  in  the  longer  leg  being  higher  than 
that  in  the  shorter  will  preponderate  and  run  out ;  and  the  pres- 
sure of  the  atmosphere  will  force  the  liquor  up  the  shorter  leg 
to  supply  its  place. 

P.  To  what  height  may  a  fluid,  (water  for  instance,)  be  made 
to  rise  in  a  syphon  ? 

S.  To  any  height  less  than  that  of  a  column  of  the  fluid  of 
equal  weight  with  a  column  of  the  atmosphere  of  the  same 
base  ;  which,  when  water  is  the  fluid,  is  about  33  feet. 


Of  Optics, 

P.  What  is  understood  by  optics  ? 

S.  That  science  which  treats  of  light  and  vision,  with  the  con- 
struction and  uses  of  optical  instruments. 

P.  What  may  we  conceive  light  to  be  ? 

S.  An  extremely  subtile  elastic  fluid,  composed  of  inconceiva- 
bly-small particles,  emanating  with  prodigious  velocity,  in  all  di- 
rections, from  the  sun,  and  other  luminous  bodies;  or,  by  re- 
flection, from  other  bodies,  and  thus  rendering  them  visible  to 
the  eye. 

P.   With  what  velocity  does  light  move  ? 

S.  From  the  eclipses  of  Jupiter's  satellites,  and  other  astrono- 
mical phenomena,  it  has  been  computed,  that  light  moves  over 
the  whole  diameter  of  the  earth's  annual  orbit  in  about  16  mi- 
nutes, which  is  not  much  less  than  12  millions  of  miles  in  a 
minute. 

P.  How  does  it  appear  that  light  is  a  material  substance  ? 

S.  From  its  being  perceivable  by  one  of  the  external  senses, 
and  possessing  all  the  other  essential  properties  of  matter. 

P.  Are  the  particles  of  light  of  the  same,  or  of  different  mag- 
nitudes ? 

S.  It  is  probable  that  they  are  of  different  magnitudes:  for  a 
beam  of  light  falling  obliquely  on  a  triangular  glass  prism,  will 


APPENDIX.  517 

be  differently  refracted,  and  thus,  falling  on  a  white  surface,  will 
form  a  spectrum  of  seven  different  colours,  viz.  red,  orange,  yel- 
low, green,  blue,  indigo,  violet;  and  this  cannot  be  satisfactorily 
accounted  for  on  any  other  supposition. 

P.  Explain  this  phenomena  on  the  hypothesis  of  light  being 
corrtposed  of  particles  of  different  magnitudes. 

S.  The  particles  of  light  falling  obliquely  on  the  surface  of 
the  prism,  will,  when  they  come  within  the  sphere  of  its  influ- 
ence, be  attracted  directly  towards  it,  and  thus  bent  (or  refracted) 
out  of  their  course,  less  or  more,  according  to  the  impetus  with 
which  they  move,  that  is,  according  to  their  magnitudes  :  the 
red  rays,  therefore,  being  the  least  refracted,  we  must  conclude 
to  be  composed  of  the  largest  particles,  the  violet  of  the  least, 
and  the  others  in  their  intermediate  order. 

P.  Are  not  the  different  impetus  of  the  rays  of  light  sensible 
to  the  eye  itself? 

S.  This  is  doubtless  the  case,  both  the  glare  of  the  red  colour, 
and  the  feeble  impressions  of  the  violet,  are,  for  opposite  rea- 
sons, painful  to  the  sight ;  while  the  intermediate  green,  the 
general  robe  of  nature,  is  always  viewed  with  pleasure  and 
delight. 

P.  You  have  mentioned  neither  black  nor  white  among  the 
colours  of  light. 

S.  Perfect  black  is  indeed  nothing  else  than  a  total  deprivation 
of  light,  and  white  has  been  demonstrated  to  be  the  intimate 
union  of  all  the  prismatic  colours. 

P.  How  may  the  science  of  optics  be  divided  ? 

S.  Into  two  general  parts,  viz.  catoptrics,  and  dioptics. 


Of  Catoptrics, 

P.  What  is  catoptrics  ? 

S.  That  part  of  optics  which  treats  of  reflected  light. 

P.  On  what  general  principles  may  the  various  phenomena 
of  reflected  light  be  explained  ? 

S.  On  this  single  principle,  viz.  that  from  any  polished  sur- 
face capable  of  a  copious  reflection  of  the  rays  of  light,  called  a 
mirror,  or  speculum,  the  angle  of  reflection  will  be  equal  to  the 
angle  of  incidence  ;  that  is,  the  angles  which  the  reflected  and 
incident  rays  make  with  a  line  perpendicular  to  the  surface  at 
the  point  of  reflection,  will  be  equal  to  each  other  ;  and  that 
any  object  will  appear  to  the  eye  in  the  direction  in  which  the 
ray  of  light  coming  from  the  object,  either  directly,  or  after 
refraction  or  reflection,  enters  the  eye.  Hence,  the  reflected  ray- 
will  exhibit  to  the  eye,  an  image  of  the  radiant  point  which  will 
appear  in  that  place  where  the  reflected  ray  (continued  back- 
wards if  necessary;  would  meet  a  line  drawn  from  the  radiant 
point  perpendicular  to  the  reflecting  surface. 

P.  How  many  kinds  of  mirrors  are  there  ? 

S.  Three  ;  plane,  concave,  and  convex. 


518  APPENDIX. 

P.  If  a  mirror  be  made  of  glass,  crystal,  or  any  other  transpa- 
rent substance,  and  polished  on  both  sides,  as  a  common  look- 
ing-glass, how  will  the  image  of  a  candle,  or  any  other  radiant 
point  appear  ? 

S.  There  will  be  a  number  of  partial  transmissions  and  re- 
flections of  the  rays  of  light,  from  the  first  and  second  surfaces, 
alternately  ;  part  of  the  rays  of  light  passing  through,  and  part 
being  reflected  at  each  surface  successively  ;  the  first  image 
from  the  second  surface  being  the  brightest,  that  from  the  first 
surface,  next  in  brightness,  and  so  on,  growing  fainter  and 
fainter,  till  they  become  invisible. 

P.  Will  a  number  of  images  always  appear  from  such  mir- 
rors? 

S.  When  the  two  opposite  sides  of  the  glass  are  parallel 
planes,  and  the  radiant  point  not  very  near,  all  the  images  from 
the  first  and  second  surfaces  will  coincide,  and  therefore  appear 
but  as  one  ;  but  when  the  opposite  sides  are  not  parallel,  as 
in  all  common  looking-glasses,  the  images  will  be  distinct  and 
separate. 

P.  Wrhat  effect  does  the  silvering  of  a  glass  speculum  pro- 
duce ? 

S.  The  light  from  the  second  surface,  being  prevented  by 
the  metallic  coating  from  passing  off,  will  be  more  copiously  re- 
flected ;  and  thus,  the  brightness  of  the  principal  image  will  be 
greatly  increased. 

P.  At  what  angle  of  incidence  will  there  be  the  most  copious 
reflection  ? 

S.  The  greater  the  angle  of  incidence,  the  more  copious  will 
be  the  reflection  ;  as  the  more  obliquely  the  light  strikes  the  re* 
fleeting  surface,  the  less  tendency  will  it  have  to  enter  or  pass 
through  and  be  lost. 

P.  By  what  general  equation  or  theorem  may  all  problems 
respecting  reflected  light  be  solved  ? 

S.  If  we  put  r  =  the  radius  of  curvature  of  a  concave  mir- 
ror, 
d  =  the  distance  of  the  object  or  radiant  point 
from  the  vertex  of  the  mirror, 
and  f  =  the  point  where   the    reflected   rays  cross 
each  other,  called  the  point  of  intersection, 
focus,  or  place  of  the  image, 
then,  from  the   general  principle  that  the  angle  of  reflection  is 
equal  to  the  angle  of  incidence,  the  following  general  theorem 
may  be  readily  demonstrated,  viz. 
rd 

2d— r 

P.  How  may  this  general   theorem  be  accommodated  to  all 
the  various  cases  that  may  occur?  For  instance, 
Suppose  the   object  at  a  great  distance,  or  bearing  no  sensible 
proportion  to  the  radius  of  mirror  ? 

S.  In  this  case,  d  may  be  considered  as  infinite,  and  then  r, 
with  respect  to  it,  as  notUing  ;  hence  the  equation  will  become 


APPENDIX.  519 

rd  r 

f  — =  —  ;  that  is,  the  focal  distance  equals  half  the  radius 

2d  2 

of  the  mirror.  , 

P.  Suppose  d  =  r  ? 

S.  Then  f  =  d. 

P.  Suppose  d=|r? 
irr 

S.  Then  f  =  ■ ;  that  is,  f  will  be  infinite,  or  the  reflected 

o 
rays  will  proceed  parallel  to  each  other. 

P.  Suppose  d  less  than  \  r  ? 

S.  Then  the  divisior  2d-r  will  become  negative,  and  conse- 
quently f  will  be  negative;  that  is,  the  reflected  rays  will  pro- 
ceed diverging,  as  from  a  point  behind  the  mirror,  where  the 
image  will  appear,  called  a  negative  focus, 

P.  Suppose  r  infinite,  or  the  reflecting  surface  a  plane  spe- 
culum ? 

S.,  Then  2d  will  be  infinitely  small  with  respect  to  r,  or  =  o, 
rd 

and  consequently  f  = = — d  ;  that  is,  the  image  will  ap- 

— r 
pear  behind  the  speculum,  and  at  the  same  distance  from  it  as 
the  object. 

P.  Suppose  r  to  be  negative,  that  is  the  mirror  convex  to- 
wards the  object? 

— rd 

S.  Then  f  = ,  a  negative  quantity,  or,  the  image  will 

2d-f-r 
appear  behind  the  mirror. 

P.  In  what  position  will  the  image  appear  with  respect  to  th& 
object  ? 

S.  When  the  image  appears  behind  the  speculum,  or  in  a 
negative  focus,  it  will  be  erect,  or  in  the  same  position  with  the 
the  object  ;  but  when  it  appears  before  the  mirror,  or  in  a  posi- 
tive focus,  it  will  be  inverted,  or  in  a  contrary  position  to  that 
of  the  object. 

P.  What  ratio  will  there  be,  between  the  magnitude  of  the 
image  and  that  of  the  object  ? 

S.  In  their  linear  dimensions,  they  will  be  to  each  other  as 
their  respective  distances  from  the  mirror  ;  or,  from  its  vertex, 
they  would  appear  under  equal  angles. 

P.  Will  the  rays  of  light  reflected  from  all  parts  of  a  concave 
mirror  actually  meet  in  the  same  point  ? 

S.  It  is  only  those,  coming  from  the  radiant  point,  which  fall 
near  the  vertex  of  a  spherical  mirror,  that  will  actually  meet  in 
the  same  focus.  To  obtain  this  end,  in  the  case  of  parallel  rays, 
the  mirror,  as  in  the  best  reflecting  telescopes,  should  be  ground 
to  a  parabolic  curve. 


[     520     ] 

Of  Dioptrics. 

P.  What  is  dioptrics. 

S.  That  part  of  optics  which  treats  of  transmitted  or  re- 
fracted light. 

P.  On1  what  general  principles  may  the  various  phenomena 
of  refracted  light  be  explained  ? 

S.  1.  When  a  ray  of  light  passes  obliquely  from  one  transpa- 
rent medium  into  another  of  different  density,  it  will  be  refracted, 
or  bent  out  of  its  right  course. 

2.  When  passing  from  a  rarer  to  a  denser  medium,  the  re- 
fraction, from  the  superior  attraction  of  the  latter,  will  be  towards 
a  line  perpendicular  to  the  common  surface  of  the  two  media: 
but  from  a  denser  to  a  rarer  medium,  the  refraction  will  be 
from  the  perpendicular. 

3.  The  quantity  of  this  refraction  is  different  between  differ- 
ent media,  and  with  different  angles  of  incidence  ;  but  between 
the  same  media  there  is  a  constant  ratio  between  the  sines  of 
the  angles  of  incidence  and  refraction. 

4.  The  different  coloured  rays  suffer  different  degrees  of  re- 
fraction, as  before  observed,  though  the  ratio  among  those  is 
different  in  different  media  ;  or  the  dispersive  powers  of  some 
are  greater  than  those  of  others. 

5.  The  mean  ratios  between  the  sines  of  the  angles  of  inci- 
dence and  refraction,  in  air,  glass,  water,  and  diamond  are  as 
follows,  viz. 

From  air  to  glass  as  3  to  2, 
air  to  water  as  4  :  3, 
air  to  diamond  as  5  :  2. 
P.  What  is  a  lens ;  and  of  how  many   different   forms  are 
they  ? 

S.  A  lens  is  a  round  polished  piece  of  glass,  or  other  transpa- 
rent substance,  one  or  both  surfaces  being  curved.  They  are  of 
five  different  kinds,  viz. 

Plano-convex, 
Plano-concave, 
Double-convex, 
Double-concave, 
Concavo-convex,  or,  meniscus. 
P.  By  what  general  theorem  or  equation  may  all  the  problems 
respecting  refracted  light  be  solved  ? 

S.  If  we  put — 
r  =  the  radius  of  curvature  of  the  side  of  a  double-convex  lens 

next  to  the  object, 
e  =  the  radius  of  curvature  of  the  opposite  side, 
d  =  the  distance  of  the  object,  or  radiant  point,  from  the  near- 
est surface  of  the  lens,  from   which  the  rays  proceed   di- 
verging, 
i  :  R  =  the  ratio  of  the  sines  of  the  angle  of  incidence  and  re- 


APPENDIX.  521 

fraction,  of  the  rays  of  light,  in  entering  the  first  surface 
of  the  lens :  consequently — 

R  :  i  =  the  ratio  in  passing  out  of  the  second  surface, 
R 

b  =  

i—R 

t  =  the  thickness  of  the  lens  at  its  vertex,  and 

f  =  the  distance  of  the  focus  of  the  rays  of  mean  refraction, 
from  the  nearer  surface  of  the  lens  ;  then,  from  the  forego- 
ing general  principles,  it  may  be  readily  demonstrated  that 
bidrg-fbRrgt — dRgt 


f  = 


idr-fidg— idt+Rdt+Rrt— birg 

P.  How  may  this  general  theorem  be  applied  to  all  the  vari- 
ous cases  that  may  occur  ?  For  instance — 

If  both  sides  of  the  lens  be  of  equal  curvature  ? 

S.  Then,  call  r  and  g  equal. 

P.  If  the  lens  be  a  double-concave  ? 

S.  Then,  let  r  and  g  be  both  considered  as  negative,  and  con- 
sequently change  the  sign  of  every  term  containing  but  one  of 
these  letters. 

P.  If  one  side  only  be  concave  ? 

S.  Then,  change  the  sign  of  every  term  containing  the  letter 
which  represents  the  radius  of  curvature  of  the  concave  side. 

P.  If  one  side  of  the  lens  be  plane  ? 

S.  Then,  let  its  radius  of  curvature  be  considered  as  infinite, 
and  consequently  all  the  terms  in  which  it  is  not  found,  expunged, 
as  being  infinitely  small  with  respect  to  it :  the  infinite  quantity 
being  then  expunged  as  a  common  factor  both  in  the  numerator 
and  denominator,  the  result  will  give  the  value  of  f. 

P.  If  the  thickness  of  the  lens  be  inconsiderable  with  respect  to 
the  other  quantities  ? 

S.  Then,  let  it  be  considered  as  nothing,  and,  consequently, 
all  the  terms  in  which  it  is  found,  expunged. 

P.  If  the  distance  be  incomparably  great  with  respect  to  the 
other  quantities  ? 

S.  Then,  let  it  be  considered  as  infinite,  and  proceed  accord- 
ingly. 

P.  If  the  rays  of  light  fall  on  the  lens,  converging  towards  a 
point  on  the  other  side  ? 

S.  Then,  this  point  must  be  considered  as  the  place  of  the 
object  from  which  the  rays  would  diverge,  and  therefore  d,  as  a 
negative  quantity. 

P.  To  illustrate  by  a  few  particular  examples — 

Suppose  a  double-concave  glass  lens,  placed  in  air,  both 
sides  of  equal  curvature,  the  distance  of  the  object  considered  as 
infinite,  or  the  rays  proceeding  from  it  as  parallel,  and  the  thick- 
ness of  the  lens  inconsiderable,  what  will  the  general  equation 
become  ? 

S.  Here  i  =  3,  R  =  2,  b  =  2,  r  =  g,  t  =  o,  and  d  infinite* 

VOL.  IV.  3  ¥ 


522  APPENDIX. 

Hence,  f  =  r,  the  principal  focus,  or  focus  of  parallel  rays. 

P.  Suppose  the  lens  a  plano-concave,  and  all  the  other  parts  as 
in  the  last  example  ? 

S.  Then  f  =  2  r. 

P«  Suppose,  as  in  example  1st,  only  the  lens  a  double-concave, 
and  the  sides  of  equal  curvature  ? 

S.  Then  f  =  —  r.  That  is,  the  rays,  after  passing  through 
the  lens,  will  diverge  as  from  the  centre  of  the  curve  on  the  side 
of  the  object. 

P.  Suppose,  as  in  example  1st,  but  the  distance  finite? 
dr 

S.  Then  f  = 

d  —  r 

P.  What  is  meant  by  an  achromatic  lens? 

S.  One  composed  of  two,  and  sometimes  of  three  pieces  of 
glass,  flint,  and  crown,  which  are  of  different  dispersive  powers, 
the  convex  surface  of  one  exactly  fitted  into  and  touching  the 
concave  surface  of  the  other,  and  so  adjusted  that  all  the  dif- 
ferent rays  of  light  will  be  converged  into  the  same  focus,  and 
form  an  untinged  or  colourless  image  of  the  object;  and  hence, 
by  its  inventor,  Mr.  Dollond,  termed  achromatic. 

P.  In  what  position  will  the  image  appear,  with  respect  to  the 
object  ? 

S.  The  image  will  be  inverted  with  respect  to  the  object,  in 
a  positive  focus,  or  behind  the  lens  ;  but  erect  in  a  negative 
focus,  or  before  the  lens. 

P.  What  ratio  will  there  be  between  the  magnitude  of  the 
image  and  that  of  the  object  ? 

S.  In  their  linear  dimensions,  they  will  be  to  each  other  as 
their  respective  distances  from  the  lens  ;  that  is,  from  the  cen- 
tre of  the  lens  they  would  appear  under  equal  angles. 


Of  the  Eye  and  of  Vision* 

P.  Give  a  short  description  of  the  human  eye. 

S.  The  eye,  or  organ  of  vision,  is  nearly  of  a  spherical  figure  ; 
and  is  composed  of  three  coats  or  integuments,  inclosing  three 
transparent  humours.  The  outer  coat  is  called  the  sclerotica,  and 
its  anterior  part,  the  cornea.  The  next  is  called  the  choroides, 
which  serves  as  a  lining  to  the  former;  the  anterior  part  of  this 
is  called  the  iris,  which  is  of  different  colours  in  different  sub- 
jects, and  is  perforated  with  a  circular  hole  called  the  pupil.  The 
iris  is  composed  of  two  sets  of  muscular  fibres  ;  one  circular,  and 
the  other  radial,  by  the  action  of  which  the  pupil  is  dilated  or 
contracted,  for  the  purpose  of  admitting  more  or  less  light,  ac- 
cording as  this  is  weak  or  strong.  The  third,  or  inner  coat,  is  a 
fine  retiform  expansion  of  tiu  optic  nerve,  and  hence  termed  the 
retina.  On  this,  probably,  are  formed  the  images  of  external  ob- 
jects ;  whence,  in  a  manner  unknown  to  us,  ideas  of  those  ob- 


APPENDIX.  523 

jects  are  excited  in  the  mind.  The  three  humours  are,  1.  The 
aqueous  humour,  which  is  a  thin  transparent  fluid  resembling  wa- 
ter, lodged  in  the  anterior  part  of  the  eye,  in  contact  with  the 
cornea  ;  and  in  this  the  iris  floats.  2.  The  crystalline  humour, 
or  lens,  of  die  consistence  of  hard  jelly,  in  form  of  a  double-con- 
vex lens,  inclosed  in  a  fine  transparent  membrane,  which,  by  ra- 
dial fibres,  called  the  ligamentum  ciliare,  join  it  to  the  outer  cir- 
cumference of  the  iris,  and  thus  form  the  anterior  chamber  of 
the  eye.  3.  The  vitnous  humour,  resembling  melted  glass,  or 
the  white  of  an  egg  ;  occupying  the  posterior  chamber,  and 
composing  by  far  the  greatest  part  of  the  globe  of  the  eye. 

P.  Trace  the  progress  of  the  rays  of  light  from  an  external 
object,  to  the  image  formed  on  the  bottom  of  the  eye. 

S.  The  rays  of  light  from  an  external  object,  entering  by  the 
pupil,  pass  successively  through  the  three  different  humours  of 
the  eye,  which,  being  of  different  refractive  and  dispersive  pow- 
ers, will  form,  in  the  manner  of  Dollond's  compound  lens,  an 
achromatic  image  of  the  object  on  the  retina,  whence  corre- 
sponding ideas  will  be  excited  in  the  mind  ;  it  being  a  condition 
of  distinct  vision,  that  the  image  of  the  object  be  formed  on  or 
near  the  retina  ;  and  for  this  purpose,  the  rays  of  light,  on  en- 
tering the  eye  of  a  young  person,  in  a  sound,  healthy  state, 
must  be  nearly  parallel,  or  the  object  not  nearer  than  six  inches. 

P.  What  is  the  nature  of  defective  vision  in  old  age,  and  how 
is  it  remedied  ? 

S.  In  old  age,  the  anterior  part  of  the  eye  becomes  too  flat ; 
and  the  images  of  near  objects  will  therefore  be  formed  beyond 
the  retina.  In  this  case,  to  procure  distinct  vision,  the  object 
must  be  removed  to  a  greater  distance,  or  a  convex  glass  inter- 
posed between  the  object  and  the  eye;  by  which  means,  the  focal 
distance  being  diminished,  the  image  will  be  brought  forward  to 
the  retina. 

P.  What  is  the  nature  of  defective  vision  in  pur-blind,  or  near- 
sighted persons  ;  and  how  is  it  remedied  ? 

S.  In  these  persons,  the  anterior  part  of  the  eye  is  too  protu- 
berant, and  the  image  of  objects  at  a  moderate  distance  will  be 
formed  before  the  retina.  In  this  case,  to  procure  distinct  vision, 
the  object  must  be  brought  nearer  to  the  eye,  or  a  concave  glass 
used  ;  by  which  means,  the  focal  distance  being  increased,  the 
image  will  be  carried  back  to  the  retina.  In  eyes  of  this  con- 
stitution, it  is  observed  that  old  age  contributes  to  the  remedy 
of  this  defect. 


Of  the  Rainbow. 

P.  How  is  the  rainbow  formed  ? 

S.  By  the  refraction  and  reflection  of  the  rays  of  light  from  the 
sun,  incident  upon  falling  drops  of  rain  in  the  opposite  part  of 
the  heavens. 


524  APPENDIX. 

P.  How  is  the  interior  bow  produced  ? 

S.  By  two  refractions  and  one  intermediate  reflection;  the  ray 
of  light  being  incident  on  the  drop  above,  or  outside  of  the  axis  of 
the  drop  directed  to  the  eye,  and  consequently  emergent  from 
the  drop  below,  or  inside  of  the  axis.  The  colours  from  the  outer 
to  the  inner  verge  of  the  bow  will  therefore  appear  in  the  order 
of  their  respective  refragibilities,  viz.  R.  O.  Y.  G.  B.  I.  V. 

P.  How  is  the  exterior  bow  produced  ? 

S.  By  two  refractions  and  two  intermediate  reflections ;  the 
rays  of  light  being  incident  on  the  drop  below,  or  inside  of  the 
axis,  and  consequently  emergent  from  the  drop  above  or  outside 
of  the  axis.  The  colours  will  therefore  be  in  an  inverted  order 
to  those  in  the  interior  bow,  and  will  appear  fainter,  from  the 
greater  loss  of  light  by  the  additional  reflection. 

P.  What  angles  will  the  several  parts  of  the  bows  make  re- 
spectively with  the  axis  ? 

S.  It  is  found  by  calculation,  from  the  laws  of  optics,  and  con- 
firmed by  observation,  that  the  angles  which  the  several  parts  of 
the  respective  bows  make  with  the  incident  ray,  or,  which  is  the 
same  thing,  with  the  axis  of  the  cone,  of  which  the  eye  is  the 
vertex,  and  the  bow  the  base,  are  as  follows,  viz. 

The  inner  verge  of  the  interior  bow  •            •         40  2 

The  outer  verge  of              do.  •           '•                42  17 

Consequently,  breadth  of    do.  •             •            2  15 

The  inner  verge  of  the  exterior  bow  .                 50  42 

The  outer  verge  of             do.  •            •         54  22 

Consequently,  breadth  of  do.  •            •                   3  40 

And  space  between  bows            •  •            •             8  25 

P.  In  what  circumstances  will  a  rainbow  be  visible? 

S.  1.  The  sun  must  be  shining  on  the  spectator,  and  on  falling 
drops  of  rain  from  a  cloud  in  the  opposite  part  of  the  heavens. 

2.  To  see  any  part  of  the  exterior  bow,  the  sun's  altitude 
must  not  exceed  54°  22'.  To  see  any  part  of  the  interior  bow, 
the  sun's  altitude  must  not  exceed  42*  17'.  But  the  less  the 
sun's  altitude  is,  the  greater  portion  of  the  rainbow  will  be  visi- 
ble, and  vice  versa. 

Of  Microscofies, 

P.  What  is  a  microscope  ? 

S.  It  is  an  instrument  for  the  purpose  of  viewing  more  dis- 
tinctly, and  increasing  the  apparent  magnitude  of  small,  near 
objects. 

P.  How  many  kinds  of  microscopes  are  there  ? 

S.  Two  general  kinds  ;  simple,  and  compound. 

P.  Describe  the  simple  microscope. 

S.  It  is  no  more  than  a  single  convex  lens,  of  a  small  focal 
distance,  generally  set  in  a  frame,  and  placed  between  the  eye 


APPENDIX.  525 

and  the  object,  at  the  distance  of  the  focus  of  parallel  rays  from 
the  object. 

P.  Trace  the  progress  of  the  rays  of  light  from  the  object  to 
the  eye,  viewing  it  through  this  single  lens. 

S.  The  rays  of  light  issuing  from  any  point  in  the  object  will, 
after  passing  through  the  lens,  proceed  in  a  parallel  direction ; 
and,  thus  falling  on  the  eye,  will  be  converged  to  the  retina ;  and 
therefore  produce  distinct  vision. 

P.  What  will  be  the  apparent  position  and  magnitude  of  an 
object  viewed  through  a  single  microscope  ? 

S.  The  object  will  appear  in  its  natural  position,  and  will  be 
magnified  in  its  linear  dimensions,  in  the  ratio  of  six  inches,  (or 
the  distance  at  which  the  object  might  be  seen  distinctly  by  the 
naked  eye,  to  the  focal  distance  of  the  lens. 

P.  Describe  the  compound  microscope. 

S.  The  compound  microscope  is  composed  of  two  convex 
lenses,  viz.  an  object-glass  and  an  eye-glass,  placed  in  a  tube  ; 
the  former  being  of  a  much  smaller  focal  distance  than  the  latter. 
The  object  to  be  viewed  is  placed  a  little  beyond  the  principal 
focus  of  the  object-glass  ;  so  that  the  rays  of  light  coming  from 
the  object,  after  passing  through  this  lens,  may  converge,  and 
form  an  image  on  the  other  side.  This  image  is  then  viewed 
through  the  eye-glass,  placed  at  its  principal  focal  distance  from 
the  image  ;  by  which  means  the  rays  of  light  will  fall  parallel  on 
the  eye,  and  thus  produce  distinct  vision. 

P.  Are  there  not  three  glasses  in  some  compound  micro- 
scopes ? 

S.  A  broad  convex  lens,  called  an  amplifying  lens,  is  frequently- 
interposed  between  the  other  two,  a  little  farther  from  the  eye- 
glass than  its  focal  distance.  The  rays  of  light  from  the  object 
will  then,  after  passing  through  the  object-glass,  fall  converging 
on  the  amplifying  lens,  and  by  it  be  further  converged,  and  form 
an  image  to  be  viewed  through  the  eye-glass,  placed  at  its  focal 
distance  as  before. 

P.  What  is  the  use  of  this  amplifying  lens  ? 

S.  Chiefly  to  enlarge  or  amplify  the  field  of  view  of  the  ob- 
ject. 

P.  What  will  be  the  position  of  the  image,  and  its  apparent 
magnitude,  with  respect  to  the  object,  in  this  microscope  ? 

S.  The  image  will  be  inverted,  and  magnified  in  the  com- 
pound ratio  of  the  distances  of  the  image  and  of  the  object  from 
the  object  glass,  and  of  six  inches  to  the  focal  distance  of  the 
eye-glass. 

P.  Are  there  not  some  microscopes  with  two  glasses,  through 
which  the  object  docs  not  appear  inverted  ? 

S.  Those  commonly  used  for  reading  off  the  graduations  on 
the  sextant,  and  for  other  similar  purpose,  where  a  great  magni- 
fying power  is  not  necessary,  are  frequently  of  this  kind. 
They  are  composed  of  two  convex  lenses  of  moderately  small 
focal  distances,  and  placed  near  to  each  other.     The  object  to 


526  APPENDIX. 

be  viewed  is  placed  nearer  to  the  object-glass  than  its  principal 
local  distance  ;  so  that  the  rays  of  light  after  passing  through  it 
may  fall  on  the  eye-glass  with  such  a  degree  of  convergtncy 
as  if  proceeding  from  its  principal  focus;  they  will  then  fall  pa- 
rallel on  the  eye,  and  thus  produce  distinct  vision.  Through 
this  microscope,  the  object  will  appear  erect,  as  no  image  will 
be  formed  ;  and  will  be  magnified  in  the  compound  ratio  of  the 
distance  of  the  object  from  the  focus  of  the  eye-glass  to  its 
distance  from  the  object-glass ;  and  of  six  inches  to  the  focal 
distance  of  the  eye-glass. 

P.  In  what  manner  may  the  object  be  illuminated  so  as  to 
render  it  the  more  distinct  ? 

S.  For  this  purpose,  the  microscope  may  be  directed  to- 
words  a  candle,  a  lamp,  or  the  clear  sky  ;  or  the  light  may  be 
reflected  on  the  object,  in  a  concentrated  state,  by  a  concave 
mirror;  or  it  may  be  thrown  on  it  after  passing  through  aeon- 
vex  lens,  placed  nearly  at  its  focal  distance  from  the  object ;  or 
the  light  of  the  sun  may  be  reflected  on  it  in  a  concentrated 
state  by  a  plane  mirror  and  convex  lens ;  or  the  light  of  a  can- 
dle, or  of  a  lamp,  may  be  thrown  on  it  in  a  concentrated  state, 
by  a  convex  lens,  and  the  image  received  on  a  white  screen  or 
wall  in  a  dark  room.  In  the  last  form,  when  the  sun  is  made  use 
of  to  illuminate  the  object,  the  instrument  is  called  a  solar  micro- 
scope  ;  and  when  a  candle  or  lamp  is  made  use  of,  the  instru- 
ment is  called  a  inagic  lanthorn* 

Of  Telesco/ies. 

P.  What  is  a  telescope  ? 

S.  It  is  an  instrument  for  increasing  the  angular  or  apparent 
magnitude  of  distant  objects,  and  thence  viewing  them  more 
distinctly. 

P.  How  many  kinds  of  telescopes  are  there  ? 

S.  Two  general  kinds  ;  refracting,  and  reflecting. 

P.  Describe  the  most  simple  form  of  the  refracting  telescope. 

S.  In  its  most  simple  form,  it  consists  of  two  convex  lenses, 
an  object-glass,  (which,  in  the  best  telescopes,  is  an  achroma- 
tic lens)  and  an  eye-glass,  placed  in  a  tube,  at  the  sum  of  their 
focal  distances  ;  the  latter  being  of  a  smaller  focal  distance  than 
the  former.     This  is  the  common  astronomical  telescope. 

P.  Trace  the  progress  of  the  rays  of  light  from  a  distant  ob- 
ject to  the  eye,  as  viewed  through  this  telescope. 

S.  The  rays  of  light  passing  through  the  object-glass  nearly 
parallel,  will  be  converged  to  its  focus,  and  there  form  an  image, 
which  will  be  viewed  through  the  eye-glass,  placed  at  its  focal 
distance  from  this  image. 

P.  In  what  position  will  the  image  appear,  and  how  magnifi- 
ed in  this  telescope  ? 

S.  The  image  will  be  inverted  with  respect  to  the  object,  and 
magnified  in  the  ratio  of  the  focal  distance  of  the  object-glass  to 
that  of  the  eye-glass. 


APPENDIX.  527 

P.  By  what  means  may  the  image  be  erected  in  this  tele- 
scope ? 

S.  1.  By  using  a  double-concave  eye-glass,  placed  at  a  dis- 
tance from  the  object-glass,  equal  to  the  difference  of  their  fo- 
cal distances  ;  for  then  the  rays  of  light,  after  passing  through 
the  object-glass,  will  fall  upon  the  eye-glass  with  such  a  degree 
of  convergence  as  that  they  meet  in  its  negative  focus  ;  conse- 
quently, after  passing  through  the  eye-glass,  they  will  fall  on 
the  eye  parallel,  and  thus  produce  distinct  vision  ;  and  the 
object  will  appear  erect,  because,  in  fact,  no  image  is  formed. 

This  is  the  kind  of  telescope  first  used  by  Galileo,  and  which 
still  retains  the  name  of  the  Galilean  telesco/ie.  The  chief  defect 
of  this  telescope  is  the  smallness  of  its  field  of  view. 

2.  By  using  three  eye-glasses  placed  at  the  sum  of  their  re- 
spective focal  distances  ;  for  then  the  rays  of  light  from  each 
point  of  the  inverted  image  formed  in  the  focus  of  the  object- 
glass,  after  passing  through  the  first  eye-glass,  will  proceed 
parallel ;  the  pencils  will  cross  each  other,  and  thus  by  the  second 
eye-glass  will  be  converged  to  a  focus,  and  form  a  second  and 
erect  image,  to  be  viewed  through  the  third  or  outer  eye-glass. 
In  telescopes  of  this  construction,  the  field  of  view  will  be  conside- 
rably enlarged,  though  the  object  will  appear  more  obscure 
than  in  the  Galilean,  by  reason  of  the  loss  of  light  at  each  re- 
fraction. 

P,  How  many  different  kinds  of  reflecting  telescopes  are 
there  ? 

S.  There  may  be  reckoned  four  viz.  the  Newtonian,  the 
Gregorian,  the  Cassigrain,  and  Herschel's  telescope. 

P.  Describe  the  Newtonian  reflector. 

S.  It  consists  of  a  large  concave  metallic  speculum,  placed  at 
the  bottom  of  a  tube,  its  axis  coinciding  with  that  of  the  tube  ; 
a  small  plane  speculum  placed  in  the  middle  of  the  tube,  be- 
tween the  great  mirror  and  its  focus,  at  an  angle  of  45°  with 
its  axis  ;  and  a  small  tube  containing  an  eye-glass  fixed  in  the 
side  of  the  great  tube,  at  right  angles  to  its  axis,  directly  op- 
posite the  plane  speculum  ;  its  focus  and  that  of  the  great  mir- 
ror being  equally  distant  from  the  plane  speculum. 

P.  Trace  the  progress  of  the  rays  of  light  from  a  distant  ob- 
ject through  this  telescope. 

•  S.  The  rays  of  light  from  every  visible  point  of  the  distant 
object,  entering  the  mouth  of  the  telescope  and  falling  on  the 
great  mirror  nearly  parallel,  will  thence  be  reflected  converging 
towards  its  focus ;  and  thus  falling  on  the  plane  speculum, 
will  be  reflected  from  it  with  the  same  degree  of  convergence, 
and  from  an  image,  to  be  viewed  through  the  eye-glass,  placed 
at  its  focal  distance  therefrom. 

P.  What  will  be  the  position  of  the  image,  and  magnifying 
power  in  this  telescope  ? 

S.  One  image  only  being  formed,  it  will  be  inverted  ;  and  the 
magnifying  power  will  be  as  the  focal  distance  of  the  great 
mirror  to  that  of  the  eye-glass. 


52*  APPENDIX. 

P.  Give  an  account  of  the  construction  and  properties  of  the 
Gregorian  telescope. 

S.  This  telescope  is  composed  of  two  metallic  concave  mirrors, 
a  great  and  a  small,  placed  in  a  large  tube  facing  each  other, 
at  somewhat  more  than  the  sum  of  their  focal  distances  apart ; 
together  with  an  eye-glass.  The  great  mirror  is  placed  in  the 
bottdm  of  the  tube,  with  a  perforation  through  the  vertex 
t>f  it,  and  the  small  one,  on  a  moveable  foot,  near  the  mouth  ;  the 
eye-glass  being  placed  in  a  small  tube,  outside  of  the  large  one, 
and  near  the  perforation  in  the  great  mirror.  The  rays  of 
light  from  any  distant  object  entering  the  telescope  and  falling 
on  the  great  mirror  nearly  parallel,  will  be  reflected  converg- 
ing to  its  focus,  where  an  inverted  image  of  the  object  will  be 
formed.  The  rays,  crossing  each  other  in  this  focus,  will 
proceed  to  the  small  mirror  on  which  they  will  fall  diverging, 
and  will  thence  be  reflected  converging,  and  thus  passing 
through  the  perforation  in  the  great  mirror  will  form  a  se- 
cond or  erect  image,  to  be  viewed  through  the  eye-glass, 
placed  at  its  focal  distance  therefrom  ;  the  magnifying  power 
©f  this  telescope,  will  be  in  the  compound  ratio  of  the  focal  di- 
stances of  the  great  and  small  mirrors,  and  of  the  great  mirror 
and  eye-glass. 

P.  Give  a  short  account  of  the  Cassegrain  telescope. 

The  Cassegrain  telescope  differs  from  the  Gregorian  only  in1 
this,  that  instead  of  the  small  mirror  being  concave,  it  is  convex, 
and  placed  between  the  great  mirror  and'  its  focus.  The  rays 
of  light,  therefore,  from  ihe  great  mirror,  will  fall  on  the  small 
one  converging,  and  from  it  be  reflected  less  convergent,  and 
form  an  inverted  image  in  the  focus  of  the  eye-glass,  from  which 
it  is  viewed.  The  chief  advantage  of  this  construction  consists 
in  its  being  shorter  than  the  Gregorian  of  the  same  magnifying 
power,  by  twice  the  focal  distance  of  the  smaller  mirror. 

P.  Give  a  short  description  of  HerschePs  reflector. 

S.  This  has  only  one  reflecting  mirror,  viz.  the  great  one, 
which  is  placed  in  the  bottom  of  the  tube,  a  little  obliquely,  so 
that  its  axis  would  pass  near  one  side  of  the  mouth  of  the  tube, 
where  the  eye-glass  is  placed,  at  its  focal  distance  from  the  image 
formed  by  the  reflection  from  the  great  mirror ;  the  observer 
turning  his  back  lo  the  object.  In  this,  as  in  the  Newtonian 
telescope,  the  object,  with  a  single  convex  eye-glass,  will  appear 
inverted  ;  and  will  be  magnified  in  the  ratio  of  the  focal  distance 
of  the  mirror  to  that  of  the  eye-glass. 

P.  What  is  the  peculiar  advantage  of  this  construction  above 
the  others  ? 

S.  The  prevention  of  the  loss  of  light,  by  having  but  one  re- 
flection, and  no  perforation  in  the  mirror;  the  objects,  especially 
small  celestial  ones,  will  therefore  be  seen  move  distinctly  and 
better  defined. 

P.  In  what  manner  may  the  field  of  view  be  enlarged  in  any 
kind  of  telescope  ? 


APPENDIX.  529 

S.  By  using  an  amplifying  lens,  generally  a  plano-convex,  as 
in  the  compound  miscroscope,  between  which  and  the  eye-giass 
the  image  is  formed.  A  tripple  eye-glass,  has  also  the  same 
advantage. 

Of  Magnetism, 

P.  What  is  meant  by  magnetism  ? 

S.  That  species  of  attraction  which,  especially  in  any'consi- 
derable  degree,  is  found  only  in  iron,  steel,  or  the  calces  or  ores 
of  iron. 

P.  What  is  a  magnet? 

S.  Any  piece  of  ore  or  steel,  possessing  this  attractive  power 
or  magnetic  virtue,  in  any  considerable  degree,  is  called  a  mag- 
net;  if  a  piece  of  ore,  it  is  termed  a  natural  magnet ',  or  load-stone  ; 
if  a  piece  of  steel,  to  which  this  virtue  has  been  communicated  by 
art,  it  is  termed  an  artificial  magnet. 

P.  Explain  a  few  of  the  most  common  terms  and  phenomena 
relative  to  magnetism.     For  instance, — 

Magnetic  meridian  ? 

S.  If  a  magnet,  of  an  oblong  form,  be  suspended  by  its  centre 
of  gravity,  and  suffered  to  move  freely,  it  will  finally  settle  in 
the  plane  of  a  vertical  circle,  called  the  magnetic  meridian. 

P.  Magnetic  needle  ? 

S.  A  small  artificial  magnet,  balanced  on  a  centre,  which  will 
then  settle  in  the  magnetic  meridian  ? 

P.  Variation  of  the  needle  ? 

S.  The  angle  which  the  magnetic  meridian  makes  with  the 
meridian  of  the  place  is  called  the  variation  of  the  needle,  or  of 
the  compass.  This  is  different  in  different  parts  of  the  world, 
and  varies  from  time  to  time. 

P.  Line  of  no-variation  ? 

S.  An  irregular  curve-line',  surrounding  the  earth  from  north 
to  south,  and  passing  through  all  the  places  where  the  magnetic 
meridian  coincides  with  the  true,  is  called  the  line  of  no-varia- 
tion. On  the  east  of  this  line  the  variation  is  west,  and  on  the 
west  of  it  it  is  east.  This  line  at  present  passes  through  the 
western  parts  of  Pennsylvania. 

P.  Diurnal  variation  ? 

S.  In  the  fore  part  of  the  day,  especially  in  the  summer  sea- 
son, and  in  warm  climates,  the  magnetic  needle  verges  a  little 
towards  the  west,  and  returns  to  its  former  situation  in  the  after- 
noon ;  and  this  is  called  the  diurnal  variation  of  the  needle.  It 
is,  however,  very  inconsiderable,  seldom  exceeding  a  quarter  of 
a  degree. 

P.  Dip  of  the  needle  ? 

S.  A  magnetic  needle,  balanced  horizontally  before  it  is  ren- 
dered magnetic,  will,  after  this,  lose  its  equilibrium,  the  north 
end  in  the  northern  hemisphere,  when  it  can  move  freely,  dipping 
below  the  horizon,  and  vice  versa.     The  quantity  of  this  dip  en- 

VOL.  IV.  3  Z 


5  SO  APPENDIX. 

creases  with  the  latitude,  though  according  to  some  ratio  not  yet 
sufficiently  ascertained.  In  Philadelphia,  the  dip  is  at  present 
about  70°. 

P.  Magnetic  equator? 

S.  An  irregular  curve-line,  surrounding  the  earth  from  east  to 
west,  and  passing  through  all  those  places  where  the  needle  has 
no  dip. 

P.  Poles,  and  equator  of  a  magnet? 

S.  The  extremities  of  a  magnet  which,  when  it  is  suffered  to 
move  freely,  point  towards  the  north  and  south,  are  called  the 
north  and  south  poles  respectively  ;  and  a  section  of  the  magnet 
equidistant  from  the  two  poles  is  called  its  equator. 

P.  Magnetic  attraction  and  repulsion  ? 

S.  If  one  end  of  a  magnet  be  brought  near  to  a  piece  of  iron, 
or  any  other  ferruginous  body,  a  mutual  attraction  will  take  place; 
and,  if  suffered  to  move  freely,  they  will  approach  each  other  with 
increasing  velocity,  and  finally  adhere  together:  but  if  the  equa- 
tor of  the  magnet  be  thus  presented,  no  such  attraction  will  be 
perceived.  While  a  piece  of  iron,  Sec.  is  in  contact  with  a  mag- 
net, or  within  the  sphere  of  its  attraction,  it  has  itself  all  the  pro- 
perties of  a  magnet ;  but  when  removed  without  the  sphere  of 
magnetic  attraction,  it  will,  if  iron  or  soft  steel,  lose  its  magnetic 
virtue,  but  if  hard  or  tempered  steel,  it  will  retain  its  virtue,  and 
thus  become  a  permanent  magnet.  Permanent  magnetism  may, 
however,  be  more  fully  communicated  to  a  piece  of  hard  steel,  by 
placing  it,  for  some  time,  between  the  contrary  poles  of  two  strong 
magnets  ;  or  by  reiterated  friction  properly  applied.  The  like  poles 
of  two  magnets  will  repel  each  other,  but  their  contrary  poles 
will  mutually  attract. 

P.  Does  a  magnet  lose  any  of  its  virtue  by  communicating 
magnetism  ? 

S.  No,  it  is  rather  improved  thereby. 

P.  Does  all  pieces  of  iron  naturally  possess  the  magnetic  vir- 
tue? 

S.  An  oblong  piece  of  iron,  in,  or  nearly  in,  the  direction  of 
the  dipping  needle,  will,  though  most  frequently  in  a  small  de- 
gree, possess  magnetic  virtue  ;  the  lower  extremity,  in  the  nor- 
thern hemisphere,  being  the  north  pole,  and  the  upper  extremity 
the  south  pole.  And  if  the  piece  of  iron  be  of  any  considerable 
length,  as  a  lightning-rod,  a  stove-pipe,  or  the  like,  it  will  be  di- 
vided into  a  number  of  magnets,  though  of  different  lengths,  and 
increasing  upwards.  But  an  oblong  piece  of  iron,  in  the  direc- 
tion of  the  magnetic  equator,  or  at  right  angles  to  the  dipping 
needle,  will  seldom  exhibit  any  signs  of  magnetism. 

P.  On  what  hypothesis  may  all  the  principal  phenomena  of 
magnetism  be  explained  ? 

S.  On  that  of  M.  ^Epinus,  viz. 

1.  That  there  exists  in  all  magnetic  bodies  a  substance  which 
may  be  called  the  magnetic  fluid  ;  the  particles  of  which  strongly 
repel  each  other,  with  a  force  decreasing  as  the  square  of  the  dis- 
tance increases.  , 


APPENDIX.  531 

2.  That,  between  the  particles  of  iron,  in  any  of  its  states,  and 
those  of  this  magnetic  fluid,  there  exists  a  mutual  attraction, 
whfch  decreases  according  to  the  same  law. 

3.  That  the  particles  of  iron  mutually  repel  each  other,  ac- 
cording to  the  same  law  ;  though  this  repulsion  does  not  coun- 
teract the  aggregate  attraction  of  the  particles,  on  which  its  tex- 
ture depends. 

4.  That  the  magnetic  fluid  moves  without  any  considerable 
obstruction,  through  the  pores  of  iron  or  soft  steel ;  but  is  more 
and  more  obstructed  in  its  motion,  as  the  temper  is  harder;  and 
in  hard  tempered  steel,  as  well  as  in  the  ores  of  iron,  it  is  moved 
with  the  greatest  difficulty. 

5.  In  a  piece  of  iron  exhibiting  no  signs  of  magnetism,  this 
fluid  is  conceived  to  be  equally  diffused,  or  in  its  natural  state. 

6.  In  a  magnet,  this  fluid  is  conceived  to  be  redundant  in  one 
extremity  (most  probably  the  northern)  and  deficient  in  the  other 
extremity  ;  consequently,  in  the  middle  section,  or  equator  of  the 
magnet,  the  magnetic  fluid  will  be  in  its  natural  state. 

7.  The  earth  probably  contains  a  great  magnet,  continually 
acting,  by  its  attractive  and  repulsive  powers,  on  all  bodies  in 
which  this  fluid  is  contained  ;  this  great  central  magnet  having, 
its  deficient  extremity  towards  the  north. 

P.  Apply  this  hypothesis  to  the  explanation  of  a  few  of  the 
magnetic  phenomena. 

S.  1.  When  the  north  or  redundant  end  of  a  magnet  is  pre- 
sented to  one  extremity  of  a  piece  of  iron  or  steel,  the  magnetic 
fluid  contained  therein  wiil  be  repelled,  by  the  redundant  mag- 
netic fluid  in  the  magnet,  to  the  opposite  extremity  ;  and  the 
piece  of  metal,  if  tempered  steel,  will  become  a  permanent  mag- 
net, the  fluid  not  readily  returning  through  the  pores  of  har- 
dened steel.  But,  for  the  opposite  reason,  if  the  piece  of  metal 
be  iron  or  soft  steel,  the  fluid  will  readily  return  on  withdrawing 
the  magnet,  and  no  permanent  signs  of  magnetism  will  conti- 
nue. 

2.  The  contrary  poles  of  two  magnets  will  attract  each  other, 
from  the  mutual  attraction  between  the  redundant  magnetic  fluid 
in  the  one,  and  the  deficient  particles  of  metal  in  the  other. 

3.  The  like  poles  of  two  magnets  will  repel  each  other,  from, 
the  mutual  repulsion  between  the  particles  of  the  fluid  in  the 
two  redundant  ends,  or  that  of  the  metal  in  the  two  deficient 
ends. 

4.  The  direction  of  the  magnetic  needle,  both  with  respect  to 
its  azimuth  and  dip,  is  occasioned  by  the  mutual  attraction  be- 
tween the  deficient  extremity  of  the  great  central  magnet  and 
the  redundant  extremity  of  the  needle,  and  vice  versa. 

5.  Heating  a  magnet  will  impair  or  destroy  its  magnetic  vir- 
tue ;  as  the  fluid  will  then  find  less  difficulty  in  moving  from  the 
redundant  to  the  deficient  end. 

6.  The  diurnal  variation  is  probably  occasioned  by  the  heat- 
ing of  the  eastern  hemisphere  in  the  forenoon,  which,  weakening 


532  APPENDIX. 


its  magnetic  attraction,  will  suffer  the  north  end  of  the  needle 
to  verge  towards  the  west,  and  vice  versa. 

7.  The  temporary  magnetism  of  a  piece  of  iron,  while  placed 
vertical,  or  nearly  in  the  direction  of  the  dipping  needle,  may  be 
owing  to  the  mutual  attraction  between  the  magnetic  fluid  in  the 
piece  of  iron,  and  the  deficient  or  negative  extremity  of  the  great 
central  magnet. 

8.  A  piece  of  iron  will  acquire  a  more  powerful  temporary 
magnetism  while  in  contact  with  one  extremity  of  a  magnet  than 
a  piece  of  tempered  steel,  because  the  magnetic  fluid  will  move 
more  readily  through  the  pores  of  the  former  than  through  those 
of  the  latter. 

Of  Electricity, 

P.  What  is  electricity  ? 

S.  That  species  of  attraction,  with  other  phenomena,  which 
was  first  discovered  in  amber,  electron,  from  which  the  term 
electricity  is  derived,  but  which  is  now  known  to  belong  to  many 
other  bodies  ;  friction  being  generally  necessary  to  produce  elec- 
trical phenomena. 

P.  On  what  hypothesis  may  all  the  principal  phenomena  of 
electricity  be  satisfactorily  accounted  for? 

S.  On  that  of  Dr.  Franklin,  as  more  fully  explained  by  M. 
JEpinus,  viz. 

1.  All  bodies  are  possessed  of  a  certain  elastic  fluid,  sui  gene- 
ris, called  the  electric  fluid,  the  particles  of  which,  like  those  of 
every  other  elastic  fluid,  repel  each  other,  with  a  power  decreas- 
ing as  the  square  of  the  distance  increases. 

2.  Between  the  particles  of  this  fluid,  and  those  of  some  ingre- 
dients in  all  other  bodies,  there  exists  a  mutual  attraction,  which 
decreases  according  to  the  same  law. 

3.  Through  the  pores  of  some  bodies  the  electric  fluid  passes 
with  facility,  or  meets  with  little  obstruction  ;  they  are  hence 
termed  conductors  of  electricity.  But  through  the  pores  of  other 
bodies  it  moves  with  difficulty,  or  is  wholly  obstructed  ;  and  these 
are  hence  termed  van-conductors  of  electricity  ;  though  among  bo- 
dies there  is  a  gradation  from  the  most  perfect  conductors  to  the 
most  perfect  non-conductors. 

4.  By  sundry  operations,  both  of  nature  and  of  art,  the  equi- 
librium of  the  electric  fluid  is  destroyed,  becoming  redundant  in 
one  body  or  part  of  a  body,  and  deficient  in  another.  Where 
the  electric  fluid  is  redundant,  the  body  is  said  to  be  in  a  plus 
or  fiositive  state  of  electricity  ;  where  it  is  deficient,  the  body  is 
said  to  be  in  a  minus  or  negative  state  ;  and  where  it  is  neither 
redundant  nor  deficient,  tl\e  body  is  said  to  be  in  its  natural 
state. 

5.  Friction  weakens  the  attraction  between  the  particles  of  the 
body  rubbed  and  those  of  the  electric  fluid  it  contains  ;  hence, 
when  two  bodies  are  rubbed  together,  and  one  of  them  more  af- 


APPENDIX.  533 

fected  by  the  friction  than  the  other,  the  latter  having  its  attrac- 
tion for  the  electric  fluid  less  weakened  than  the  former,  will  at- 
tract a  portion  of  its  electricity  ;  and  if  one  of  the  bodies  be  a  non- 
conductor, they  will  be  found  in  different  states  of  electricity  ;  but 
if  they  be  both  conductors,  the  equilibrium  will  be  instantly  res- 
tored as  soon  as  disturbed.  On  this  account  all  non-conductors 
are  called  electrics,  and  all  conductors,  non-electrics  ;  being 
more  or  less  perfect  in  the  latter  respect,  according  as  they  are 
so  in  the  former  respect. 

P.  What  bodies,  or  media,  are  found  to  be  non-conductors,  or 
electrics  ;    and  what,  conductors,  or  non-electrics  ? 

S.  1.  All  vitreous,  and  resinous  substances,  and  gems;  all 
animal  excrescences,  as  hair,  feathers,  horn,  silk,  wool,  &c. 
all  vegetable  substances  when  deprived  of  moisture,  as  dry- 
paper,  baked  wood,  Sec.  dry  air,  and  some  non-electrics,  as  ice, 
in  a  very  low  degree  of  temperature,  are  electrics  or  non-con- 
ductors. 

2.  All  metals,  water,  or  bodies  abounding  with  moisture, 
animal  fluids,  charcoal,  black-lead,  a  vacuum,  flame,  and  elec- 
trics in  a  high  degree  of  temperature,  as  melted  glass,  hot  air, 
&c.  are  non-electrics  or  conductors. 

P.  Please  to  describe  and  explain  some  of  the  most  common 
phenomena  of  electricity,  on  the  hypothesis  just  laid  down. 
For  instance, — exciting  electricity  by  friction  ? 

S.  If  a  piece  of  smooth  glass,  as  a  tube,  a  globe,  or  the  like, 
be  rubbed  with  the  dry  hand,  or,  what  is  much  better,  with  a 
piece  of  leather  or  oiled  silk,  smeared  over  with  an  amalgam  of 
zinc  and  mercury  with  the  addition  of  a  little  tallow,  then,  as 
the  smooth  glass  will  be  less  affected  by  the  friction  than  the 
rough  rubber,  it  will  attract  from  this  a  part  of  its  electricity  ; 
and  if  the  rubber  be  insulated,  that  is,  cut  off  by  a  non-conduc- 
tor from  any  communication  with  the  earth  or  any  other  con- 
ductor, it  will  be  electrified  negatively,  and  the  glass,  positively. 
But  if  a  piece  of  rough  glass,  silk,  sulphur,  sealing-wax,  or  any 
other  resinous  substance,  be  rubbed  with  a  piece  of  fine  flannel, 
or  rather  of  cat-skin,  then,  the  finely-polished  pile  of  the  rubber 
being  less  affected  by  the  friction  than  the  rough  glass,  &c.  the 
former  will  be  electrified  positively,  and  the  latter,  negatively. 

P.  Electrical  attraction  and  repulsion  ? 

S.  1,  If  a  positively-excited  electric,  as  a  smooth  glass-tube,  be 
brought  near  to  one  extremity  of  a  conductor,  as  a  gunbarrel, 
then,  the  redundant  fluid  in  the  excited  electric  will,  by  its  re- 
pulsive force,  drive  the  fluid  in  the  conductor  to  the  opposite 
extremity  ;  and,  for  a  like  reason,  if  the  electric  be  excited  ne- 
gatively, it  will,  from  its  attractive  force  on  the  fluid  contained 
in  the  conductor,  draw  it  towards  itself.  Hence,  an  excited 
electric  will  produce  a  contrary  state  of  electricity  in  the  part  of 
a  conductor  nearest  to  it. 

2.  If  an  excited  electric  be  brought  near,  or  in  contact  with  a 
conductor,  and  passed  over  it,  the  electric  fluid  will,  from  its  pre- 
valent attraction  to  the  body  which  has  the  least  of  it,  be  com- 


534  APPENDIX. 

municated  from  one  to  the  other  ;  and  the  conductor,  if  insu- 
lated, that  is,  suspended  by  silk  lines  or  supported  by  glass  pil- 
lars, or  the  like,  will  become  electrified  in  the  same  state  with 
the  excited  electric.  For  when  the  excited  electric  is  positive, 
the  electric  fluid  will  pass  from  it  to  the  conductor  ;  but  when 
negative,  it  will  pass  to  it  from  the  conductor. 

3.  If  two  light  bodies,  as  cork  or  pith  balls,  suspended  by  silk 
threads,  and  near,  or  in  contact  with  each  other,  be  electrified 
by  an  excited  electric,  they  will  repel  each  other.  For  if  elec- 
trified positively,  the  redundant  electricity  forming  an  electric 
atmosphere  round  the  balls,  will  be  doubly  dense  between  them, 
and,  therefore,  from  the  repulsion  of  its  particles,  the  balls  will 
be  separated.  If  electrified  negatively,  the  negative  atmosphere 
surrounding  the  balls,  (that  is,  the  air  deprived  of  its  natural 
electricity)  will  be  doubly  rare  between  them,  and,  thertfore, 
from  the  prevalent  attraction  outwards,  between  the  negative 
balls  and  the  natural  electricity  of  the  surrounding  air,  they  will 
still  be  separated. 

4.  If  one  be  electrified  and  the  other  not,  or  if  one  be  positive 
and  the  other  negative,  then  they  will  attract  each  other.  For, 
in  both  cases,  as  one  body  will  have  a  greater  proportion  of  the 
electric  fluid  than  the  other,  a  mutual  attraction  between  this 
and  the  other  body  will  take  place.  When  they  come  into  con- 
tact, the  electric  fluid  will  be  equally  distributed  between  them  ; 
and  if,  in  this  case,  they  have  either  more  or  less  than  their  na- 
tural quantity,  they  will  again  repel  each  other.     Hence, 

5.  Any  light  body,  as  a  feather,  a  tuft  of  cotton,  or  the  like, 
will  be  alternately  attracted  and  repelled,  and  thus  move  back- 
wards and  forwards  between  two  bodies  in  different  states  of 
electricity. 

P.  The  charging  and  discharging  of  the  Leyden  phial  ? 

S.  The  Leyden  phial,  so  called  from  its  properties  being  first 
discovered  in  the  city  of  Leyden,  is  one  lined  on  the  inside,  and 
coated  on  the  outside,  within  about  two  inches  of  the  mouth, 
with  tin-foil,  or  any  other  conducting  substance.  The  inside  com- 
municating, by  means  of  wire  or  other  conductors,  with  an  ex- 
cited electric,  as  a  glass  globe,  plate,  or  the  like,  and  the  out- 
side with  the  earth,  or  with  the  rubber  ;  while  the  excitation 
goes  on,  the  electric  fluid  passes  from  the  electric,  if  positive, 
to  the  lining  of  the  phial,  where,  by  its  repulsive  force,  operat- 
ing through  the  glass,  it  will  expel  the  electric  fluid  from  the 
outside  coating  into  the  earth  or  rubber  ;  the  inside  lining  will 
thus  become  eletrified  positively,  and  the  outside  coating  nega- 
tively ;  and  the  glass  phial,  or  intermediate  electric,  is  then  said 
to  be  charged.  The  thinner,  therefore,  the  intermediate  elec- 
tric, the  higher  may  it  be  charged.  If  the  negative  side  be  now 
connected  with  the  excited  electric,  and  the  positive  side  with  the 
rubber,  the  phial  will  be  gradually  discharged  ;  and  if  the  ex- 
citation be  continued,  it  will  be  again  charged,  the  sides  assum- 
ing contrary  states.  If  a  communication,  of  any  good  conduc- 
tor, be  formed  between  the  opposite  sides  of  a  charged  electric, 


APPENDIX.  535 

the  redundant  fluid  in  the  positive  side  will  rush  with  violence 
to  the  negative  side,  and  that  even  before  the  communication  is 
complete,  attended  in  its  passage  through  the  air  with  an  elec- 
tric spark  and  explosion,  proportioned  to  the  quantity  of  coated 
surface,  and  the  intensity  of  the  charge  ;  and  after  this  the 
coated  electric  will  exhibit  little  or  no  signs  of  electricity. 

P.  The  electric  shock  ? 

S.  If  any  part  of  a  living  body  form  the  circuit  of  discharge 
in  the  Leyden  experiment,  a  sudden  spasmodic  shock  and 
painful  sensation  will  be  felt,  greater  or  less,  according  to  the 
quantity  and  intensity  of  the  charged  electric. 

P*  The  phenomena  of  the  electric  spark  a»d  explosion  ? 

S.  The  electric  fluid  while  passing,  in  any  considerable  quan- 
tity, from  one  body  to  another,  through  the  air,  exhibits  the 
appearance  of  a  spark  of  fire,  accompanied  with  an  audible  ex- 
plosion; the  spark,  moving  with  prodigious  velocity,  and,  when 
the  distance  is  considerable,  in  a  zig-zag  direction.  It  will,  in 
favourable  circumstances,  set  combustibles  on  fire  ;  and,  in .  its 
passage  through  bodies  that  are  but  imperfect  conductors^ 
rend  them  to  pieces. 

The  light  and  heat  of  the  electric  spark  may  possibly  be  ow- 
ing to  the  sudden  condensation  of  the  air,  or  other  elastic  me- 
dium through  which  it  passes  ;  and  perhaps  a  chemical  com- 
bination of  latent  light  and  chloric,  furnished  by  the  electric 
fluid  and  the  elastic  medium,  may  take  place  in  the  production 
of  the  visible  flame. 

The  audible  explosion  is  no  doubt  occasioned  by  the  sudden, 
concussion  of  the  air  by  the  electric  spark,  in  its  passage,  and 
its  subsequent  collapse. 

The  zig-zag  direction  arises,  in  part,  from  the  succes- 
sive changes  in  the  figure  of  the  electric  spark  or  ball  of  fluid 
fire  ;  for  when  this  becomes  oblate  in  its  direct  course,  it  will, 
from  its  increased  resistance,  in  this  direction,  glance  off  ob- 
liquely ;  and  thus,  by  successive  changes  of  figure,  as  in  a  bubble 
of  air  moving  through  water,  an  angulated  or  zig-zag  direction 
is  produced.  The  condensation  of  the  air,  in  the  direction  of 
the  spark,  will  no  doubt  contribute  to  this  phenomenon. 

P.  The  influence  of  metallic  /wints  in  attracting  and  emit- 
ting the  electric  fluid  ? 

S.  A  pointed  conductor  is,  by  experiment,  found,  to  attract 
and  transmit  the  electric  fluid  from  one  body  to  another,  in  dif- 
ferent states  of  electricity,  at  a  greater  distance,  and  in  a  more 
gradual  and  silent  manner,  than  a  conductor  in  any  other  form. 
This  may  be  accounted  for  on  the  supposition,  that  a  certain 
subtile  elastic  fluid,  (the  at  her  of  Sir  Isaac  Newton)  surrounds 
all  bodies,  and  particles  of  matter,  preventing  them  from  com- 
ing into  actual  contact.  This  xtherial  atmosphere  will,  there- 
for^, be  the  most  dense  on  a  concave  surface,  less  on  a  plane 
surface,  still  less  on  a  convex,  and  least  of  all,  or  in  fact  insen- 
sible, at  a  point.  It  follows,  that  the  resistance  from  this 
subtile  fluid,  to  the  entrance  or  escape  of  the  electric  fluid  will  be 


536  APPENDIX. 

less  when  the  conductor  terminates  in  a  point,  than  when  in  any 
other  form. 

P.  The  electrical  aura,  or  sensible  blast,  from  an  electified 
point? 

S.  While  the  electric  fluid  is  passing  either  to  or  from  a 
pointed  conductor,  it  is  accompanied  with  a  sensible  cold  blast, 
frequently  sufficient  to  extinguish  a  taper  or  small  candle  :  For, 
the  electric  fluid  in  passing  through  the  air  produces  a  partial 
vacuum,  in  form  of  a  cone  inverted  with  respect  to  the  conical 
conductor;  the  air,  in  consequence,  will  rush  in  from  behind, 
to  supply  the  vacuum,  and  thus  the  sensible  current  will  be 
produced. 

P.  Are  there  any  phenomena  in  nature,  that  may  be  consi- 
dered as  electrical  ? 

S.  Thunder  and  lightning  are  now  well  known  to  be  electri- 
cal phenomena. 

P.  Please  to  explain  these,  upon  the  general  hypothesis. 

S.  The  clouds,  by  some  operation  of  nature,  not  yet,  perhaps, 
satisfactorily  understood,  are  frequently  put  into  a  high  state  of 
electricity  ;  sometimes  positive,  sometimes  negative. 

When  an  electrified  cloud  approaches  near  to  the  earth, 
which,  from  their  mutual  attraction,  it  will  have  a  tendency  to 
do,  the  part  immediately  under  the  cloud  will,  from  the  attrac- 
tion or  repulsion  of  the  electric  fluid,  be  put  into  the  opposite 
state  of  electricity,  the  stratum  of  air  between  the  earth  and 
cloud,  thus  becoming  a  charged  electric  ;  and,  in  favourable  cir- 
cumstances, a  discharge  will  tyke  place  ;  the  redundant  fluid  in 
the  one,  rushing  to  the  deficient  or  negative  matter  of  the  other, 
and  thus  exhibiting  the  awful  phenomena  of  thunder  and  light- 
ning. 

P.  How  may  buildings,  ships,  or  other  objects,  be  preserved 
from  the  dreadful  effects  of  lightning  ? 

S.  By  means  of  a  metallic  conductor,  termed  a  lightning-rod  ; 
extending  a  few  feet  above  the  highest  part  of  the  building  or 
other  object,  and  reaching  to  a  moderate  depth  below  the  sur- 
face of  the  earth, or  into  the  water:  for  if  thus  furnished, it  will 
seldom,  if  ever,  be  struck  with  lightning. 

P.  What  are  the  best  materials  and  construction  of  a  light- 
ning-rod to  defend  a  building  ? 

S.  The  body  of  the  rod  may  be  of  iron,  say  from  \  to  \  of  an 
inch  in  thickness — the  thicker  kind  of  iron-wire  or  of  nail-rod 
will  answer  the  purpose  very  well. 

The  top  of  the  rod,  or  part  rising  above  the  building,  which 
may,  in  general,  be  about  six  feet  long,  should  be  made  a  little 
thicker  than  the  rest.  The  part  under  ground  should  also  be 
made  thicker,  and  descend  as  far  below  the  surface  as  practica- 
ble, so  as  to  reach  a  permanently-moist  earth. 

The  upper  extremity  of  the  conductor  may  terminate  in  a  hol- 
low cone  of  thin  sheet  gold,  or  of  copper,  a  few  inches  long,  fill- 
ed with  apiece  of  good  black-lead,  (which  may  be  taken  from  a 
pencil)  cut  to  a  fine  point,  so  as  to  reach  to  the  vertex  of  the 


APPENDIX.  537 

cone,  and  secured  in  its  place  by  a  paste  made  of  calcined  plais- 
ter  of  Paris  and  black-lead  dust,  rammed  in  round  the  piece  of 
black-lead.  And  round  the  lower  extremity  of  the  rod,  or  the 
part  under  ground,  there  may  be  thrown  a  few  bushels  of  charcoal. 
By  these  means,  the  conductor  will  be  rendered  much  more  per- 
fect: for,  the  black-lead  at  the  upper  extremity,  being  nearly  as 
good  a  conductor  as  metal,  and  yet,  in  a  manner,  infusible^ 
will  effectually  secure  it  against  the  frequent  accident  of  being 
melted  off  by  a  stroke  of  lightning  ;  and  the  charcoal  round  the 
lower  extremity,  from  its  conducting  power,  (being  little  inferior 
to  that  of  metal)  the  angular  or  pointed  figure  of  its  parts,  its 
quality  of  absorbing  moisture,  and  its  indestructibility  by  any 
agent,  except  fire,  will  afford  a  permanent,  copious,  and  effectual 
means  for  the  free  passage  of  the  electric  fluid,  between  the 
conductor  and  the  surrounding  earth. 

P.  What  buildings,  or  parts  of  buildings,  are  most  liable  to  be 
struck  with  lightning,  and  should  therefore  be  secured  by  con- 
ductors ? 

S.  Barns,  after  the  in-gathering  of  the  harvest,  are  observed  to 
be  more  frequently  struck  with  lightning  than  any  other  build- 
ings. This  is  probably  owing  to  the  ascent  of  vapour,  generated 
by  a  slight  fermentation  taking  place  in  the  moist  contents  of  the 
barn,  which  favours  the  descent  of  the  lightning.  For  a  similar 
reason,  a  chimney  in  which  there  is  a  fire,  is  more  exposed  to 
a  stroke  of  lightning  than  any  other.  Hence,  kitchen-chimnies* 
being  the  only  ones  in  which  fires  are  usually  kept  during  the 
summer,  (the  season  in  which  thunder  most  prevails)  should, 
when  other  circumstances  will  admit,  be  furnished  with  light- 
ning-rods. 

As  thunder-storms  generally  come  from  the  westward,  the 
lightning-rod,  to  secure  the  building,  should,  when  other  cir- 
cumstances are  equal,  be  placed  on  the  most  western  chim- 
ney. 

When  the  house  is  furnished  with  a  metal  gutter,  or  metal 
spout,  to  carry  off  the  rain,  this,  so  far  as  it  goes,  may  be  made 
a  part  of  the  lightning-rod,  or  metallic  conductor. 


Of  Astronomy* 

P.  What  is  astronomy  ? 

S.  That  science  which  describes  and  explains  the  various  phe- 
nomena of  the  heavenly  bodies. 

P.  Give  a  short  description  of  the  general  appearances  of  the 
heavenly  bodies. 

S.  The  heavenly  bodies  appear  to  consist  of  a  vast  number  of 
luminaries,  of  different  magnitudes  and  degrees  of  brightness. 

The  swn,  whose  light  occasions  our  day,  far  surpasses  in  splen- 
dour all  the  other  luminaries.  The  moon,  which  is  visible  chiefly, 
and  shines  only,  during  the  night,  emits  incomparably  less  light 

VOL.  IV.  4  A 


538  APPENDIX. 

than  the  sun,  yet  much  more  than  all  the  other  luminaries  toge- 
ther. The  sun  and  moon  are  nearly  of  the  same  apparent  mag- 
nitude ;  and  of  the  other  luminaries  a  few  only,  called  planet*, 
have  any  apparent  magnitude;  all  the  ixst,  called  Jixed  t,tarsf 
appearing  only  as  mere  luminous  points,  though  differing  in 
brightness. 

The  sun  always  appears  a  complete  luminous  circle  ;  hut  the 
moon  is  continually  changing  her  phases',  from  that  of  a  full  cir« 
cle  to  that  of  the  smallest  crescent,  accorcing  to  her  greattr  or 
less  angular  distance  from  the  sun.  Thus  the  moon,  and,  indeed, 
the  other  planets,  (which,  when  viewed  through  a  tele  scope,  exhi- 
bit similar  phases)  all  appear  to  derive  their  light  from  the  sun. 

These  luminaries  all  appear  to  he  in  the  concave  surface  of  a 
great  sphere  surrounding  the  earth  ;  and  exhibit  the  same  phe- 
nomena as  if  the  sphere  containing  them  turned  round  on  its 
axis  daily,  from  east  to  west. 

The  moon  is  continually  changing  her  position,  with  respect 
to  the  sun,  at  the  rate  of  about  12  1-2  degrees,  from  west  to  east, 
every  day  ;  and  thus  returns  to  her  former  position,  and  under- 
goes all  her  changes  of  phases,  in  about  29  1-2  days,  termed  a 
lunar  month. 

The  sun  is  also  continually  changing  his  position,  w  ith  respect 
to  the  fixed  stars,  at  the  rate  of  about  a  degree,  from  west  to 
east,  every  day  ;  and  thus  goes  through  a  whole  circle  in  the 
heavens,  in  about  36j   1-4  days,  termed  a  solar  year. 

The  planets  are  likewise  continually  changing  their  places, 
and  return  to  any  former  position,  in  different  periods.  In  their 
revolutions,  however,  through  the  starry  heavens,  they  appear, 
as  seen  from  the  earth,  to  be  sometimes  progressive,  sometimes 
stationary,  and  sometimes  retrograde. 

Two  of  these  planets,  called  Mercury  and  Venus,  are  never  seen 
in  opposition  to  the  sun,  nor  at  any  very  great  angular  distance 
from  him  ;  but  all  the  other  plant ts  are  occasionally  seen  in  op- 
position, and  in  ail  other  positions  with  respect  to  the  sun.  '1  he 
stars,  however,  newer  appear  sensibly  to  change  their  positions 
with  respect  to  each  other,  and  are  on  this  account  called  Jixed 
stars. 

Neither  the  sun,  moon,  nor  planets,  appear  constantly  to  de- 
scribe the  same  diurnal  paths  in  the  heavens;  their  places  of 
rising,  setting,  and  meridian  altitudes,  continually  changing, 
and  going  through  all  their  varieties  of  change  in  this  respect, 
in  different  periods  ;  but  the  fixed  stars  undergo  no  such  change 
in  their  diurnal  paths. 

P.  On  what  hypothesis  may  these  phenomena  of  the  heavenly 
bodies  be  explained  ? 

S.  These,  and  all  other  phenomena  of  the  heavenly  bodies,  may 
he  satisfactorily  explained,  on  the  Copernican  or  Newtonian  hy- 
pothesis or  system  of  the  universe. 

P.  Please  to  give  a  brief  view  of  this  system. 

S.  1.  Round  the  sun,  as  a  common  centre,  there  revolves,  in 
different  orbits,  and  in  different  periodical  times,  a  number  of 


APPENDIX.  539 

opaque  bodies,  nearly  spherical,  resembling  the  earth,  (which  is 
one  of  them)  called  primary  planets. 

2.  Round  some  of  the  primary  planets  there  revolve  also  in 
different  orbits,  and  in  different  periodical  times,  other  smaller 
and  similar  bodies   called  moo?is,  satellites,  or  secondary  filantts. 

3.  All  the  orbits  of  the  primary  planets  round  the  sun,  and  of 
the  secondaries  round  their  primaries,  are  nearly  circular,  though, 
in  reality,  more  or  less  ellifnical ;  the  sun  and  primary  planets 
respectively  being  placed  in  one  focus  of  the  elliptical  orbit. 

4.  The  planets,  both  primary  and  secondary,  revolve,  in  their 
respective  orbits,  in  the  same  direction,  namely,  from  west  to  east; 
and  all,  except  in  one  instance,  nearly  in  the  same  plane. 

5.  Besides  the  periodical  revolution  of  the  planets  round  their 
respective  centres  of  motion,  some  of  thtm  it  is  certain,  and  all 
of  them  it  is  probable,  perform  a  rotation  round  their  oiim  axes,  in 
different  times,  from  west  to  east ;  tht  ir  axes  of  rotation  making 
different  angles  with  their  respective  orbits. 

6.  Besides  the  primary  and  secondary  planets,  there  is  another 
order  of  bodies,  called  comets,  that  revolve  round  the  sun  in  very 
excentric  elliptical  orbits,  in  very  different  planes,  and  very  dif- 
ferent directions.  Few  of  the  periodical  times  of  these  bodies  are 
yet  accurately  ascertained;  as  they  are  never  visible,  unless  for 
a  short  time  when  in  or  near  their  perihelion,  (or  nearest  dis- 
tance from  the  sun)  at  which  time  they  generally  appear  with  a 
lucid  or  shining  tail  on  the  side  opposite  to  the  sun. 

7.  All  the  above  bodies  in  the  solar  system,  whether  planets 
or  comets,  in  their  revolutions  round  their  respective  centres,  ob- 
serve this  general  law,  viz.  "  That  the  squares  of  their  periodical 
times  are  directly  proportional  to  the  cubes  of  their  mean  dis- 
tances from  the  centre  of  motion."  It  therefore  follows,  from  the 
doctrine  of  central  forces,  "  that  the  gravity  or  mutual  attrac- 
tion between  any  two  bodies  in  the  system,  must  decrease  as  the 
square  of  the  distance  increases." 

8.  The  sun  is  a  dense,  and  probably  an  opaque  body,  sur- 
rounded with  a  luminous  atmosphere,  which  is  the  source  of 
solar  light  and  heat  to  the  whole  system. 

P.  What  account  do  you  give  of  the  fixed  stars? 

S.  They  are,  most  probably,  all  great  luminaries,  resembling 
the  sun,  and  each  the  centre  of  a  system  of  bodies  revolving 
round  it,  similar  to  those  which  compose  the  solar  system.  '1  hese 
stars  are  probably  at  as  great  a  distance  from  each  other  as  that 
of  the  nearest  of  them  from  the  sun  ;  and  this  distance  is  truly 
immense.  I 

P.  What  can  prevent  these  systems  from  obeying  the  general 
laws  of  gravity,  and  falling  together? 

S.  The  innumerable  systems  composing  the  stupendous  fabric 
of  the  universe,  are,  it  is  more  than  probable,  themselves  also  in 
motion  round  some  common  centre,  and  thus  prevented  from 
approaching  each  other;  which,  by  their  mutual  attraction,  they 
must  otherwise  do.  This  hypothesis  is  not  only  analogous  to  the 


540  APPENDIX. 

general  laws  of  nature  in  central  motions,  but  is  now,  in  some 
measure,  verified  by  astronomical  observations. 

P.  Explain  a  few  of  the  most  common  astronomical  terms 
relating  to  the  sphere.     For  instance — 

What  is  meant  by  a  great  circle  of  the  sphere  ? 

S.  That  circle,  whose  plane  passes  through  its  centre. 

P.  The  poles  of  a  great  circle  ? 

S.  Two  opposite  points  on  the  surface  of  the  sphere  equidis- 
tant from  every  point  in  the  circumference  of  the  circle. 

P.  The  axis  of  a  sphere  ? 

S.  That  imaginary  line  passing  through  its  centre,  round 
which  it  performs  its  diurnal  rotation. 

P.  The  equator? 

S.  That  circle  which  is  at  right  angles  to  the  axis  of  rotation. 

P.  The  eclifitic  ? 

S.  That  great  circle  in  the  plane  of  which  the  earth  performs 
its  annual  revolution'round  the  sun. 

P.  A  meridian  ? 

S.  Any  great  circle  passing  through  the  poles  of  the  equator. 

P.  The  horizon? 

S.  That  great  circle  which,  extended  to  the  heavens,  is  the 
boundary  of  our  vision.  The  poles  of  this  are  called  the  zenith 
and  nadir,  the  one  above,  and  the  other  below,  the  horizon. 

P.  Azimuth  circle  ? 

S.  Any  great  circle  passing  through  the  poles  of  the  horizon ; 
that  at  right  angles  to  the  meridian  being  called  the  prime -vertical, 

P.  The  signs  of  the  ecliptic,  or  of  the  zodiac  ? 

S.  These  are  twelve  constellations  through  which  the  plane 
of  the  ecliptic  passes  ;  they  are  named  Aries  T,  Taurus  c»,  Ge- 
mini n,  Cancer  25,  Leo  SI,  Virgo  i»JJ,  Libra  =£=,  Scorpio  tit,  Sagi- 
tarius  ti  Capricornus  VJ,  Aquarius  ££,  Pisces  X.  The  begin- 
nings of  Aries  and  of  Libra,  being  those  points  in  which  the 
ecliptic  crosses  the  equator,  are  called  the  vernal,  and  autumnal, 
equinoxial  points,  respectively  ;  and  the  beginnings  of  Cancer 
and  of  Capricorn,  being  those  points  farthest  distant  from  the 
equator,  are  respectively  called  the  northern  and  southern  solstitial 
points. 

P.   Tropics  and  polar  circles  ? 

S.  The  tropics  are  circles  parallel  to  the  equator,  and  touching 
the  ecliptic  in  the  extreme  points,  Cancer  and  Capricorn ;  from 
which  they  are  respectively  named.  The  polar  circles  are  also 
parallel  to  the  equator,  and  at  the  same  distance  from  the  poles 
that  the  tropics  are  from  the  equator,  and  are  denominated  the 
north  and  south  polar  circles  respectively. 

P.  Zones  ? 

S.  These  are  divisions  of  the  surface  of  the  earth  (or  any 
other  planet)  by  the  tropics  and  polar  circles.  They  are  five  in 
number,  viz.  the  torrid  zone,  bounded  by  the  two  tropics  ;  the 
two  frigid  zones,  lying  round  the  poles,  and  bounded  by  their 
respective  polar  circles ;  and  the  two  temperate  zones,  lying  be- 
tween the  torrid  and  frigid. 


APPENDIX.  541 

P.  Latitude  ? 

S.  Latitude  of  a  place  on  the  surface  of  a  planet,  is  its  dis- 
tance, in  degrees  and  parts,  from  the  equator,  measured  on  the 
meridian  of  the  place ;  and  latitude  of  a  star,  which  is  fre- 
quently taken  in  a  general  sense  to  signify  any  of  the  celestial 
bodies,  is  its  distance  from  the  ecliptic,  measured  on  a  great  cir- 
cle passing  through  the  star  and  the  poles  of  the  ecliptic. 

P.  Longitude  ? 

S.  Longitude  of  a  place  is  the  distance  of  its  meridian  from 
the  prime  meridian,  or  that  from  which  longitude  is  reckoned; 
and  longitude  of  a  star  is  the  distance,  on  the  ecliptic,  from  the 
vernal  equinoxial  point,  reckoned  from  west  to  east,  or  according 
to  the  order  of  the  signs,  to  the  circle  passing  through  the 
star  and  the  poles  of  the  ecliptic. 

P.  Right  ascension  of  a  star  ? 

S.  The  distance,  on  the  equator,  from  the  vernal  equinoxial 
point,  reckoned  from  west  to  east,  to  the  meridian  passing 
through  the  star. 

P.  Declination  of  a  star  ? 

S.  Its  distance  from  the  equator,  measured  on  the  meridian 
passing  through  the  star. 

P.  Altitude  of  a  star  ? 

S.  Its  elevation  above  the  horizon,  measured  on  an  azimuth 
circle. 

P.  azimuth  of  a  star  ? 

S.  The  distance  of  the  azimuth  circle  passing  through  the 
star,  measured  on  the  horizon,  from  the  meridian. 

P.  Amplitude  of  a  star  ? 

S.  The  distance  of  the  point  where  it  rises  or  sets  from  the 
east  or  west  points  of  the  horizon. 

P.  Please  to  explain  a  few  terms  relative  to  the  planetary  or- 
bits :  For  instance — 

What  is  meant  by  Perihelion  ? 

S.  That  point  of  a  planet's  orbit  in  which  it  is  nearest  to  the 
sun. 

P.  Aphelion? 

S.  That  point  of  a  planet's  orbit  in  which  it  is  farthest  from 
the  sun. 

P.  Perigee,  and  Apogee  ? 

S.  Those  opposite  points  in  the  moon's  orbit  in  which  it  is 
respectively,  nearest  to,  and    farthest  from,  the  earth. 

P.  Apuis  or  Apsides  ? 

S.  The  extremities  of  the  transverse  axis  of  a  planet's  elliptical 
orbit. 

P.  Node*? 

S.  Those  opposite  points  in  a  planet's  orbit,  in  which  it  crosses 
the  ecliptic.  That  point  in  which  it  passes  from  south  to  north, 
being  called  the  ascending  node  or  dragon's  head  (&) ;  and  the 
opposite  point,  the  descending  node  or  dragon's  tail  (£3). 

P.  Excentricity  of  an  elliptical  orbit  ? 

S.  The  distance  between  the  centre  of  the  ellipsis  and  one  of 
its  foci. 


542  APPENDIX. 

P.   Heliocentric  filace  of  a  planet  ? 
S.  Its  place  among  the  stars  as  seen  from  the  sun. 
P.   Geocentric  place  ? 
S.  Its  place  as  seen  from  the  earth. 

P.  Enumerate  the  several  planets  in  the  Solar  system,  in  the 
order  of  their  distance  from  the  sun. 
S.   1.  Mercury,  nearest  to  the  sun  ; 

2.  Venus  ; 

3.  The  earth  with  one  moon  ; 

4.  Mars  ; 

5.  Jupiter,  with  four  moons  ; 

6.  Saturn,  with  a  double  ring  and  seven  moons  ; 

7.  Herschel,  with  six  moons*. 

P.  What  phenomena  result  from  the  order  of  the  planets  in 
the  solar  system  in  their  annual  revolution  round  the  sun  ? 

S.  1.  Mercury  and  Venus,  being  inferior  planets,  that  is,  their 
orbits  being  within  the  earth's  orbit,  will  never  be  seen  in  oppo- 
sition to  the  sun,  nor  at  any  great  angular  distance  from  him; 
the  greatest  elongation  of  Venus,  exceeding,  however,  that 
of  Mercury. 

2.  All  the  other  planets,  having  their  orbits  without  that  of 
the  Earth,  will  occasionally  appear  in  opposition,  and  in  all 
other  positions  with  respect  to  the  sun. 

3.  The  motion  of  the  planets  among  the  stars,  as  seen  from 
the  earth,  will  be  sometimes  direct,  sometimes  retrograde;  and 
sometimes  they  will  appear  stationary.  Namely,  the  inferior 
planets  will  appear  direct,  when  in  the  superior  or  opposite 
parts  of  their  orbits  ;  retrograde,  when  in  the  inferior  or  nearest 
part  ;  and  stationary,  when  at  their  greatest  elongation.  The 
superior  planets  will  appear  direct,  when  the  earth  is  in  the  op- 
posite part  of  its  orbit  with  respect  to  them  ;  retrograde,  when 
the  earth  is  in  the  nearest  part  of  its  orbit  ;  and  stationary,  when 
the  earth,  with  respect  to  them,  is  stationary. 

4.  The  planets  will  change  phages  and  apparent  magnitudes, 
according  to  their  position  with  respect  to  the  sun,  and  distance 
from  the  earth. 

P.  What  phenomena  result  from  the  earth's  orbit  being  ellip- 
tical, and  the  sun  placed  in  one  of  its  foci  ? 

1.  The  sun  will  change  his  appareni  magnitude,  being  great- 
est when  the  earth  is  in  its  perihelion,  or  about  the  miucile  of 
winter,  and  least,  when  in  its  aphelion,  or  about  the  middle  of 
summer. 

2.  The  motion  of  the  earth  in  its  orbit  will  be  unequable,  be- 
ing slower  or  quicker,  according  to  its  greater  or  less  distance 
from  the  sun;  and  this  is  one  source  of  the  equation  Gf  time, 
or  ditference  between  the  times  pointed  out  by  a  well-regulated 
clock,  and  by  the  sun. 


*  For  a  m  >re  particular  view  of  the  solar  system,  see  the  table  at  the 
tnd  of  this  article. 


APPENDIX.  543 

3.  It  follows,  that  the  earth  will  be  considerably  longer  in  de- 
scribing the  aphelion,  then  the  perehelion  part  of  its  orbit  ;  and 
hence  our  summer  half-year  will  exceed  (by  about  eight  days) 
our  winter  half-year. 

4.  From  a  sensible  ratio  between  the  velocity  of  the  earth  in 
its  annual  orbit,  and  that  of  light,  the  stars  will  each  appear  to 
describe  small    ellipses    in   the  heavens,  called  their   aberration, 

P.  What  phenomena  result  from  the  diurnal  rotatory  motion 
of  the  planets  round  their  axes  ? 

5.  1.  From  this  arises  the  spheroidal  figure  of  the  earth,  and  of 
all  the  other  planets  in  which  this  motion  has  been  discovered  ; 
the  matter  of  the  pianet  being  thereby  thrown  out  or  rendered 
more  protuberant  in  the  equatorial,  and  consequently  flatter  in 
the  polar  parts. 

2.  The  diurnal  rotation  of  the  earth  from  west  to  east,  pro- 
duces the  apparent  diurnal  motion  of  the  sun,  and  other  heaven- 
ly bodies,  from  east  to  west  ;  and  hence  the  succession  of  day  and 
night. 

P.  How  has  the  rotatory  motion  of  the  planets  been  discover- 
ed ? 

S.  From  the  regular  motion  of  certain  spots  on  their  surface, 
seen  by  the  aid  of  the  telescope:  That  of  the  moon  is  known 
from  her  always  presenting  the  same  face  to  the  earth — Hence 
the  time  of  her  diurnal  rotation  is  exactly  equal  to  that  of  her 
monthly  revolution  round  the  earth  c  And  there  is  reason  to  be- 
lieve that  it  is  a  general  law — that  all  satellites  or  secondary  pla- 
nets, constantly  present  the  same  side  towards  their  primaries. 

P.  What  phenomena  result  from  the  obliquity  of  the  equator 
to  the  ecliptic  ? 

S.  1 .  The  continual  changes  in  the  apparent  diurnal  path  of  the 
sun,  and  consequently  in  the  length  of  day  and  night;  with  the 
diversity  of  seasons. 

2.  This  also,  as  well  as  the  elliptical  orbit  of  the  earth,  con- 
tributes to  the  difference  between  the  mean  time  per  clock,  and 
the  apparent  solar  time. 

P.  What  phenomena  result  from  the  planets'  orbits,  being  all 
in  different  planes  from  that  of  the  earth,  or  the  ecliptic  ? 

S.  1.  Their  greatest  declinations,  both  north  and  south,  will  ex- 
ceed those  of  the  sun,  by  the  quantity  of  the  angle  which  their 
respective  orbits  makes  with  the  ecliptic. 

2.  From  the  moon's  orbit  crossing  the  ecliptic,  eclipses  of 
the  sun  and  moon  will  be  less  frequent ;  for  an  eclipse  of  the 
sun  can  never  happen,  when  the  moon  is  more  than  17°  from  her 
node  ;  nor  one  of  the  moon,  when  she  is  more  than  12°  ;  called 
their  respective  ecliptic  limits  :  whereas,  if  the  moon's  orbit  coin- 
cided wun  the  ecliptic,  there  would  be  an  eclipse  at  every  full 
and  change. 

P.  What  phenomena  result  from  the  spheroidal  figure  of  Ihe 
earth  ? 

S.  From  the  greater  attraction  of  the  sun,  and  of  the  moon, 
to  the  equatorial  parts  of  the*  earth,  a  great  number  of  seeming 
irregularities  in  the  motions  of  the  heavenly  bodies  are  pro- 
duced. 


544  APPENDIX. 

1.  The  recession,  of  the  equinoxes,  and  firecession  of  the  stars, 
a  slow  motion  by  which  the  equinoxial  points  of  the  ecliptic 
recede,  or  fall  backwards,  about  5Q\"  per  year,  and,  conse- 
quently, the  stars  increase  in  right  ascension,  more  or  less,  accord- 
ing to  their  situation. 

2.  The  nutation  of  the  earth's  axis,  a  slow  motion  in  the  axis 
of  the  earth,  by  which  the  extremities  or  poles  describe  in  about 
18  years,  7  months,  (the  lunar  period  or  revolution  of  the  moon's 
nodes,)  a  small  ellipse  whose  transverse  axis, =  19.  1"  and  con- 
jugate=14.  2" ,  thus  producing  corresponding  apparent  mo- 
tions in  the  stars. 

3.  The  degrees  of  latitude,  as  well  as  gravity,  will  increase 
from  the  equator  to  the  poles. 

P.  How  may  the  magnitude  of  the  earth  be  ascertained  ? 

S.  1.  By  proper  instruments,  let  the  meridian  altitude  of  a 
star  be  accurately  taken  at  two  convenient  places,  on  the  same 
meridian,  at  a  considerable  distance  from  each  other,  about  a 
degree,  for  instance  ;  and  thus  the  arch  in  circular  measure 
will  be  known. 

2.  Actually  measure  this  distance  on  the  surface  of  the  earth  ; 
or,  by  measuring  a  base-line,  and  taking  angles,  calculate  the 
distance  trigonometrically. 

3.  Then,  having  the  length  of  this  given  arch  of  the  meridian, 
we  may,  by  the  rule  of  proportion,  find  that  of  the  whole  cir- 
cumference, and  thence  the  diameter  and  magnitude  of  the  earth. 

P.  From  the  earth's  diameter,  how  would  you  find  its  distance 
from  the  sun  ? 

S.  From  corresponding  observations  of  the  transit  of  Venus 
over  the  sun's  disk,  made  at  distant  places,  the  sun's  parallax, 
that  is  the  angle  under  which  the  earth's  semidiameter  would 
appear  at  the  sun,  may  be,  and  actually  has  been,  pretty  accu- 
rately ascertained  ;  and  then,  from  this  angle  and  the  earth's 
semidiameter,  the  distance  may  be  found  by  the  solution  of  a 
right-angled  plane  triangle. 

P.  From  the  distance  of  the  earth  from  the  sun,  how  may  that 
of  all  the  other  planets  be  found  ? 

S.  Their  several  periodical  times  have  already  been  accurate- 
ly ascertained,  and  hence  their  distances  will  be  found  from  the 
general  proportion,  that  the  squares  of  the  periodical  times  are 
as  the  cubes  of  the  distances. 

P.  How  may  the  jnagnitudes  of  the  several  planets  be  found  ? 

S.  By  the  resolution  of  a  plane  triangle,  from  their  respective 
distances  and  apparent  magnitudes. 

P.  How  may  their  relative  densities  be  found  ? 

S.  1.  By  comparing  the  periodical  times  and  distances  of  the 
satellites  of  one  planet,  with  those  of  the  satellites  of  another 
planet,  the  ratio  of  their  attracting  forces  may  be  readily  found. 
In  the  same  manner,  may  be  found  the  relative  attracting  force 
of  the  sun  and  of  any  planet  having  a  satellite. 

2.  From  the  relative  attracting  forces  of  the  planets  on  their 
satellites,  may  be  found  their  relative  attracting  forces  at  their 
surfaces,  and  then  from  these  and  their respectives  magnitudes* 
their  relative  densities  may  be  readily  computed. 


APPENDIX.  545 


Of  Eclipses. 


P.  What  is  understood  by  an  eclifise,  and  how  is  it  produced  ? 

S.  The  planets  being  all  opaque,  and  nearly  spherical  bodies, 
but  much  less  than  the  sun,  will  project  a  conical  shadow  on 
the  side  opposite  to  the  sun  ;  while,  therefore,  any  planet  in  its 
orbit  passes  through  one  of  these  shadows,  it  will  be  either  par- 
tialy,  or  totally  deprived  of  the  light  of  the  sun  ;  and  this 
deprivation  of  light  is  called  an  ecli/ise. 

P.  When  is  the  sun  said  to  be  eclipsed  ? 

S.  When  the  moon,  coming  between  the  sun  and  the  earth 
at  the  time  of  a  conjunction  or  change  of  the  moon,  casts  her 
shadow  on  the  earth,  then  the  sun  is  said  to  be  eclipsed,  with 
respect  to  that  part  of  the  earth  on  which  the  shadow  or  pen- 
umbra falls. 

P.  When  is  the  moon  said  to  be  eclipsed  ? 

S.  When  the  earth,  at  the  time  of  an  opposition  or  full  moon, 
casts  its  shadow  on  the  moon  ;  and  thus,  for  the  time,  deprives 
it  of  the  sun's  light. 

P.  In  what  circumstances  will  there  be  an  eclipse  of  the  sun, 
or  of  the  moon,  at  the  time  of  a  conjunction,  or  opposition  ? 

S.  This  can  happen  only  when  the  moon  is  in  or  near  one  of  her 
nodes;  about  17°  being  the  limit  with  respect  to  an  eclipse  of 
i  the  sun,  and  about  12°  the  limit  with  respect  to  an  eclipse  of 
the  moon :  for,  when  beyond  these  limits  the  shadow  of  the 
moon  will  pass  by  the  earth,  or  the  shadow  of  the  earth  will 
pass  by  the  moon,  and,  consequently,  no  eclipse  will  take  place. 

P.  In  what  particular  circumstances  will  an  eclipse  of  the 
sun,  or  of  the  moon,  be  central  ? 

S.  When  at  the  time  of  conjunction,  or  opposition,  the  spec- 
tator is  in  the  same  right  line  with  the  centers  of  the  sun  and 
moon. 

P.  When  will  an  eclipse  of  the  sun  be  annular  ? 

S.  This  will  be  the  case,  when  it  is  central,  or  nearly  so,  and 
the  apparent  diameter  of  the  moon  less  than  that  of  the  sun ; 
for  then,  at  or  near  that  part  of  the  earth  over  which  the  axis  of 
the  conical  shadow  passes,  an  annulus  or  ring  of  solar  light 
will  appear  round  the  body  of  the  moon. 

P.  When  will  an  eclipse  of  the  sun  be  total  ? 

S.  When  it  is  central  or  nearly  so,  and  the  apparent  diameter 
of  the  moon  greater  than  that  of  the  sun. 

P.  When  will  an  eclipse  of  the  moon  be  total  ? 

S.  When  the  moon,  at  the  time,  is  so  far  within  the  ecliptic 
limit,  that  she  will  wholly  pass  through  the  shadow  of  the  earth  ? 
P.  When  will  an  eclipse  be  partial  ? 

S.  When  the  moon  is  so  near  the  ecliptic  limit,  that  the  axis, 

though  not  the  whole,  of  the  conical  shadow,  will   pass  by  the 

moon  or  earth,  at  the  time  of  the  eclipse.     An  eclipse   of  the 

sun  will  also  always  be  partial  from  any  point  within  the  pen* 

VOL.  IV.  4  B 


546  APPENDIX. 

umbra  of  the  moon,  and  without  this  prenumbra  the  eclipse 
will  be  invisible. 

P.  How  frequently  will  eclipses  of  the  sun   and  moon  occur? 

S.  In  any  year,  the  number  of  eclipses  cannot  be  less  than 
two,  (both  of  the  sun)  nor  more  than  seven  ;  very  seldom  more 
than  six;  the  most  usual  number  is  four. 

P.  How  do  you  explain  this  irregularity  in  the  number  of 
eclipses  ? 

S.  1.  The  moon's  nodes  move  backwards  annually,  194  de- 
grees ;  and  of  course  the  sun  will  pass  from  one  node  to  the 
other  in  173  days. 

2.  When  the  sun  is  approaching  either  node  and  within  17 
degrees  of  it  at  the  tune  of  new  mcon,  there  will  be  an  eclipse 
of  the  sun;  and,  at  the  subsequent  Opposition,  the  moon  will  be 
eclipsed  in  the  opposite  node,  and  come  round  to  the  next  con- 
junction again,  before  the  sun  shall  have  got  past  the  ecliptic 
limit  on  the  other  hide  of  the  node  ;  and  consequently  the  sun 
will  again  be  eclipsed.  When  three  eclipses  thus  happen  while 
the  sun  is  at  one  of  the  nodes,  a  like  number  will  generally  hap- 
pen, while  at  the  other  node  ;  for  173  days,  the  lime  in  which 
the  sun  passes  from  one  node  to  the  other,  is  wiihin  4  days  of  6 
complete  lunations.  But  when  the  moon  changes  at  or  very 
near  one  of  the  nodes,  and  there  is  consequently  an  eclipse  of 
the  sun,  she  cannot  be  near  enough  to  the  other  node  at  the 
next  full  to  be  eclipsed.  There  will  therefore  be  no  other  eclipe 
till  after  six  lunations,  when  the  sun  will  have  reached  the  op- 
posite node,  and  will  then  be  eclipsed. 

In  the  former  case,  therefore,  there  will  be  six  eclipses,  viz. 
four  of  the  sun,  and  two  of  the  moon,  in  the  course  of  the  year; 
and  in  the  latter  case,  only  two,  .and  these  both  of  the  sun. 

P.  Since  there  are  more  eclipses  of  the  sun  than  of  the  moon, 
why  are  there  more  visible  eclipses  of  the  latter,  than  of  the 
former  ? 

S.  Because  eclipses  of  the  sun  are  visible  only  over  a  small 
part  of  the  earth's  surface,  but  eclipses  of  the  moon  are  visible 
over  the  whole  hemisphere. 

P.  In  what  time  would  there  be  a  regular  period  of  eclipses,  ex- 
actly corresponding  in  circumstances  to  the  eclipses  of  a  former 
period  ? 

S.  In  223  mean  lunations,  after  the  sun,  moon,  and  nodes  have 
been  once  in  a  line  of  conjunction,  the  same  eclipes,  will  again 
return  with  very  little  variation  for  many  ages. 

P.  Are  not  Jupiter's  satellites  frequently  eclipsed? 

S.  The  magnitude  of  Jupiter  being  so  great  with  respect  to 
the  distance  of  his  satellites,  the  first,  second,  and  third  of  them, 
are  eclipsed  in  every  revolution;  but  the  fourth,  on  account  of 
its  great  distance  from  Jupiter,  and  the  inclination  of  its  orbit, 
is  seldom  eclipsed. 

P.   What  is  meant  by  an  occultation  ? 

S.  When  the  moon,  in  her  motion  from  west  to  east,  comes 
between  the  spectator  and   any   of  the   stars  or   planets,  there 


APPENDIX.  547 

is  said  to  be  an  occultation  of  that  star,  or  planet  ;  the  com- 
mencement of  the  occultation  being  termed  tiie  immersion,  uiid 
the  end  of  it,  the  emersion^ 

P.   What  is  meant  by  a  transit  ? 

S.  When  either  Mercury,  or  Venus,  at  the  time  of  its  infe- 
rior conjunction,  is  in  or  near  one  of  its  nodes,  it  will  appear  to 
pass  over  the  sun's  disc,  in  the  form  of  a  black  spot  ;  the  be- 
ginning of  the  transit  being  termed  the  ingress,  and  the  end,  the 
egress.     These  transits,  however,  very  seldom  occur. 

P.  To  what  practical  use  may  the  observation  of  eclipses,  oc- 
cupations, and  transits  be  applied? 

S.  To  the  determination  of  the  longitude  of  places. 

P*  How  may  the  longitude  be  found,  by  observing  the  eclips- 
es of  Jupiter's  satellites  ? 

S.  The  times  of  all  the  visible  immersions  and  emersions  of 
those  satellites  are  calcinated  from  tables  of  their  motions,  for 
the  meridian  of  Greenwich,  and  inserted  in  the  English  nautical 
almanac  always  some  years  in  anticipation  ;  and  therefore  the 
difference  between  iliis  Greenwich  time,  and  that  observed  un- 
der any  other  meridian  when  the  same  phenomenon  takes 
place,  will  be  the  longitude,  in  time,  of  that  meridian  from 
Greenwich. 

P.  How,  by  an  eclipse  of  the  sun,  or  an  occultation  of  a  star  ? 

S.  From  astronomical  tables,  the  tine  of  any  of  these  pheno- 
mena may  be  calculated  for  the  meridian  of  Greenwich,  and 
then  the  difference  between  this  and  the  time  of  observation 
under  any  other  meridian,  cleared  from  the  effects  of  parallax 
and  refraction,  will  be  the  longitude,  in  time,  of  the  place  of 
observation. 


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[      550     ] 


Of  the   Tides. 

P.  Give  a  brief  description  of  the  principal  phenomena  of  the 
tides. 

S.  1.  On  the  shores  of  the  ocean,  and  in  bays  and  rivers  com- 
municating  therewith,  the  waters  flow  in  and  rise  for  about  six 
honis,  and  then  flow  out  and  fall  during  the  same  space  of  time. 
There  are,  therefore,  two  tides  of  flood,  and  two  of  ebb,  every 
24  hours,  or  rather  every  lunar  day,  which  is  about  24  hours  48 
minutes. 

2.  Twice  every  lunar  month,  viz.  a  little  after  the  full  and 
change  of  the  moon,  the  tide  of  flood  is  higher,  and  that  of  ebb 
lower  than  at  other  times,  and  hence  called  spring-tides  ;  and  a 
little  after  the  first  and  last  quarters  of  the  moon,  the  difference 
between  high  and  low  water  is  less  than  at  any  other  time  ;  these 
are  hence  call  d  neafi-tides. 

3.  At  certain  seasons  of  the  year,  namely,  a  little  before  the 
time  of  the  venial,  and  a  little  after  that  of  the  autumnal  equi- 
noxes, the  spring-tides  will  be  higher  than  at  other  times. 

4.  On  or  near  the  equator,  any  tide,  and  the  one  immediately 
succeeding  it,  will  be  nearly  alike  in  quantity  ;  but  in  places  of 
any  considerable  latitude,  these  tides  will  be  frequently  different. 

5.  On  the  shores  of  an  extensive  ocean,  especially  at  the  bot- 
tom of  wide-mouthed  bays,  as  the  bay  of  Funday,  the  tides  are 
most  considerable  ;  but  in  inland  seas  or  lakes,  as  the  Baltic,  the 
Caspian,  &.c.  they  are  scarce  pt rceptihle* 

P.  How  may  all  these,  or  any  other  phenomena  of  the  tides, 
be  explained  ? 

S.  Ttoey  may  all  be  satisfactorily  explained  on  the  Newtonian 
theory  of  the  solar  system. 

P.  Give  a  brief  view  of  this  theory  as  applicable  to  the  tides. 

S.  1.  The  tides  are  producetl  chiefly  by  the  attraction  of  the 
moon,  though  in  part  by  that  of  the  sun  also,  on  the  waters  of 
the  ocean. 

2.  Attraction  decreasing  as  the  square  of  the  distance  increa- 
ses, it  follows,  that  the  waters  on  the  surface  of  the  e;irth  directly 
under  the  attracting  body,  will  be  more  attracted  than  the  body 
(or  centre)  of  the  earth  itself,  and  tnis  again,  more  than  the 
waters  on  the  opposite  surface  ;  the  terraqueous  globe  thus  as- 
suming an  egg-like  figure,  or  that  of  a  prolate  spheroid.  Hence 
there  will  be  two  tides  of  flood,  or  places  of  high  water,  at  the 
same  time  ;  viz.  one  under  the  attracting  body,  from  an  excess  of 
attraction,  raising  the  waters  upward;  and  the  other,  opposite 
the  attracting  body,  from  a  deficiency  of  attraction,  leaving,  as 
it  were,  the  waters  behind. 

3.  As  the  moon,  from  the  diurnal  rotation  of  the  earth  and 
her  own  proper  motion  in  her  orbit,  makes  an  apparent  revolution 
round  the  earth,  from  east  to  west,  in  about  24  nours4b  minn'es, 
called  a  lunar  day,  there  will  of  course  be  two  tides  of  flood,  and 


APPENDIX.  551 

consequently  two  of  ebb  in  that  time;  and  hence  the  time  Of  high 
water  at  any  place,  on  any  day,  will  be  about  48  minutes  later 
than  on  the  day  immediately  preceding. 

4.  When  the  attractions  of  the  sun  and  moon  conspire,  or  act 
in  the  same  direction,  as  at  the  time  of  opposition  aid  tonjunc- 
tion,  (full  anil  change)  the  tides  thus  produced,. by  their  joint  in- 
fluence, must  be  higher  than  at  other  times:  but  when  the  at- 
tractions of  the  sun  and  moon  are  at  right  angles  to  each  other, 
as  at  the  quadratures,  or  times  of  first  and  last  quarters,  then  the 
place  of  high  water  from  the  influence  of  the  moon  will  be  the 
place  of  low  water  from  that  of  the  sun,  and  vice  versa  ;  and  the 
tides  thus  produced,  from  the  difference  of  their  influence,  must 
be  lower  than  at  other  times.  Hence  the  origin  uf  spring  and 
neap  tides. 

5.  As  one  of  the  apexes,  or  most  elevated  points  of  high  water, 
must  be  in  the  same  parallel  with  the  diurnal  track  of  the  moon, 
and  the  other  apex  diametrically  opposite,  it  follows,  that  when 
the  moon  is  in  or  near  the  equator,  both  the  tides,  in  a  lunar  day, 
must  be  nearly  of  the  same  height ;  but  when  the  moon  has  any 
considerable  declination,  the  tide  immediately  follow  ing  the  moon, 
on  the  same  side  of  the  equator  with  thr  moon,  will  be  the  high- 
er, a"nd  the  opposite  one  the  lower;  but  at  the  equator  itself,  both 
tides  will  be  in  the  same  circumstances-  Where  the  co-latitude 
of  the  place  does  not  exceed  the  moon's  d<  clination,  there  will  be 
but  one  tide  in  the  lunar  day  ;  for,  if  the  latitude  be  of  the  same 
name  with  the  declination,  there  will  be  only  the  superior  tide; 
if  of  different  names,  there  will  be  only  the  inferior  one  ;  the  other 
tides  not  reaching  these  points. 

6.  When  the  sun  and  moon  are  both  in  the  equator,  which  is 
the  case  at  the  time  of  the  equinoxes,  the  tides  will  move  west- 
ward with  the  greatest  velocity,  and  the  spring-tides  would,  on 
this  account,  be  then  the  greatest ;  but  as  the  height  of  these  tkles 
will  also,  in  some  measure,  be  influenced  by  the  distance  of  the 
sun,  they  would,  on  this  account,  be  greatest  when  the  earth  is 
in  its  perihelion,  that  is,  in  the  month  of  December.  Hence,  from' 
the  combined  influence  of  these  causes,  the  time  of  the  greatest 
spring-tides  will  actually  be  sometime  before  the  vernaj,  and 
after  the  autumnal,  equinoxes. 

7.  When  a  spring-tide  is  higher  than  usual,  from  the  moon 
being  then  in  or  near  her  perigee,  the  next  sj .nng-tioe  will  be  * 
lower  than  usual,  from  the  moon  being  then  in  or  near  her  apo- 
gee. * 

P.  What  will  be  the  average  ratio  between  the  influence  of  the 
moon  and  that  of  the  sun  in  raising  tides  ? 

S.  in  the  ratio  of  about  9  to  2. 

P.   How  does  this  appear? 

S.  From  observation  it  is  found  that  the  average  ratio  between 
the  height  of  a  spring-tide,  caused  by  the  sum  of  the  attractions 
of  the  sun  and  moon,  and  that  of  a  neap-tide,  caused  by  their 
difference,  in  any  given  place,  is  nearly  as  1 1  to  7  ;  that  is,  as 
the  sum  and  difference  of  9  and  2. 


552  APPENDIX. 

P.  Ts  not  the  time  of  high  water  generally  after  the  moon 
passes  the  meridian,  and  that  of  spring-tide  after  the  full  or 
change?     How  ma\     his  be  accounted  for? 

S.  "i.t  time  ol  .ugh  water  at  any  place  where  the  tide  meets 
with  iv>  parth  ular  obstruction,  will  be  generally  about  three  hours 
after  .the  moon  has  passed  the  meridian  ;  and  the  time  of  the 
highest  spring-tide,  about  three  tides  after  the  full  or  change: 
for  e\>  i\  i  itural  cans  must  take  some  time  to  produce  its  ulti- 
mate effect ;  or,  the  efTet  c  must  always  follow  the  cause. 

P.  Why  are  there  little  or  no  tides  perceptible  in  inland  seas 
or  lakes  ? 

S.  The  natural  extent  of  a  tide  from  the  place  of  high  to  that 
of  low  water  is  90°.  In  inland  seas  or  lakes,  therefore,  a  tide 
cannot  be  raised,  for  want  of  a  sufficient  extent  of  water. 

P.  Why  arc  the  tides  much  less  considerable  on  the  shores  of 
small  islands  in  the  open  ocean,  than  on  the  shores  of  conti- 
nents, or  in  rivers,  bays,  or  narrow  seas? 

S.  This  difference  is  owing  to  the  greater  obstruction  which 
the  flowing  tide  meets  with  in  the  latter  case  than  in  the  former; 
especially  where  the  obstruction  is  opposed  to  the  general  direc- 
tion of  the  tide  from  east  to  west.  Perhaps  the  obstruction  or 
other  influence  of  the  Gulfih-stream,  along  the  western  coast  of 
North  America,  may  also  contribute  towards  the  height  of  the 
tides  on  that  coast. 

P.  Are  there  not  considerable  irregularities  sometimes  in  the 
tides? 

S.  At  places  where  the  tides  come  through  two  or  more  dif- 
ferent passages,  and  meet  with  less  interruption  through  one 
passage  than  through  another,  it  may  arrive  at  different  times; 
and  in  this  case  there  may  be  two  or  more  tides  of  flood  succeed- 
ing each  other  at  small  intervals  of  time.  Winds  also  and  fresh- 
es will,  in  certain  situations,  have  considerable  effects. 


Of  Winds. 

P.  How  many  general  kinds  of  wind  are  there  ? 
S.  They  may  chiefly  be  reduced  under  four  different  heads, 
viz. 

1.  Trade-winds, 

2.  Monsoons, 

3.  Sea  and  land  breezes, 

4.  Irregular  or  variable  winds. 
P.  Give  a  brief  description  of  the  trade-winds. 

S.  These  prevail  chiefly  in  the  Atlantic  and  Pacific  oceans, 
within  the  torrid  zone.  On  the  north  side  of  the  equator,  or  ra- 
ther of  a  space  between  the  2d  and  5th  degrees  of  north  latitude, 
the  wind  blows  constantly  from  the  north-east,  varying,  however, 
a  point  or  two  on  either  side  ;  and  on  the  south  side  of  this  space, 
the  wind  blows  constantly  from  the  south-east ;  subject  to  a  like 


APPENDIX.  553 

variation.  In  the  Atlantic  ocean,  the  track  of  the  trade-winds 
extends  farther  north,  and  they  become  more  easterly,  and  de- 
crease in  strength,  as  you  proceed  westward. 

P.  How  do  you  account  for  the  phenomena  of  the  trade-winds  ? 

S.  I.  The  surface  of  the  earth  under  the  torrid  zone  being 
more  heated  by  the  sun  than  the  other  regions,  the  air  will  there 
be  rarefied ;  and,  ascending  to  the  superior  parts  of  the  atmosphere, 
the  colder  and  denser  air,  from  the  northern  and  southern  re- 
gions, will  rush  in  to  supply  the  deficiency. 

2.  As  the  rarefying  cause,  the  sun,  is  continually  moving  from 
east  to  west,  the  wind  will,  in  consequence,  have  this  general 
direction. 

S.  But  the  air  coming  from  northern  and  southern  latitudes, 
where  its  eastern  rotatory  motion  is  less  than  that  at  or  near  the 
equator,  will,  when  it  arrives  there,  have  comparatively  a  western 
direction,  and  this,  combined  with  its  real  motion  from  the  north 
or  south,  will,  with  the  western  motion  of  the  sun,  produce  the 
N.  E.  and  S.  E.  trade-winds. 

P.  Why  is  the  boundary  between  the  N.  E.  and  S.  E.  trade- 
winds  north  of  the  equator? 

S.  This  is  probably  owing  to  the  sun's  being  longer  north  of 
the  equator  than  south  of  it,  and  thus  making  the  'track  of  the 
sun's  greatest  influence  somewhat  north  of  the  equator.  Besides, 
the  northern  hemisphere  containing  much  more  land  than  the 
southern,  will,  of  consequence,  have  a  greater  average  heat. 

P.  Give  a  short  description  of  the  monsoons. 

S.  These  winds  are  found  chiefly  in  the  Indian  ocean,  and  blow 
generally  six  months  in  one  direction,  and  six  in  the  opposite  di- 
rection. They  may  be  reduced  to  two  general  classes,  viz.  those 
on  the  north  side  of  the  equator,  and  those  on  the  south  side. 

1.  Between  the  3d  and  10th  degrees  of  south  latitude,  the  S. 
E.  trade-wind  continues  from  April  till  October,  but  during  the 
rest  of  the  year,  the  wind  blows  from  N.  W.  Between  Sumatra 
and  New-Holland,  the  monsoon  blows  from  the  S.  while  the  sun 
is  north  of  the  equator ;  and  from  the  N.  while  it  is  south  of  the 
equator.  Between  Africa  and  Madagascar,  the  monsoons  during 
the  same  periods  are  S.  W.  and  N.  E. 

2.  Over  all  the  Indian  ocean,  to  the  northward  of  the  3d  de- 
gree of  south  latitude,  a  N.  E.  wind  blows  while  the  sun  is 
south  of  the  equator,  and  a  S.  W.  wind  while  it  is  north  of  the 
equator.  From  Borneo,  along  the  coast  of  Malacca,  and  as  far 
as  China,  the  monsoon  blows  nearly  from  the  S.  while  the  sun  is 
north  of  the  equator,  and  from  the  N.  while  it  is  south.  Regular 
monsoons  are  also  found  in  the  Red-Sea,  blowing  N.  W.  and 
S.  W.  the  direction  of  the  coast  of  Arabia.  Monsoons  also  pre- 
vail on  some  parts  of  the  coast  of  South  America. 

P.  How  do  you  explain  the  phenomena  of  the  monsoons  ? 

S.  1.  While  the  sun  is  passing  daily  over  any  region,  the  dry 
land  will  be  more  heated,  and  consequently  the  air  more  rare- 
fied, than  the  water  or  the  air  over  it  will  be.    This  arises  chiefly 

VOL.  IV,  4,0 


554  APPENDIX. 

from  water  being  a  transparent  body  ;  and  therefore  admitting 
the  solar  rays  readily  to  pass  through  it.  It  may  also,  in  part, 
be  owing  to  the  copious  evaporation  from  the  surface  of  water, 
by  which  its  increase  of  temperature  will  be  moderated:  to  this 
end  the  continual  motion  of  the  water  will  likewise  contribute. 

2.  The  rarefied  air  over  the  land  will  ascend  to  the  superior 
parts  of  the  atmosphere,  and  a  current  from  the  colder  and 
denser  air  over  the  neighbouring  waters  must  of  consequence 
take  place,  to  supply  the  deficiency. 

3.  Wherever,  therefore,  two  considerable  tracks  of  landv  one 
north  and  the  other  south  of  the  equator,  are  separated  by  wa- 
ter, a  monsoon  must  necessarily  take  place  ;  always  blowing  to- 
wards the  land  over  which  the  sun  daily  passes ;  and  these  are 
generally,  if  not  always,  the  situations  of  those  countries  be- 
tween which  the  monsoons  are  observed  to  blow. 

P.  Describe  the  phenomena  of  sea  and  land  breezes. 

S.  These  are  observable  in  all  maritime  countries  of  any  con- 
siderable extent  between  the  tropics ;  and  frequently  beyond 
those  limits.  The  sea-breeze,  or  that  from  the  sea  towards  the 
land,  generally  sets  in  about  10  in  the  morning,  and  blows  till  6 
in  the  evening  ;  about  7,  the  land-breeze  begins,  and  blows  to- 
wards the  sea  till  8  in  the  morning,  when  it  dies  away. 

P.  How  do  you  account  for  these  phenomena? 

S.  On  the  same  general  principles  on  which  the  monsoons  are 
explained;  viz.  1.  The  land,  during  the  presence  and  influence 
of  the  sun,  will  be  more  heated  than  the  neighbouring  water ; 
and,  consequently,  the  rarefied  air  over  the  land  will  ascend,  and 
its  place  be  supplied  by  the  colder  and  denser  air  over  the  water. 
Hence,  the  sea-breeze  during  the  day. 

2.  During  the  night,  in  the  absence  of  the  sun,  the  air  over 
the  land  will  be  more  cooled  than  that  over  the  water,  ( 1)  because 
the  air  that  had  ascended  during  the  day,  being  cooled  and  con- 
densed in  the  higher  parts  of  the  atmosphere,  or  on  the  summits 
of  the  high  lands,  will  now  descend  again  to  the  surface  ;  and 
(2)  because  the  surface  of  the  water,  as  soon  as  it  cools,  will  de- 
scend and  be  replaced  by  the  warmer  water  from  below  :  hence-, 
a  land-breeze  during  the  night  will  ensue. 

P.  What  account  do  you  give  of  the  irregular,  or  variable 
winds  ? 

S.  Such,  with  few  exceptions,  are  the  wiuds  in  the  temperate 
and  frigid  zones ;  though  almost  in  all  countries,  certain  winds 
are  prevalent  during  certain  seasons  of  the  year;  but  the  theory 
of  these  winds  is  still  but  imperfectly  known. 

P.  What  do  you  say  with  respect  to  the  velocity  of  the  wind? 

S.  It  varies  from  the  most  gentle  breeze,  just  perceptible, 
moving  at  the  rate  of  about  I  mile  per  hour,  to  the  most  violent 
hurricane,  with  the  velocity  of  100  miles  per  hour.  It  moves, 
however,  with  different  velocities,  and  sometimes  in  different  di- 
rections, in  different  strata  of  the  atmosphere:  the  velocity,  as 
has  been  tully  ascertained  by  aeronauts  in  their  balloons,  being: 


APPENDIX.  555 

generally  much  greater  in  the  superior  than  in  the  inferior  parts 
of  the  atmosphere.  .     , 

Of  Chronology, 

P.  What  does  chronology  treat  of  ? 

S.  It  treats  of  time  with  respect  to  its  measures  ;  including  its 
various  distinctions,  divisions,  and  subdivisions. 

P.  How  is  time  measured  ? 

S.  It  is  measured  or  regulated  by  the  motion  of  the  heavenly 
bodies. 

P.  What  are  the  usual  distinctions  of  time  ? 

S.  It  is  usually  distinguished  into  apparent.solar,  mean-solar, 
sidereal,  and  lunar  time. 

P.  Give  a  brief  account  of  apparent  solar  time, 

S.  1.  Apparent  solar  noon,  at  any  given  place,  is  the  moment 
of  time  when  the  sun's  centre  is  on  the  meridian  of  the  place  ; 
an  apparent  solar  day  is  the  interval  of  time  between  one  appa- 
rent solar  noon  and  the  next ;  and  the  apparent  solar  time,  hour 
of  the  day,  or  horary  angle,  is  the  angle  (15  degrees  to  an  hour) 
which  the  meridian  passing  through  the  centre  of  the  sun  makes 
with  the  meridian  of  the  place. 

2.  The  apparent  solar  days  are  not  all  equal  to  each  other 
throughout  the  year.  This  inequality  arises  from  two  causes ; 
one  is,  that  the  earth,  moving  in  an  elliptical  orbit,  does  not  de- 
scribe equal  arches  in  equal  times  ;  and,  therefore,  as  the  earth, 
in  a  solar  day,  must  make  one  complete  rotation  round  its  axis, 
and  so  much  of  another  as  will  correspond  with  the  earth's  di- 
urnal arch  in  the  ecliptic,  and  since  these  arches  are  unequal, 
being  least  when  the  sun  is  in  its  aphelion,  and  greatest  when  in 
its  perihelion,  it  follows,  that  the  days,  on  this  account,  must  be 
unequal  also.  The  second  cause  of  this  inequality  is,  the  inclina- 
tion of  the  ecliptic  to  the  equator ;  whence  equal  arches  in  the 
ecliptic,  in  which  the  earth  moves,  will  not  correspond  with  equal 
arches  of  the  equator,  on  which  time  is  measured. 

P.  What  is  meant  by  mean  solar  time  ? 

S.  It  is  that  pointed  out  by  a  well-regulated  time-piece,  going 
with  an  equable  motion  throughout  the  whole  year. 

P.  What  is  meant  by  the  equation  of  time  ? 

S.  It  is  the  difference  between  apparent  and  mean.  Four  times 
in  the  year,  viz.  on  the  16th  April,  16th  June,  1st  September; 
and  25th  December,  this  equation  will  be  nothing,  or,  the  appa- 
rent and  mean  time  will  be  the  same  ;  but  when  greatest,  it  will 
amount  to  upwards  of  16  minutes. 

P.   What  is  sidereal  time  ? 

S.  That  measured  by  the  apparent  diurnal  motion  of  the  stars. 
A  sidereal  day,  therefore,  is  the  interval  of  time  from  the  pas- 
sage of  any  fixed  star  over  the  meridian,  till  it  passes  that  meri- 
dian again.  These  days  are  all  equal,  and  3'  55 ".9  o+  time  less 
than  a  mean  solar  day. 


556  APPENDIX. 


P.  What  is  lunar  time? 

S.  That  measured  by  the  motion  of  the  moon.  A  lunar  day 
is  the  interval  of  time  between  the  moon's  passing  the  meridian 
on  any  day  and  the  next  succeeding  day.  These  days  are  une- 
qual, but,  on  an  average,  exceed  a  mean  solar  day  about  48  mi- 
nutes. 

P.   At  what  time  is  the  day  usually  considered  as  beginning? 

S.  The  civil  day  begins  at  midnight ;  ihe  astronomical  day  at 
the  succeeding  noon ;  and  the  nautical,  or  tea  day,  at  thfe  pre- 
ceding noon. 

P.   What  is  a  sidereal  year? 

S.  The  time  in  which  the  earth  performs  a  complete  revolution 
through  the  whole  circle  of  stars  in  the  ecliptic  =  365d.  6h.  9m. 
17s. 

P.  What  is  a  trofiical  year  ? 

S.  The  time  in  which  the  earth  performs  a  complete  revolu- 
tion through  all  the  artificial  signs  of  the  ecliptic,  =  365d.  5h. 
48m.  48s. 

P.  What  is  the  reason  of  this  difference  between  the  sidereal 
and  tropical  years  ? 

S.  The  earth  being  of  an  oblate  spheroidal  figure,  and  the 
ecliptic  inclined  to  the  equator,  the  attraction  of  the  sun  and 
moon  on  the  accumulation  or  redundance  of  matter  at  the  equa- 
tor, will  cause  the  equinoctial  points  to  move  backwards  (called 
the  recession  >  of  the  equinoxes)  at  the  rate  of  501"  yearly, 
through  which  the  earth  will  move  in  20m.  29s*  the  difference 
between  the  sidereal  and  tropical  years. 

P.  Give  an  account  of  the  origin  of  old  and  neiv  stile. 

S.  The  mean  Julian  civil  year  consists  of  365d.  6h.  three  years 
containing  each  363,  and  every  fourth  year  366  days.  The  sup- 
plementary day  was  added  to  the  month  of  February  by  counting 
twice  the  23d  day  ;  (which  in  the  old  Roman  calendar  was  called 
the  sixth  of  the  calends  of  March)  and  hence  this  year  was  called 
bissextile,  or,  on  another  account,  leap-year.  This  Julian  year 
exceeds  the  true  or  tropical  year,  according  to  which  the  seasom 
take  place,  11m.  12s.  which  in  131  years  will  amount  to  a  whole 
day  ;  and  this  constitutes  the  difference  between  the  Julian,  or 
old  stile,  and  the  Gregorian,  or  new  stile. 

P.  When  was  the  Julian  calendar  corrected,  and  on  what  oc- 
casion ? 

S.  In  the  year  325,  when  the  council  of  Nice  appointed  the 
time  for  the  celebration  of  Easter,  (viz.  the  first  Sunday  after  the 
full  moon,  immediately  succeeding  the  time  of  the  vernal  equi- 
nox) the  equinoxes  happened  on  the  21st  of  March  and  the  21st 
of  September.  In  the  year  1582,  Pope  Gregory  XIII  observing 
that  the  Julian  had  got  a-head  of  the  tropical  year  10  days,  ordered 
that  so  many  should  be  then  struck  out  of  the  calendar;  and  in 
the  year  1753,  when  the  British  government  adopted  the  new 
stile,  1 1  days  were  struck  out.  To  prevent  any  irregularity  from 
taking  place  in  future,  it  was  ordered,  that  three  days  should  be 
struck  out  of  the  Julian  calendar  every  400  years,  by  reckoning 


APPENDIX.  S5T 

1700,  1800,  1900,  2100,  &c.  or  every  centurial  year  not  divisible 
by  4,  a  common  year,  instead  of  a  leap-year,  which  it  would  other- 
wise be.  There  is  therefore  now  a  difference  of  12  days  between 
the  old  stile  and  the  new. 

P.  How  may  the  leap-years  be  found? 

S.  The  first  year  of  the  Christian  aera  was  the  first  after  leap- 
year  ;  hence  divide  the  given  year  by  4,  and,  if  nothing  remain,  it 
will  be  leap-year;  except  the  centurial  years  not  divisible  by  4, 
which,  according  to  the  Gregorian  account,  must  be  considered 
as  the  3d  after  leap-year:  but  if  any  thing  remain,  it  will  point 
out  the  year  after  leap-year, 

P.  What  is  meant  by  the  Dominical  letter? 

S.  In  calendars,  it  has  been  customary  to  prefix  the  first  seven 
letters  of  the  alphabet  to  the  several  days  of  the  year,  succes- 
sively ;  that  opposite  the  first  day  of  the  year,  being  A,  that  op- 
posite the  second,  B,  and  so  on  to  G.  The  same  letter,  there- 
fore, would  continually  correspond  to  the  same  day  of  the  week 
throughout  the  year ;  and  thus  the  letter  corresponding  to  the 
Lord's  day  (Uominicus  dies)  was  called  the  Dominical  letter. 

P.  In  what  order  will  the  Dominical  letters  occur,  from  year  to 
year  ? 

S.  A  common  year  containing  52  weeks  and  1  day,  and  a  bis- 
sextile 52  weeks  and  2  days  ;  the  Dominical  letter  will  therefore 
fall  back,  in  the  order  of  the  alphabet,  one  letter  every  common 
year,  and  two  letters,  after  the  last  of  February,  every  bissextile  ; 
hence,  on  this  account,  it  is  frequently  termed  lea/:*year, 

P.  How  would  you  find  the  Dominical  letter  for  any  given  year 
of  the  Christian  sera? 

S.  To  the  given  year  add  its  fourth  part;  rejecting  fractions, 
divide  the  sum  by  7,  and  the  remainder  taken  from  7,  will  leave 
the  number  of  the  Dominical  letter  in  the  order  of  the  alphabet ; 
viz.  1  =  A,  2  =  B,  Sec.  But  in  leap-years,  the  letter  thus  found 
will  be  the  Dominical  letter  till  the  latter  end  of  February,  and 
the  one  next  preceding  this  will  be  the  Dominical  letter  for  the 
rest  of  the  year.  This  rule  will  hold  good  for  any  year  of  the 
18th  century;  but  for  the  19th  century,  (1800  being  a  common, 
instead  of  a  leap-year)  the  Dominical  letter  will  be  the  next  sue* 
ceeding  that  found  by   the  rule. 

P.  What,  for  example,  will  the  Dominical  letter  be  in  1807? 

b.  4)1807 

451 


7)2258 

S24...4 
7 

3-f.l=4=D. 
P.  Having  the  Dominical  letter  for  any  given  year,  how  would 
you  find  on  what  day  of  the  week  anv  given  day  of  any  month 
vould  fall? 


558  APPENDIX. 

S.  By  the  following  distich — 

"  At  Dover  Dwells  George  Brown  Esquire, 
Good  Christopher  Finch  And  David  Friar." 

P.  How  is  this  to  be  applied? 

S.  The  first  letter  of  each  of  these  twelve  words  is  the  letter 
which,  in  the  calendar,  belongs  to  the  first  day  of  its  respective 
month,  from  January  (At)  to  December  (Friar).  Hence,  count- 
ing from  the  letter  of  the  first  day  of  the  month  to  the  Dominical 
letter,  v-e  will  have  the  day  of  that  month  on  which  the  first 
Sunday  falls. 

P.  On  what  day  of  the  week,  for  example,  will  the  4th  of  July 
fall  in  the  year  1807  ;  the  Dominical  letter  being  D  ? 

S.  July,  the  7th  month,  Good-,  G,  A,  B,  C,  D; — the  first  Sunday 
will  be  the  5th,  therefore  the  4th  will  be  on  Saturday. 

P.  What  is  meant  by  a  cycle-,  in  chronology  ? 

S.  A  certain  period  of  time,  wherein  the  same  circumstances, 
to  which  the  cycle  has  a  reference,  will  regularly  return. 

P.  What  are  the  most  noted  astronomical  cycles? 

S.  The  solar  cycle,  I  he  Metonic,  or  lunar  cycle,  and  the 
Dionysian  cycle  or  period. 

P.  What  is  the  solar  cycle? 

S.  It  is  a  period  of  28  years,  after  which  the  same  day  of  any 
month  will  happen  on  the  same  day  of  the  week  as  on  the  same 
year  of  a  former  cycle. 

P.  How  would  you  find  what  year  of  this  cycle  corresponds 
to  any  given  year  of  the  Christian  aera? 

S.  The  first  year  of  the  Christian  aera  was  the  9th  of  the  solar 
cycle  :  hence,  add  9  to  the  given  year,  divide  the  sum  by  28,  and 
the  remainder  will  be  the  year  of  the  cycle  required;  0  corre- 
sponding to  the  28th,  or  last  year  of  the  cycle. 

P.  What  year  of  the  solar  cycle,  for  example,  is  1807? 

S.  1807 

9 

28)1816(64 
168 


136 
112 


24,  year  of  the  solar  cycle. 

P.  What  is  the  Metonic,  or  lunar  cycle? 

S.  A  period  of  19  years,  in  which  the  moon's  age  will  be  the 
same,  as  on  the  same  day  of  the  same  month,  in  the  same  year 
of  a  former  cycle. 

P.  How  would  you  find  what  year  of  this  cycle  corresponds 
to  any  given  year  of  the  Christian  sera  ? 

S.  The  first  year  of  the  Christian  asra  was  the  first  year  of  this 
cycle  ;  hence,  add  1  to  the  given  year,  divide  the  sum  by  19,  and 
the  remainder  will  be  the  year  of  the  cycle  required  ;  0  corr* 


APPENDIX.  559 

spomling  to  the  19th,  or  last  year  of  the  cycle.  The  year  of  the 
lunar  cycle  is  also  frequently  called  the  prime-,  or  golden  number* 

P.  Find  the  golden  number  for  the  year  1807. 

S.  1807 

1 

19)1808(95 
171 

95 

3,  golden  number. 

P.  What  is  the  Dionysian  period  ? 

S.  A  period  compounded  of  the  solar  and  lunar  cycles,  con- 
taining 532  years;  (28x19)  after  which  the  day  of  the  month, 
the  day  of  the  week,  and  the  moon's  age,  will  all  return  together 
in  the  same  order  as  in  the  former  period. 

P.  What  is  the  epact? 

S.  It  is  the  moon's  age,  reckoned  from  the  change,  at  the 
beginning  of  the  year. 

P.  How  may  this  be  found  ? 

S.  A  lunar  year,  or  12  lunar  months,  contains  about  354  days, 
or  1 1  days  less  than  a  common  solar  year  ;  hence,  should  the  solar 
and  lunar  years  begin  together,  and  consequently  the  epact=0, 
the  next  year  the  epact  would  be  1 1 ,  the  next  22,  the  next  (33 — 30) 
3,  and  so  on,  through  the  cycle  of  19  years;  after  which  the 
epacts  would  again  return  in  the  same  successive  order.  Hence, 
divide  the  year  of  the  Christian  sera  by  19,  multiply  the  remain- 
der by  1 1,  divide  the  product  by  30,  and  the  remainder  will  be 
the  epact. 

P.  What  is  the  epact  for  1807  ? 

S.  19)1807(95 

171 

97 
95 

2 
11 

30)22  remainder,  the  epact. 
P.  How  may  the  moon's  age  be  found,  on  any  given  day. 
S.  By  adding  together  the  epact,  the  day  of  the  month,  and 
the  number  corresponding  to  the  month,  viz, 

Jan.  Feb.  Mar.  Ap.  M.  Jun.  Jul.  Aug.  Sep.  Oct.  Nov.  De«. 
Com.  year,     02       0       2244       6       7       8      10     10 
Leap-year,       02        1        3345       7        8       9      10      11 
and  then  the  sum,  or  its  excess  above  30,  will  be  the  moon's  ageA 
nearly. 


360  APPENDIX. 

P.  Suppose  the  4th  July,  1807? 
S.  22  epact 

4  day  of  month 

4  num.  for  July 

30,  moon's  age,  or  day  of  change. 
P.  How  would  you  find  the  time  of  the  moon's  passage  over  tl 
meridian  on  any  given  day  of  her  age  ? 

S.  From  her  age  subtract  one-fifth  part,  and  the  remainder  will 
be  the  time  of  her  passage,  nearly. 
P.  Suppose  the  moon's  age  30  days  ? 
S.  5)30 

6 

24,  or  noon,  the  time  of  passage,  nearly 


FINIS. 


GENERAL  INDEX. 


JV.  B.  The  Roman  Characters  denotes  the  Volume; 
thus  i.  ii.  iii.  iv.  The  Arabic  Numbers,  the  Page : 
thus,  1,  2,  3,  &c. 


^LCID  Liquors-,  the  variety  in  their  freezing,  ii.  62. 

Agency  active  in  nature,  iii.  213. 

Air,  a  fluid,  the  nature  and  properties  of  it,  i.  45.  Effects  pro- 
duced by  it,  i.46.  Its  resistance,  i.  47,  49.  Its  weight,  i.  56. 
Its  pressure  in  every  direction,  i.  50,  67.  Its  force  equally 
extended,  i.  52.  By  it  is  explained  the  suction  of  animals,  i. 
60.  Its  weight  sustains  the  column  of  mercury  in  the  baro- 
meter, i.  69.  Its  vast  pressure  on  the  earth,  i.  71.  Its  elas- 
ticity proved,  i.  72,  76.  And  differently  accounted  for,  i.  30. 
Is  caused  by  fire,  i.  81.  Is  capable  of  raising  great  weights, 
i.  74.  The  air  in  timber  causes  it  to  swim,  i.  79.  Conti- 
nued varieties  in  nature  caused  by  air,  i.  80.  Its  elasticity  is 
always  equal  to  the  force  which  compresses  it,  i.  83.  Its 
elasticity  undestroyed,  i.  94.  Air  expands  by  heat  and 
contracts  by  cold,  i.  96.  Is  the  cause  of  winds,  i.  98. 
The  benefits  of  fresh  and  cool  air,  i.  100.  Carries  off 
smoke,  i.  101.  Enlivens  fire,  i.  102.  Different  currents 
of  air  in  chimnies  in  summer  time,  i.  109.  Effects  of  con- 
densed air,  i.  111.  Produces  fountains  and  jets,  i.  116. 
Different  calculations  of  the  dilatation  of  air,  i.  118.  Raises 
water  thirty-four  feet,  i.  120.  Cannot  be  totally  exhausted 
from  the  receiver  of  the  air-pump,  i.  133.  The  quantities 
of  air  exhausted  at  every  stroke  are  not  equal,  but  are  per- 
petually diminished,  i.  136.  The  different  methods  of  ex- 
tracting it  from  the  bodies  which  contain  it,  i.  178.  By 
heat,  by  cold,  by  exhaustion,  and  by  dissolution,  i.  178. 
Air  is  contained  in  water,  i.  178.  In  eggs,  i.  179.  In  wood 
i.  180.  The  pressure  of  it  may  produce  the  finest  anaton.ical 
injections,  i.  182.  Air  condensed  retards  fermentation,  i. 
184.     Air  is  a  resisting  medium,  according  to  the  surface 

VOL.  IV.  4  D 


562  GENERAL  INDEX. 

exposed,  i#  184.     Supports  the  flight  of  birds,  i.   186.     Is 
of  great  importance  in  the  theory  of  gunnery,  i.    Ib7.     Its 
great  resistance  to    cannon   balls,  iii.    169.     Its   use   in   the 
animal  economy,  i.  187,  189.     Is  necessary  for  combustion, 
i.   189.     Is  diminished  by  combustion,  i.    192.     Vast  quan- 
tities are  consumed  by  fires  and  various  oth.tr  means,  i.  193. 
Nourishes    the   blood,  i.    198.     Passes    into    the    buries   of 
birds,    i.     202.     Is    the    medium    of   sound,    i.    208,  209. 
Dense    air    conveys     the     sound    best,     i.     2u8.       Air    is 
thrown    into    an  undulating   motion    by    sound,  i.  2u6.     la 
what  manner  the  pulses  ol  the  air  are  propagated  by  sound, 
i.  206.      The   various  effects   in   the    atmosphere    rest 
from  the  air,  i.  241.  Connexion  between  air  and  tire,  i.  24fl 
The    benefits    resulting    from    air,    i.   244.     Is    a    general 
agent,  i.  244.     Hippocrates,  his  remark  on    it,  l.  245.     Is 
not  a  solvent  of  water,  i.  547.     Rarefied  air  a  *  ause  of  dry- 
ness, ii.  76.     Very  different  currents  of  air  in    tne  atmos- 
phere, iii.  403.     Hot   air,  its    power   to   raise    weights,  iii. 
410.     Air  produced  by  fire  and  light,  iv.  371.     The  upper 
regions    of   it    extremely    dry,  iv.  445.      See    Fire,  Light, 
Water.     An  account  of  the  discoveries  of  the  different  airs 
i.  430.     The  manner  of  conveying  them  from   one    vess 
to  another,  i.  435.     Methods  of  extracting  air   from  seve 
ral  subjects,  i.  436. 

Air,  alkaline  acid,  its  nature  and  properties,  i.   506. 

Air,  at?nospheric,  is  a  mixture  of  different  fluids,  i.  ^59 
•  Particularly  of  two  elastic  fluids  of  opposite  qualities, 
Contains  about  seventy-two  parts  phlogisticated  air,  am 
twenty-eight  parts  vital  air,  i.  474.  Is  a  uniform  eom- 
pound,  i.  475.  Air  presses  on  fire,  i.  360.  Air  supplies 
the  fire,  i.  362.  Vital  air,  on  inflammation,  disengages 
much  fire,  i.  362.  Explanation  of  Arganci's  lamps,  i.  3o3. 
Air,  its  efYecis  on  ignited  iron,  i.  363.  Contains  great 
quantities  of  fire,  i.  423. 

Air,  fixed,  its  nature  and  properties,  i.  481,  490.  Is  found  in 
subterraneous  places,  and  is  produced  from  fermentations,  i. 
482.  From  the  respiration  of  animals,  i.  482.  Is  com- 
bined with  various  substances,  i.  483.  Its  effects  whcH 
breathed  by  various  animals,  i.  483.  Is  absorbed  by  water, 
i.  *434.   Tiie  analogy  between  this  and  phlogiston,  i.  51 4,  5  16. 

Air,  fiuor  acid,  an  account  of,  i.   o06. 

Air,  hefiatic,  an  account  of,  i.  500.  Its  nature  and  properti  es  i. 
501. 

Air,  inflammable,  its  nature  and  properties,  how  obtained,  i.  492. 
Is  prouueed  from  water  by  means  of  fire,  i.  494.  Different 
manner  of  its  burning,  i.  498.  Inflammable  air  from 
marshes,  i.  503.  A  useful  apparatus  for  making  it,  by  the 
Editor,  ii.  91. 

Air,  cretaceous  iuj-ammable,  bow  obtained,  i.   502. 

Air,  pure  injiammable,  its  nature  and  properties,  i.  497.  Can 
form  fire- works  without  smoke,  i.  499.  Being  mixed  with 
different  airs,  it  produces  different  colours,  i.  499. 


GENERAL  IND£X.  563 

Air,  nitrous,  obtained  by  the  spirit  of  nitre  poured  on  various 
metals,  i.  476.  Is  a  combination  of  phlogisticated  and  vital 
air,  i.  477.    Its  nature  and  properties,  i.  479. 

Air,  phosphoric,  its  nature  and  properties,  i.  501. 

Air,  phlogisticated,  called  also  azotic  gas,  or  atmospherical  me- 
phitis, is  variously  obtained,  i.  468.  Is  light,  tasteless,  in- 
soluble, i.  469.  Is  improper  for  respiration  or  combustion, 
i.  476. 

Air,  sulphureous  acid,  its  nature  and  properties,  i.  505. 

Air  vital,  its  singular  effects  on  fire,  i.  364.  Amazingly  increas- 
es its  power,  i.  4  13.  An  account  of  it,  i.  449.  Is  extricat- 
ed by  heat  from  various  substances,  i.  450.  Also  by  light 
from  vegetables,  i.  452.  But  in  different  quantities,  i.  454. 
Water  differently  impregnated  promotes  the  emission  of  it, 
i.  45  5.  The  quantity  of  it  extricated  is  a  test  of  the  quan- 
tity of  food  taken  in  by  the  plant,  i.  457.  Is  extricated  from 
some  metallic  calces,  i.  462.  Its  weight  supports  combus- 
tion, i.  459.  The  change  it  produces  on  phosphorus,  i. 
460.  Many  combustible  substances  become  acid  by  vital  air, 
i.  461.  Metallic  substances  increase  in  weight  as  they  ab- 
sorb vital  air,  i.  462.  It  forms  one  third  of  the  atmosphere  ; 
is  the  acidifying  principle,  i.  462.  Is  necessary  for  respira- 
tion, i.  464.  It  takes  from  the  blood  its  superabundant 
mephitis,  and  imparts  its  own  fire,  i.  464.  The  quantity  of 
it  absorbed  in  respiration,  i.  465.  It  gives  the  red  colour  to 
the  blood  in  the  lungs,  i.  466.  By  respiration  it  passes  from 
an  aerial  to  a  concrete  form,  and  is  the  source  of  animal 
heat,  i.  467.  Its  effects  when  taken  medicinally,  i.  468.  Is 
a  constituent  part  of  certain  bodies,  i.  516. 

Air-gun,  Editor's  description  of,  i.  115. 

Air-pumps,  description  of,  i.  39.  History  of,  i.  43.  An  account 
of  them,  i.  39,  40.  An  account  of  their  improvement,  i.  127 
By  Mr.  Smeaton,  i.  129.  By  Dr.  Prince,  who  removed  the 
valves,  i.  130.  Means  to  obtain  accurate  exhaustion,  i.  144. 
American  air-pump,  i.  38.  Common  double  barrel,  i.  29. 
Editor's  caution  in  cleaning  them,  i.  41. 

Animals  possess  a  natural  language,  i.  200.  Retain  the  same  de- 
gree of  heat  in  different  climates,  i.  262.  The  wisdom  ex- 
hibited in  their  form  and  magnitude,  iii.  271. 

Ancients  supposed  that  nature  dreaded  a  vacuum,  i.  64.  But  this 
was  confined  within  certain  limits,  i.  65.  Their  knowledge 
in  natural  philosophy,  ii.  420.  Their  knowledge  of  glasses, 
ii.  430.  The  just  ideas  which  some  of  them  entertained  of 
God,  iii.  61.    Their  opinion  of  the  soul,  iii.  61. 

Archimedes  set  fire  to  the  Roman  ships  at  Syracuse,  by  means 
of  his  burning  glasses  of  plane  mirrors,  i.  410.  ii.  205.  His 
knowledge  in  hydrostatics,  iii.  340. 

Aristotle,  the  influence  of  his  authority,  which  impeded  the  pro- 
gress of  truth,  i.  65.  His  idea  of  a  plenum,  iii.  33.  His  just 
remark  concerning  the  Creator,  iv.  253. 

.irithmetic,   mecanical,  principles  of.  iii.  240. 


564  GENERAL  INDEX. 

Armillary  sphere,  iii.  482.  The  appearances  of  the  starry  hea- 
vens illustrated  by  it,  iii.  496. 

Astronomy,  observations  on  ;  its  design,  iii.  457.  Its  general  prin- 
ciples, iii.  458.  Corrects  appearances,  iv.  1.  Copernican 
system,  iv.  2,  4,  145.  Ptolemaic,  iv.  3,  68.  Remarks  on  phy- 
sical astronomy,  iv.  214.  The  different  analogies  of  the  hea- 
venly motions  which  have  been  pointed  out,  iv.  215.  Kep- 
ler's laws  of  it,  iv.  225. 

Atmosphere,  height  of ;  manner  of  ascertaining  it,  i.  84.  Does 
not  refract  light  above  forty-five  miles,  i.  87.  The  height 
not  accurately  known,  i.  87.  Horseley's  conjecture  con- 
cerning its  infinitude,  i.  88.  Is  a  mixture  of  all  vegetable, 
mineral,  and  animal  substances,  i.  240.  Great  causes  al- 
ways acting  in  it,  iv.  408. 

Atoms,  considerations  concerning  them,  iii.  19.  Are  indivisi- 
ble, iii.  20.    Are  indefinitely  small,  iii.  22. 

Attraction,  how  differently  used  by  authors,  iii.  29.  Observations 
on  it,  iii.  32.  How  falsely  applied,  iii.  33.  Instances  of  at- 
traction otherwise  explained,  iii.  35.  In  a  column  of  quick- 
silver, iii.  37. 

Attraction  of  cohesion  examined,  i.  275. 

Aurora  borealis,  the  different  appearances  of,  iv.  465.  Its  flashes 
cross  the  magnetic  meridian  at  right  angles,  iv.  466.  Where 
they  converge,  iv.  466. 

B 

Bacon,  Friar,  is  said  to  have  discovered  the  telescope,  ii.  428. 
His  superior  character  and  attainments,  ii.  429. 

Bacon,  Lord,  his  reflexions  on  the  philosophy  of  Aristotle,  i.  18. 
Discovered  the  elasticity  of  the  air,  i.  75,  His  observations 
on  the  senses,  i.  11.  On  the  tendency  of  true  philosophy, 
i.  281.  On  the  scriptures  and  the  creatures,  i.  281.  On  se- 
cond causes,  i.  282.  His  method  of  reasoning  on  natural  phi- 
losophy ;  his  novum  organum,  i.  1,2.  His  uncommon  me- 
rit, i.  2.  His  character  of  a  true  philosopher,  i.  3.  His  ac- 
count of  the  idols  of  the  mind,  i.  5.  The  idols  of  the  tribe, 
i.  5.  Idols  of  the  cave,  i.  12.  The  idols  of  the  market,  i. 
16.  The  idols  of  the  theatre,  i.  17.  His  account  of  different 
erroneous  systems;  the  sophistical,  ci.  17.  The  empirical, 
and  the  superstitious,  i.  18.  His  new  logic,  or  art  of  inter- 
preting nature,  i.  23.  His  comparison  of  natural  philosophy 
.  to  a  pyramid,  i.  33.  His  suggestions  for  a  history  of  winds, 
iv.  455. 

Balance,  its  properties,  iii.  241.  How  it  should  be  constructed, 
iii.  242.  Its  fulcrum,  iii.  244.  The  axis  to  be  placed  higher 
than  the  centre  of  gravity,  iii.  245.  Weights  to  be  used 
with,  iii.  247.   Helsham's  property  of  the  balance,  iii.  248. 

Balloons,  air,  iii.  399.  Dr.  Black  and  Mr.  Cavallo  disco\ered 
the  principle  of  them,  iii.  401.    Were  discovered  and  also 


GENERAL  INDEX.  565 

executed,  by  the  Montgolfiers,  with  rarefied  air,  iii.  401. 
The  firstf  ascended  from  Paris  with  M.  Pilatre  de  Rozier  and 
the  Marquis  d'Arlandes,  iii.  403.  Of  air  balloons  filled 
with  inflammable  air,  iii.  404.  Description  of  them,  iii.  404, 
An  account  of  different  excursions  made  in  them  by  Messrs. 
Charles  and  Roberts,  and  Mr.  Baldwyn,  iii.  406.  Will  rise 
from  only  the  rarefaction  of  common  air,  iii.  412.  Colleet 
many  vapours  in  the  atmosphere,  iii.  412.  Are  unmanage- 
able from  their  size,  iii.  413. 

Barometer  arose  from  the  Torricellian  vacuum,  i.  63.  The  man- 
ner of  filling  its  tube,  i.  66.  Was  first  used  by  Pascal  to  mea- 
sure mountains,  i.  67.  Applied  as  a  gage  to  the  air-pump,  i. 
79.  On  the  best  form  for  it,  iv.  413.  The  principal  requisites 
of  a  good  barometer,  iv.  413.  To  boil  the  mercury  in  the  ba- 
rometer tube,  iv.  414.  It  requires  a  gage  to  regulate  the  quan- 
tity cf  mercury  in  the  cistern,  iv.  416.  Is  influenced  by  heat 
and  cold,  iv.  416.  Of  the  portable  barometer ;  how  to  use  it, 
iv.  419.  Its  defects,  iv.  419.  Of  the  best  portable  barometer, 
iv.  420.  How  to  use  it,  iv.  421.  Of  the  scale  of  correction  of 
the  excesses  of  heat  and  cold,  and  their  influence  on  the  baro- 
meter, iv.  421.  M.  de  Luc's  remarks  on  them,  iv.  415,479. 
The  variations  diminish  as  you  approach  the  equator,  iv.  480. 
Remarks  from  the  barometer,  iv.  481. 

Battery,  electrical,  iv.  3 1 3.  Cautions  in  the  using  of  it,  iv.  313.  Ef- 
fects produced  by  it,  iv.  314 — 316. 

Battering  rams  of  the  ancients,  iii.  99. 

Bell,  the  manner  of  its  sounding  explained,  i.  209. 

Birds,  the  manner  of  their  flying,  i.  185  ;  iii.  1 13.  Provision  made 
for  this,  in  the  air  passing  from  their  lungs  into  every  part  of 
their  bodies,  i.  202. 

Black,  Dr.  his  doctrine  of  latent  and  sensible  heat,  i.  305.  His 
experiments  on  the  melting  of  ice,  i.  309.  His  discoveries  of 
airs,  i.  432. 

Blindness,  calamity  of,  ii.  250. 

Blood  is  purified  in  the  lungs,  and  nourished  by  air;  receives  its 
Vermillion  colour  from  vital  air,  i.  464. 

Boerhaave,  his  idea  of  fire,  i.  257. 

Bones,  their  great  strength,  iii.  301. 

Boyle,  Hon.  Mr.  his  distinguished  character,  i.  44. 

Breezes,  land  and  sea,  an  account  of  them,  iv.  461. 

Buffon  by  plane  mirrors  burnt  planks  at  a  distance,  ii.  206. 

Burning  glasses,  or  convex  lenses,  collect  the  rays  of  the  sun,  ii. 
172.  Were  known  to  the  ancients,  ii.  172.  Effects  produced 
by  M.Tschirnhausen's  burning  glass,  ii.  172.  Mr.  Parker  ob- 
served a  rotatory  motion  in  the  rays  at  the  focus  of  his  burn- 
ing glass,  ii.  174. 


Camera  obscura,  its  construction  and  use)  ii.  175.     Observations 
upon  it,  ii.  179. 


566  GENERAL  INDEX. 

Cataract. in  the  eye,  how  caused,  ii.  302.  Means  used  to  disperse 
it,  ii.  303. 

Catoptrics,  on  the  laws  of  reflexion  of  light,  ii.  135. 

Centre  of  the  solar  system,  iv.  241. 

Charcoal,  reasonings  upon  experiments  made  with  it  for  and 
against  phlogiston,  i.  5  14.  M.  Lavoisier's  mistakes,  from  con- 
sidering it  as  a  simple  principle,  i.  5  18. 

Chimnies.  High  chimnies  draw  best,  i.  103.  Causes  of  chimnies 
smoking,  i.  104.  The  want  of  a  fresh  current  of  air,  i.  104. 
Chimnies  too  large,  i.  105.  Funnels  too  short,  i.  106.  The  ac- 
tion of  two  chimnies  on  each  other,  i.  107.  The  position  of  the 
room  door,  i.  107.  Communication  of  funnels,  i.  107.  Nar- 
rowness of  funnels,  i.  108.  Low  situation  of  the  house,  i.  108. 
Violence  of  winds,  i.  108.  Chimnies  modern  inventions,  i. 
1 1 1. — See  Air,  Fire.  Count  Rumford's  improvements  on,  i. 
153.  Common  fire-places  capable  of  improvement,  i.  153. 
All  smoky  chimnies  may  be  cured,  i.  154.  Sketches  of  his 
improvements,  i.  160.  His  practical  directions  for  workmen, 
i.  163.    His  directions  for  laying  out  the  work,  i.  171. 

Clarke,  Dr.  his  idea  of  the  cause  of  motion,  iii.  216. 

Clocks  differ  in  different  degrees  of  heat,  i.  264.  The  principle! 
on  which  they  are  constructed,  iii.  191.  Their  irregularities, 
iii.  192. 

Clouds,  indications  of  the  weather  from  them,  iv.  483. 

Coals,  objections  against  them  at  first,  i.  111.  The  smoke  from 
them  considered  by  the  Editor  as  injurious  to  the  atmos- 
phere of  cities,  i-  111. 

Cohesion  is  produced  by  fire,  i.  278. 

Cold,  artificial,  accounted  for,  i.  312.  Cold  is  produced  by  the  ab- 
sorption of  fire,  i.  338.  It  depends  on  the  degree  and  the  ra- 
pidity of  the  evaporation,  i.  342. 

Cold  is  the  sensation  caused  by  fire  passing  from  one  body  to 
another,  i.  285.  Effects  of  extreme  cold  on  the  animal  frame, 
ii.  44.  Produces  great  sierility,  ii.  44.  It  is  not  extreme  cold, 
but  humidity,  which  is  fatal  to  plants,  ii.  49.  The  extreme 
degrees  of  cold,  ii.  5  3.  On  the  sources  of  cold,  iv.  468.— 
See  Fire,  Evaporation. 

Collision,  a  means  of  exciting  fire,  i.  403.  Effects  produced  by 
this,  i.  404.     Its  use  in  New  Holland,  i.  405. 

Colours  differently  absorb  the  rays  of  heat,  i.  55  1.  The  colouring 
substance  is  formed  by  the  agency  of  light  on  the  vegttabies, 
i.  381.  On  different  teints  of  the  rays  of  light}  "■  S26-  sir. 
Isaac  Newton's  theory  of  light  and  colours,  ii.  326 — See 
Newton,  The  seven  colours  exhibited  by  the  prism,  ii.  329. 
The  order  of  the  colours,  ii.  330.  The  rays  of  different  co- 
lours are  of  different  refrangibility,  ii.  332.  The  colours  of 
the  rays  are  not  changed  by  refraction,  ii.  333,  341.  The 
due  mixture  of  the  primary  colours  produces  whiteness,  ii. , 
340.  Illustrated,  ii.  341.  The  similarity  between  the  seven 
primary  colours  and  the  seven  notes  in  the  scale  of  mtiMC, 
ii.  344.     Different  experiments  on  colours  with  the  prism* 


GENERAL  INDEX.  567 

ii.  345.  Illustrated  by  the  phenomenon  of  the  rainbow,  ii. 
3  lb.  The  different  colours  appear  under  different  angles,  ii. 
35  1.  The  different  colours  observed  in  a  soap  bubble,  ii.  356, 
360.  On  the  circles  seen  in  glass  plates,  ii.  357.  Different 
colours  exhibited  by  reflected  or  by  transmitted  light,  ii.  358. 
The  colours  of  bodies  depend  in  some  degree  on  the  thick- 
ness of  the  particles  that  compose  them,  ii.  361.  The  colours 
of  different  bodies  arise  from  their  reflecting  one  colour,  and 
imbibing  all  the  rest,  ii.  365.  The  colours  tinging  shadows 
in  summer  explained,  ii.  368.  The  colours  of  the  atmosphere, 
clouds,  kc.  ii.  369.  The  excellency  of  the  colours  used  by 
the  ancients,  ii.  370.  Mr.  Delaval  found  that  the  tinging 
matter  of  all  vegetables  was  always  black  when  viewed  by 
reflexion,  ii.  372.  The  colouring  particles  of  bodies  appear 
black  when  they  are  dense,  ii.  374.  When  the  colouring  mat- 
ter is  extracted  from  vegetables,  &c.  they  appear  white,  ii. 
376.  Colours  are  destroyed  by  the  rays  of  the  sun,  ii.  377. 
Animal  and  metallic  colours  are  produced  in  the  same  man- 
ner as  vegetable,  ii.  378.  The  production  of  colour  depends 
on  fire,  ii.  38  3.  Colours  are  emitted  from  some  phosphoric 
substances,  ii.  395.  They  are  produced  by  fire,  ii.  395.  Are 
not  sensations,  but  secondary  qualities  of  bodies,  ii.  422. — 
See  Light,  Fire, 

Colure,  equinoctial  and  solstitial,  ii.  491. 

Combustion,  an  effect  of  fire,  i.  356.  The  requisites  for  combus- 
tion, i.  358.  In  combustion  the  pure  air  is  converted  into 
fixed  air  and  aqueous  vapours,  and  gives  out  its  fire,  i.  424. 

Comets  are  regular  parts  of  one  great  system,  iv.  207.  Their  use 
unknown,  and  the  knowledge  of  them  imperfect,  iv.  207. 
Their  number,  orbits  and  motion,  velocity  and  size,  iv.  208. 
Their  form  and  tails,  iv.  209. 

Conductors,  and  non-conductors,  of  electricity,  iv.  263. 

Congelation,  phenomena  of,  explained,  i.  310. 

Constellations. — See  Stars. 

Copernicus,  his  system,  iv.  1.  Was  probably  known  to  the  an- 
cients, iv.  2.  A  view  of  it,  iv.  4.  The  truth  of  his  system 
proved  by  the  planetarium,  iv.  136. 

Crawford,  Dr.  his  excellent  treatise  on  animal  heat,  i.  415,423; 
and  on  combustion,  i.  425. 

Creation,  remarks  on  it,  iv.  370.  The  order  of  it,  iv.  371.  Is  a 
theatre  for  the  divine  goodness,  iv.  373. 

Cruelty  to  animals  justly  condemned,  i.  190. 

Crystallization  explained,  i.  400. 

CuJ./iing,  the  nature  and  operation  of  it,  i.  54. 


Dalton,  Mr.  his  account  of  the  aurora  borealis,  iv.  465.    His  re- 
marks on  the  weather  from  the  barometer,  iv.  481. 
Darkness  at  our  Saviour's  crucifixion  supernatural,  iv.  107. 
Day,  astronomical  or  solar,  iv.  123.  Sidereal,  iv.  123. 


568  GENERAL  INDEX. 

Delaval,  Mr.  his  experiments  on  the  permanent  colours  of  opake 
bodies,  ii.  370.  He  found,  that  in  transparent  coloured  sub- 
stances the  colouring  matter  does  not  reflect  any  light,  ii. 
372.  An  account  of  his  experiments,  ii.  373.  He  concludes 
that  the  colouring  particles  do  not  reflect  any  light,  but  that 
objects  are  reflected  by  a  medium  diffused  over  the  surfaces 
of  the  plates,  ii.  374.  His  observations  on  the  colours  of 
animal,  earthy,  and  metallic  bodies,  ii.  377.— See  Colours, 
Light, 

Descartes,  his  error  concerning  the  universe,  iii.  215. 

Digits,  iv.  94.  - 

JDiodorus  Siculus,  his  weak  idea  concerning  the  speech  of  men  at 
the  beginning,  i.  200. 

Diofitrics,  or  the  laws  of  refraction,  ii.  135. 

Discoveries  of  printing,  mariner's  compass,  gunpowder,  ii.  389. 

Dista?ice,  the  appearance  of,  depends  on  the  brightness  of  ob- 
jects, ii.  310.  On  the  number  of  intervening  objects,  ii-  312. 
Different  degrees  suggested  by  different  apparent  magni- 
tudes of  objects,  ii.  313. 

Diving  bell,  description  of  Dr.  Halley's,  iii.  416.  The  improve- 
ment of  this  by  Tried  wald,  iii.  418;  by  •Mr.  Smeaton,  iii. 
418. 

Diving  chest,  invented  by  Mr.  Smeaton,  iii.  418. 

Dollond,  Mr.  John,  the  inventor  of  the  achromatic  telescope,  ii. 
463.  The  principles  on  which  it  is  founded,  ii.  465.  The 
discovery  attributed  to  Euler,  ii.  467.  The  invention  also 
ascribed  to  Mr.  Hall,  ii.  468. 


E 

Earth,  the  equatorial  diameter  greater  than  the  polar  diameter, 
iii.  45.  Its  size,  motion,  distance,  &c.  iv.  19.  Its  revolution  ; 
its  figure,  iv.  40.  The  proofs  of  this,  iv.  40.  Its  diurnal  mo- 
tion, iv.  43.  The  reasons  for  this,  agreeable  to  analogy,  iv. 
45.  The  phenomena  arising  from  this  diurnal  motion,  iv. 
45.  Is  half  illuminated  at  a  time,  iv.  46.  Of  the  earth's  an- 
nual motion,  iv.  49.  It  partakes  of  various  degrees  of  heat 
and  cold,  iii.  62.  The  earth  enlightens  the  moon,  iv.  90.  The 
shadow  of  the  earth  forms  a  cone,  iv.  93.  The  eclipses  of  it, 
iv.  99.  Its  monthly  motion  about  the  common  centre  of  gra- 
vity between  that  and  the  moon,  iv.  182.  The  matter  of 
earth,  how  formed,  iv.  433.  The  use  of  the  earth,  iv.  434. 
The  magnetism  of  the  earth,  iv.  455.  Earth  a  source  of 
heat,  iv.  467.  Distance  from  it  a  cause  of  cold,  iv.  468.  The 
excellency  of  its  distribution  into  seas  and  mountains,  iv. 
474. 

Ebullition  is  caused  by  the  action  of  fire  on  the  bubbles  of  air  m 
any  fluid,  i.  329. 

Echo,  nature'of,  i.  218.  Explanation  of  its  effects,  i.  220. — Sec 
Sound, 


GENERAL  INDEX.  569 

£clipses  were  formerly  superstitiously  regarded,  iv.  91.  Are  ex- 
plained, iv.  92.  Of  the  moon,  iv.  92.  Eclipses  total  and  cen- 
tral, iv,  94.  Of  their  limits,  iv.  104.  Of  their  periods,  iv. 
108. 

•Ecliptic,  the  sun's  annual  path,  iii.  47 S.  The  obliquity  of  this, 
iii.  479.  Is  divided  into  twelve  signs  of  thirty  degrees  each, 
iii.  490. 

Ecles,  Mr.  his  system  of  electricity,  iv.  264. 

Eggs,  incubation  of,  greatly  assisted  by  means  of  air,  i.  180. 

Elasticity  (see  Air,  Water)  is  caused  by  fire,  i.  81.  Elasticity  of 
bodies,  iii.  202.  Supposed  to  lie  caused  by  motion,  iii.  203. 
Its  effects  in  different  handy-works,  iii.  210.  In  gunnery  and 
rockets,  iii.  2  10. 

Electricity  first  discovered  in  amber,  iv.  259.  The  uses  derived 
from  it,  iv.  259.  Is  a  fluid  universally  diffused,  iv.  260. 
Electrical  appearances,  iv.  261.  Electricity  is  vitreous  or 
resinous,  iv.  261,268,  281,  298,  299,  367,  369,  374.  These 
are  two  distinct  and  active  powers,  iv.  265.  They  exist  to- 
gether conjoined,  in  their  natural  state,  iv.  265.  Electricity 
is  from  the  separation  from  these  two  powers,  iv.  265.  268. 
Electrical  atmospheres,  iv.  267.  Of  the  electrical  machine, 
and  its  mode  of  action,  iv.  267,  269.  Cautions  in  using  it, 
iv.  271,  272.  Conductors  must  be  insulated  before  they  are 
electrified,  iv.  269.- The  brilliancy  of  the  spark  depends  on 
tiie  pressure  of  the  atmosphere  and  medium,  iv.  269,  288. 
The  solar  and  electric  fluid  are  probably  the  same,  iv.  272. 
On  the  momentum  of  its  force,  iv.  272.  On  its  attraction 
and  repulsion,  iv.  273.  On  the  afflux  and  efflux  of  the  two 
powers,  iv.  276.  Gives  a  rotatory  motion  to  small  light 
balls,  iv.  27S.  The  electric  fluid  is  universally  disseminated, 
and  in  continued  action,  iv.  280.  The  smallest  motion  in 
nature  disturbs  its  equilibrium,  iv.  281.  Observations  on 
Franklin's  system  of  electricity,  iv.  282.  On  the  electric 
•  spark,  iv.  285.  On  the  use  of  points,  iv.  286.  The  electric 
spark  will  fire  spirits  of  wine,  iv.  289.  The  sparks  are  of 
different  colours,  iv.  290,  306.  Motions  produced  by  elec- 
tricity, iv.  290.  Promotes  evaporation,  iv.  292.  Is  a  sub- 
ject of  general  curiosity,  iv.  293.  To  ascertain  the  quantity 
of  the  electric  matter,  iv.  296.  The  powers  are  reciprocally 
exchanged,  iv.  301.  It  melts  wires,  beginning  in  the  mid- 
dle, iv.  305,  316.  The  electric  matter  is  only  luminous  in  a 
divided  state,  iv.  303.  It  perforated  a  quire  of  paper  in. 
opposite  directions,  iv.  315.  Is  discovered  in  rain,  hail, 
snow,  iv.  322.  The  immense  quantity  of  it,  iv.  325.  Is 
real  matter,  proved,  iv.  331.  Its  resemblance  to  fire  and  light, 
iv.  331,  332.  It  produces  heat,  iv.  333;  and  accelerates 
evaporation,  iv.  334.  Is  communicated  by  the  same  sub- 
stances which  communicate  heat,  iv.  334.  Electricity  i;> 
procured  by  beat  and  liquifaction,  iv.  335.  Raises  the  ther- 
mometer, iv.  336.  The  electric  state  of  the  air  in  Russia, 
VOL.  IV.  4F. 


570     %  GENERAL  INDEX. 

iv.  338.  Heat  in  summer  becomes  electric  fluid  in  winter, 
iv.  338.  Luminous  experiments,  iv.  339.  The  similar  ef- 
fects on  the  solar  and  electric  light,  by  different  media,  iv. 
344.  On  animal  electricity  ;  electricity  the  principal  of  ani- 
mal heat  and  motion,  iv.  354.  The  electric  fluid  and  the 
solar  fire  are  the  same,  iv.  3  5  8.  Its  agency  in  animated 
nature,  iv.  3j9.  Its  influence  on  health 'and  our  feelings,  iv. 
360.  Remarks  on  animal  electricity,  iv.  360.  Electricity 
may  he  the  same  as  the  animal  spirits,  iv.  362.  Experiments 
-and  results  from  animal  electricity,  iv.  365 — 368.  Remarks 
.  on  atmospherical  electricity,  iv.  474.  Its  diurnal  variations, 
iv.  476. — See  Sun,  J' ire. 

Electrometer,  that  described  by  Mr.  Bennet,  iv.  280.  Experi- 
ments with  it,  iv.  28  1,  475. 

Elements^  active  and  passive,  iv.  354.  The  importance  of  this 
agency,  iv.  405. 

Engines,  constructions  of  different  sorts,  iii.  272.  Compound  en- 
gines, iii.  273.     How  to  compute  their  powers,  iii.  374. 

Equator,  of  the,  iii.  464,  488. 

Equatorial  instrument  or  universal  sun-dial,  description  of,  iv.  180. 

Equinoxes,  precessions  of,  iv.  12  1. 

Ether,  by  evaporation,  is  capable  of  freezing  water,  ii.  5  1.  Re- 
duced mercury  29°  below  the  freezing  point,  ii.  52. 

J'.vajioration  an  effect  of  fire,  i.  323.  Different  effects  resulting 
from  it,  i.  32  7,  339.  Cools  liquors,  i.  330.  Is  the  same  in 
open  air  and  in  vacuo,  i.  332.  Evaporation  of  ether  freezes 
water,  i.  310.  Effects  of  it,  i.  543.  Produces  ice  in  the  East 
Indies,  i.  342.  lis  effects  on  the  health  of  the  body,  i.  343. 
Spontaneous  evaporations  are  also  caused  by  fire,  i.  347. 
And  are  assisted  by  motion,  i.  348.  Evaporation  proceeds 
from  grass,  vegetables,  trees,  and  shrubs,  i.  34  9.  In  a  cer- 
tain degree  it  contributes  to  health,  i.  350.  Eaws  of  evapo- 
ration by  M.  de  Luc,  ii.  77.  Is  a  dissolution  of  water  by  fire, 
iv.  435.  Is  a  great  source  of  cold,  iv.  469.  Remarks,  on 
evaporation,  iv.  4  70. — See  Fire,  Water. 

Eudiometer  tube,  and  measure,  i.  437* 

Exhaustion,  successive  degrees  of,  i.  132.    ' 

Experimentalist,  character  of,  i.  175 — 177. 

Eye,  greatly  assisted  by  optical  instruments,  ii.  133.  Can  be 
adapted  to  very  different  degrees  of  light,  and  size  of  objects, 
ii.  149.  Benefits  derived  from  it,  ii.  250.  Short  descrip- 
tion of  the  eye  and  its  various  parts,  external  and  inter- 
nal, ii.  25  1.  Its  orbit,  brows,  ii.  251.  Of  the  two  eye-lids, 
ii.  252,  254.  Of  the  lachrymal  gland,  in  254.  Of  the 
muscles  of  the  eyes,  ii.  255.  Of  the  motions  of  the  eye,  ii. 
257.  Of  its  globe,  ii.  258.  Of  its  coats,  sclerotica,  choroides, 
ii.  259.  Of  the  iris,  ii.  260.  The  pupil,  ii.  260.  Of  the  retina, 
ii.  262.  Of  the  optic  nerve,  ii.  263.  The  advantages  derived 
from  two  eyes,  ii.  2  56.  Of  the  three  humours,  ii.  263. 
The  aqueous,  ii.  264.     The  crystalihe,  ii.  254.     And  the 


GENERAL  INDEX.  571 

vitreous,  ii.  265.  Of  theligamentum  ciliafe,  ii.  266.  Of  the 
artificial  eye,  ii.  270.  The  different  degrees  of  sensation  in 
different  people,  ii.  279.  Distinct  vision  formed  on  the  re- 
tina and  near  it,  ii.  282.  The  eye  accommodates  itself  to 
different  distances,  ii.  285,  305*  Illustrated,  ii.  286.  How 
accounted  for  by  various  authors,  ii.  287.  The  eye  sees 
best  when  surrounded  by* darkness,  ii.  291.  Is  enabled  (o 
see  with  a  very  small  quantity  of  light,  ii.  292.  Of  the  de- 
fects of  the  eye  ;  of  the  long-sighted,  ii.  294.  Of  the  acciden- 
tal conformations  of  the  eye  by  habit,  ii.  295.  Rales  for  pre- 
serving the  sight,  ii,  298.  Maladies" 6f  the  eyes,  how  occa- 
sioned, ii.  299.  Two  remarkable  cases,  ii.  300.  Of  couched 
eyes  ;  require  convex  glasses,  ii.  303.  Its  powers  are  limit- 
ed, ii.  310.  Mistakes  made  by  those  who  have  but  one  eye, 
ii.  310.  The  analogy  between  the  eye  and  the  understanding, 
ii.  316.  Reflexions  on  the  wonderful  powers  of  the  eye,  ii. 
S23.  Lyes  of  animals  differently  phosphoretic,  ii.  392. — See 
Light* 


Filtration,  Mr.  Peacock's  new  apparatus  for,  ii.  29. 

Fire,  produces  elasticity,  i.  81.  Fire  and  air  are  different  condi- 
tions of  the  same  elementary  matter,  i.  242.  Plato's  idea 
of  it,  i.  245.  The  different  opinions  entertained  of  it  by  the 
ancients  and  moderns,  i.  249.  Proved  to  be  a  real  material 
substance,  acting  in  a  fluid  form,  i.  250,  256.  Is  not  creaU 
ed  by  motion,  i.25l.  Is  the  cause  of  heat,  i.  255.  Absolute 
heat  and  relative  heat,  i.  256.  Boerhaave's  idea  of  fire;  is 
universally  diffused,  i.  256.  Penetrates  all  bodies,  i.  257. 
Continually  tends  towards  an  universal  equilibrium,  except 
in  animal  bodies,  i.  258,262.  Is  differently  conducted  in  dif- 
ferent bodies,  from  their  different  capacities  of  retaining  it, 
i.  258.  Is  retarded  by  soft  substances,  i.  260.  Is  rapidly 
conveyed  by  fluids,  i.  260.  Dilates  all  bodies,  i,  263.  Liberat- 
ed, manifested,  or  thermometric  fire,  i.  264.  Different 
metals  are  differently  expanded  by  fire,  i.  265.  The  very 
great  force  of  fire  in  expanding  them,  i.  273.  Fire  the  grand 
agent  in  nature  to  dissolve  and  to  unite  all  things,  i.  276. 
Proved  to  be  the  cause  of  cohesion,  i.  278.  Acts  in  two 
different  modes,  by  dilating  and  compressing,  i.  279.  Is  the 
cause  of  cold,  which  is  occasioned  by  fire  passing  from  one 
body  to  another,  i.  285.  Latent  fire,  i.  290.  A  method  to 
discover  the  quantity  of  fire  contained  in  different  bodies,  i. 
292.  The  action  of  fire  depends  on  the  re-action  of  the  in- 
cumbent air,  i.  300.  Fire  is  the  cause  of  fluidity,  i.  303. 
Is  extricated  whenever  water  is  congealed,  i.  511.  Was 
formerly  made  an  object  of  worship,  i.  322.  Is  a  fluid  never 
at  rest,  i.  322.  Is  the  cause  of  vapours,  i.  323.  Its  extreme 
violence  when  confined,  i.  £23.  Is  the  cause  of  ebullition,  i. 
327.    Much  fire   is   absorbed   in  vapours,  i.    333.   Fire   ex- 


572  GENERAL*  INDEX. 

panels  or  separates  the  parts  of  water,  i.  335.  Specific  fire 
of  any  body,  i.  337.  Fire  is  differently  absorbed  by  different 
coloured  substances,  i.  351.  See  Colours,  Fire  maintains  its 
dimensions,  although  greatly  pressed  by  the  air,  i.  361.  The 
genera)  effects  it  produces  acting  on  different  suhstanci!--  i. 
384.  Man  alone  understands  the  use  of  fire,  i.  389.  Its  effect 
on  gunpowder,  i.  393.  On  fulminating  powder,  i.  395,  and 
fulminating  silver,  i.  396.  Produces  solutions,  i.  397.  Crys- 
tallization, i.400.  Clarification,  i.  401.  Odours,  i.  402.  Fire  is 
collected  by  collision,  403.  By  fermentation,  i.  406.  By  pi, "re- 
faction, i.  407.  By  the  action  of  the  solar  rays,  407.  B>  the 
resistance  of  the  parts  of  a  body  on  which  light  falls,  i.  409. 
"Methods  of  increasing  or  diminishing  the  action  of  fire,  i. 
412.  Different  bodies  contain  different  quantities  of  fire,  i. 
417.  Fire  constantly  tends  to  diffuse  itself  over  al)  hoc  its, 
till  they  are  brought  to  the  same  temperature,  i.  417.  It  is 
contained  in  all  bodies  at  the  common  temperature  of  thi 
atmosphere,  i.  418.  Atmospherical  air  contains  a  great 
quantity  of  fire,  i.  423.  Fire  is  distinguished  into  diffusible 
and  constitutional,  i.  426.  Its  agency  in  vegetation,  i.  429. 
Causes  the  grand  differences  in  bodies,  i.  441.  Is  distin- 
guished into  sensible  heat,  the  latent  fire  of  fluidity,  and  the 
latent  fire  of  elasticity,  441..  Opinion  of  the  Pythagoreans 
on  fire,  iv.  359.  Fire  is  imponderable,  ii.  58.  Fire  is  the 
agent  of  all  dissolution,  iv.  69.  Great  quantity  of  fire  is  re- 
quisite to  raise  water  into  steam,  iv.  70.  Is  subject  to  the 
same  laws  of  inflexibility  and  refrangibility  as  the  rays  of 
light,  iv.  215.  Is  the  means  of  producing  colours,  iv.  383. 
Is  retained  in  many  bodies  under  the  form  of  heat  and  light, 
iv.  379.  Fire  is  necessary  for  producing  the  prismatic  co- 
lours, iv.  395.  The  similarity  between  fire  and  light,  iv.  413. 
Is  the  most  important  agent  in  nature,  iv.  346.  Natural  life 
depends  on  its  activity,  iv.  346,  352.  Is  the  active  element 
within  all  bodies,  iv.  35  1.  Has  been  called  by  different 
names,  iv.  350.  Is  every  where  present,  iv.  351.  Is  only  an 
instrumental  cause,  iv.  353.  Its  effects  on  the  heart,  iv.  356. 
Its  influence  in  the  animal  economy,  iv.  357.  On  this  de- 
pends the  health  and  activity  of  animals  and  plants,  iv.  457. 
Theophrastus's  opinion  on  fire,  iv.  359.  The  various  effects 
produced  by  it,  iv.  360.  Elementary  fire,  or  the  matter  of 
light  first  formed,  iv.  371.  Fire,  its  expansive  power,  iv.  423. 
— See  Light,  Heat,  Colours, 

Fishes,  the  manner  of  their  breathing,  i.  2C3.  Their  manner 
of  swimming,  iii.  1 17. 

Flame,  an  account  of,  i.  365.  On  the  flames  of  candles  and  lamps, 
i.  366. 

Fluids  of  the  least  density  expand  most  by  heat,  i.  275.  Are 
caused  by  a  degree  of  fire,  iii.  342.  Are  not  to  be  explained 
on  mechanical  principles,  iii.  342.  Their  gravity  in  proprio 
loco,  iii.  343.  The  parts  of  a  fluid  gravitate  independently  of 


GENERx^L  INDEX,  s'J 

each  other,  iii.  3-15.  The  surface  of  fluids  is  on  a  level,  ill.  34  5. 
Their  pressures,  iii.  346.  Fluids  press  in  all  directions,  iii. 
347.  On  their  action  against  vessels  of  different  sizes  in 
which  they  are  contained,  iii.  348,  351.  The  bydrpstatic  pa- 
radox, iii.  353.  On  the  action  of  fluids  on  bodies  immersed 
in  them,  iii.  357.  Fluids,  when  deep,  press  equally  on  all 
sides  of  a  body,  iii.  360.  Bodies  sink  if  specifically"  heavier, 
N  or  swim  if  lighter  than  the  fluid,  iii.  364.  If  of  the  same  gra- 
vity, will  remain  in  any  part  of  the  fluid,  iii.  355.  The  Weight 
lost  by  a  solid  immersed  in  a  fluid  is  communicated  to  the 
fluid,  iii.  371.  Their  spouting  through  small  orifices,  iii.  420. 
The  velocity  is  as  that  cf  a  heavy  body  falling  from  an  equal 
height,  iii.  421.  The  quantities  di  charged,  iii.  422.  Thewatet 
contracts  in  flowing  out,  iii.  42.3.  Of  the  discharge  cf  fluids 
through  additional  tubes,  iii.  428.  Of  jets  d'eau  ;  they  ne- 
ver rise  to  the  height  of  the  reservoir  iii.  430.  The  position 
of  the  ajutage,  iii.  430.  Of  the  motion  of  fluids  in'  conduit 
pipes,  iii.  443.  The  friction  retards  the  velocity,  iii.  444.  If 
the  pipes  be  curved,  the  discharge  is  less,  iii.  445.  Of  the 
vibratory  motion  in  fluids,  iii.  448.  Of  the  oscillatory  motion 
of  waves,  iii.  449.  Of  the  resistance  of  fluids,  whether  at  rest 
or  in  motion,  iii.  450.  Mistakes  of  the  moderns  concerning 
this  subject,  iii.  45  1.  Their  ignorance  of  it,  iii.  452. — See 
Fire,  Neat, 

Fluids,  elastic,  are  combinations  of  fire  with  given  substances,  i. 
441.  M.  Lavoisier's  mistakes  concerning  them,  i.  443.  M. 
de  Luc's  observations  on  them,  i.  448.  The  difference  be- 
tween these  and  vapours,  i.  441. 

Fluidity  is  caused  by  heat,  i.  303,  absorbed  or  combined  with  the 
fluid  substance,  i.  306. 

Focus,  real  or  imaginary,  ii.   15  1. 

Forces,  of  the  composition  or  resolution  of  forces,  iii.  107. 

Forces,  deflecting,  of,  iv.  226.     Central  forces,  iv.  226. 

Fountain  by  condensed  air,  description  of,  i.  116. 

Frame,  human,  imbides  water  from  the  atmosphere  to  supply  the 
animal  moisture,  ii.  17.  The  great  quantities  of  this  daily 
exhaled,  ii.  17. 

Franklin,  Dr.  his  system  of  electricity,  iv.  282.  Observations  on 
his  theory,  iv.  283.  His  principles,  iv.  2S5.  The  negative 
and  positive  electricity  denied,  iv.  282. 

Friction,  iii.  290.  Is  a  uniformly  retarding  force,  iii.  292.  It 
does  not  increase  equally  with  the  quantity  of  matter,  iii. 
292.  The  smallest  surface  has  the  least  friction,  iii.  293. 
General  observations  on  frictions,  iii.  295. 

Fulminating  fiowders,  their  nature  and  operations,  i.  395. 

Furs,  their  property  of  retaining  heat,  i.  261. 


• 


5/4  GENERAL  INDEX. 


Gages  for  the  air-pump,  i.  139.  Their  very  different  results,  i. 
140.   Explained  by  Mr.  Cavendish,  i.  141. 

Galen,  his  just  remark  on  praise,  i.    194. 

Galileo  couid  not  discover  the  reason  why  water  coulq*  not  be 
raised  by  a  pump  above  32  feet,  i.  64.  Discovered  the  te- 
lescope, ii.  430.  Applied  the  pendulum  to  measure  time, 
iii.   178. 

Gas,  inflammable  carbonic,  an  account  of,  i.  502. 

Gas,  muriatic  acid,  its  nature  and  properties,  i.  5G3. 

Gas,  muriatic  dcfihlogisticated,  an  account  of  its  effects,  i.  504, 
Destroys  all  vegetable  colours,  i.  504. 

Gases,  permanently  elastic  fluids,  i.  441.  Are  of  different  kinds, 
i.  49T. 

Gorgium  Sidus  was  discovered  by  Dr.  Herschel,  iv.  29.  Its  size,- 
distance,  revolution,  iv.  29.  It  moons,  iv.  30.  The  length  of  its 
year,  iv.  138. 

Glass,  its  various  uses,  ii.  163.     Different  kinds  of,  ii.  465. 

Qlobe,  7iew  terrestrial,  the  advantages  of  it,  iv.  161.  The  absur- 
dities resulting  from  the  old  ones,  iv.  1 62.  The  construction 
and  use  of  this  new  globe,  iv.  1 63.  How  to  rectify  this  globe, 
iv.  164.  New  celestial  globe,  iv.  165.  Editor's  vindication 
of  globes  mounted  in  the  common  manner,  iv.  176.  Advan- 
tages of  the  new-mounted  globes  shewn  by  him,  iv.  178, 
and  of  the  common  globe,  iv.  179. 

Glow-worm,  an  account  of  the  light  in  it,  iv.  365. 

God,  the  author  of  speech,  i.  199.  His  end  in  creating  the  uni- 
verse the  greatest  possible  good,  i.  243.  The  author  of  all 
good  to  man,  i.  282.  Is  clearly  indicated  by  final  causes, 
i.  315.  The  immensity  of  his  works,  i.  27.  Is  the  author 
of  nature,  ii.  244.  The  character  of  the  Saviour,  as  delight- 
ing to  communicate  wisdom,  ii.  371.  His  divinity  and  cha- 
racter, ii.  423.  The  infinity  of  his  works,  ii.  424.  Has  im- 
pressed on  matter  some  faint  characters  of  his  own  beauty, 
iii.  61.  The  ancients  thought  him  to  be  goodness  itself,  and 
truth  itself,  iii.  60.  The  perfection  of  his  wisdom,  iii.  74. 
A  hymn  to  his  praise  on  the  excellency  of  the  soul  of  man, 
iii.  78.  His  wisdom  and  power,  as  the  divine  mechanic  ; 
iii.  83  ;  displayed  in  the  natural  and  moral  world,  iii.  83. 
Time  cannot  be  predicated  of  him,  iii.  196.  God  the  origi- 
nal cause  of  all  motion,  iii.  223.  His  mercy,  wisdom,  and 
power  are  discoverable  in  his  works,  iii.  456.  He  alone  sees 
the  whole  of  nature,  iii.  457.  He  is  glorified  by  the  inhabi- 
tants of  innumerable  worlds,  iv.  212.  Has  probably  peopled 
all  the  planetary  worlds,  iv.  214.  Is  the  real  cause  who  go- 
verns the  mundane  system,  iv.  224.  The  knowledge  of  him  is 
the  most  excellent  wisdom,  iv.  254.  He  is  one,  Jesus 
Christ,  iv.  254.    His  universal  government,  iv.  255#     Is  in- 


GENERAL  INDEX.  575 

visible  to  us,  iv.  256.  Is  praised  by  all  things  which  he  has 
made,  iv.  256.  His  perfections  seen  in  his  works,  iv.  256.  The 
manner  and  the  end  of  creation,  iv.  373.  The  perfection  of 
his  word  and  work,  iv.  488. — See  Providence,  Nature* 
Man, 

ravlcjy  and  gravitation,  an  inquiry  whether  it  be  an  essential 
property  of  matter,  iii.  25.  Considered  as  a  fact,  iii.  35. 
The  proportion  in  which  the  force  of  gravity  decreases,  iii. 
40.  Gravity  acts  universally,  but  not  uniformly  in  all  places, 
iii.  41.  A  body  falls  about  an  hundred  and  ninety-three 
inches  in  a  second  of  time,  44.  The  difference  respecting 
gravity  resulting  from  different  positions  on  the  globe,  iii. 
45.  Gravitation  extends  to  the  planets,  iii.  45.  Reflexions 
on  gravity  as  a  law,  iii.  46.  Gravity  considered  as  a  resist- 
ing and  as  a  moving  power,  iii.  47.  The  weight  of  bodies  is 
not  in  proportion  to  their  quantities  of  matter,  iii.  48. 
Powers  acting  contrary  to  gravity,  iii.  49  ;  observed  in  plants 
and  light,  iii.  50.  Of  the  centre  of  gravity  in  bodies,  iii. 
146.  Of  the  centre  of  gravity  in  the  human  body,  iii.  150. 
Cautions  arising  from  it,  iii.  151.  Of  the  situation  of  the 
centre  of  gravity,  iii.  154.  To  find  the  centre  of  gravity  of  a 
trapezium,  iii.  155  ;  of  a  pyramid,  iii.  156.  General  observa- 
tions on  gravity,  iii.  161.  It  extends  to  the  moon,  iv.  230.  It 
produces  a  small  irregularity  in  the  motion  of  the  planets, 
iv.  234. 

Gravity,  specific,  of  bodies,  iii.  35S  ;  is  as  their  density,  iii.  358. 
is  measured  by  water,  iii.  359.  All  bodies  immersed  in 
fluids  lose  the  weight  of  an  equal  bulk  of  that  fluid,  iii.  467. 
How  to  obtain  the  specific  gravity  of  bodies,  iii.  473.  To 
find  the  specific  gravity  of  solids,  iii.  378.  If  heavier  or 
lighter  than  water,  iii.  380.  Of  fluids,  iii.  382.  Different 
methods  of  ascertaining  the  specific  gravity  of  fluids,  iii* 
393.  A  table  of  specific  gravities,  iii.  390.  Of  several 
fluids  in  summer  and  winter,  iii.  398. 

Green  colour,  the  universality  and  excellency  of  it,  ii.  324. 

Gunnery,  the  great  difference  between  theory  and  practice,  iii. 
166.  Mr.  Robbing's  application  to  this  art,  iii.  167.  The 
imperfections  in  this  art,  iii.  170. 

Gunpowder,  the  substances  which  compose  it,  i.  392.  Manner 
of  making  it,  i.  392.  Derives  its  force  from  vital  air,  i.  393.. 
An  estimate  of  its  expansive  force?  i.  395. 


H 


Hail  is  water  suddenly  congealed,  ii.  46. 

Hallcy,  Dr   his  weak  solution  concerning  the  saltness  of  the  sea, 

ii.  32.    His  hypothesis  to  explain  the  variations  of  the  needle. 

iv.  398. 
Hearty  its  situation  and  mutual  action  with  the  lungs,  i.  197. 


576  GENERAL  INDEX. 

Hearts,  of  different  creatures,  in  what  manner  affected  by  heat, 

iv.  355. 
Heat,  the  effect  of  fire,  i.  255.  Absolute  and  relative,  the  differ- 
ence between  them,  i.  25  6.  Is  conveyed  through  a  medium 
more  subtile  than  common  air,  i.  257.  Animal  heat  re- 
mains universally  the  same,  although  under  the  most  op- 
posite circumstances,  i.  2G3.  The  degree  of  heat  is  mea- 
sured by  the  thermometer,  i.  285.  Different  subjects  re- 
ceive different  degrees  of  heat,  i.  295.  The  progression  of 
heat' not  easily  ascertained,  i.  295.  The  relative  heat  in 
different  bodies  marked  by  Mr.  Jones,  i.  297.  Latent  and 
sensible  heat,  i.  305,  337.  More  than  eight  hundred  degrees  of 
heat  are  absorbed  in  steam,  i.  334.  Heat  counteracts  the 
influence  of  gravitation,  i.  387.  The  quantity  of  heat  is  di- 
minished by  the  change  it  undergoes  in  the  lungs,  i.  423. 
Light  and  heat  are  different  modifications  of  the  same  mat- 
ter,^. $90.  Without  heat  bodies  do  not  emit  light,  ii.  401. 
The  different  sources  of  heat,  iv.  466. — See  Sun,  Fire,  Ther- 
mometer. 

Hersckel,  Dr.  his  discoveries  of  new  stars,  ii.  427.  His  idea  of 
the  construction  of  the  universe,  iv.  192.  That  the  visible 
universe  is  only  a  nebula,  iv.  192.  Concerning  a  sidereal 
stratum,  iv.  193.  The  great  powers  of  his  telescope,  iv. 
196.  On  the  origin  of  nebulous  strata,  iv.  197.  Discovered 
volcanoes  in  the  moon,  iv.  203. — See  Stars,  Moon. 

Hipfiocrates,  his  admirable  observations  on  air,  i.  245. 

Hire,  M.  de  la,  his  experiments  on  the  distance  which  rainwater 
penetrates  into  the  earth,  ii.  25. 

Horizon,  iii.  -S62.  Its  uses,  iii.  484.  Is  divided  into  rational  and 
sensible,  iii.  485. 

Horses,  their  advantage  in  drawing  from  their  weight,  iii.  306. 

Horselcy,  Bishop,  his  conjectures  concerning  the  infinitude  of 
the  atmosphere  of  the  earth  and  the  planets,  i.  88. 

JTcw-ch'cle  of  the,  iii.  489. 

Hurricanes,  an  account  of  those  in  the  West  Indies,  iv.  463. 
The  signs  of  their  coming,  iv.  463. 

Hydraulics  treat  on  the  motion  of  fluids,  iii.  419. 

Hydrometer,  of  measuring  the  specific  gravity  of  fluids  by  it,  iii. 
389.  A  description  of,  iii.  391.  The  requisites  for  a  good 
one,  iii.  392. 

Hydrostatics,  their  nature*  iii.  338.  The  difference  between 
theory  and  practice,  iii.  339,  Our  ignorance  relative  to  se- 
veral particulars  on  this  subject,  iii.  339.  Aerostation  simi- 
lar to  hydrostatics,  iii.  409. 

Hydrostatic  balance,  its  use  in  determining  the  quantity  of  gold, 
&c.  iii.  376.  How  it  is  constructed,  iii.  377.  It  varies 
with  the  heat  and  colu  of  the  weather,  iii.  378.  How  to  find 
the  proportion  of  alloy  mixed  with  gold,  iii.  385. 

Hygrometer,  its  use:  several  substances  affected  by  the  moisture 
or  dryness  of  the  air,  iv.   431,    An  account  of  M.  de  Luc's 


GENERAL  INDEX.  ,77 

hygrometer,  iv.  431.  A  further  description  and  figure  of  it, 
by  the  Editor,  iv.  491.  The  good  effects  of  a  hygrometer, 
iv.  432.  The  discoveries  made  by  means  of  the  hygrome- 
ter, iv.  438. 

Hygroscopic  substances,  different  kinds  of,  iv.  436. 

Jiyfiothesesy  observations  on,  i.  89.  Conjectures  discover  no 
truths,  i.  90;  but  confirm  men  in  prejudice  and  ignorance, 
i.  91 See  Truth. 


I 


Ice  absorbs  all  lire  until  it  is  wholly  melted,  i.  293.  It  requires  a 
hundred  and  forty  degrees  of  heat  to  convert  it  into  water,  i. 
210.  Is  a  combination  of  water,  when  deprived  of  its  fire,  ii. 
38.  Freezing  is  promoted  by  air  and  by  agitation,  ii.  39.  A 
bit  of  ice  produces  instant  congelation  in  water  cooled  below 
the  freezing  point,  ii.  40.  Its  great  expansive  force,  ii.  41. 
The  vast  quantities  of  it  in  the  northern  seas,  ii.  43.  Ice  is 
continually  diminishing  by  the  action  of  the  air,  ii.  45.  The 
manner  of  rivers  freezing,  ii.  47.  The  great  strength  of  ice, 
ii.  48.  Frost  does  not  penetrate  deep  into  the  earth,  ii.  49. 
Ice  may  be  produced  by  the  evaporation  of  ether,  ii.  51. 
The  chemical  properties  of  ice,  ii.  58. — See  Fire,  Water* 

Idols  of  the  mind,  Lord  Bacon's  account  of  them,  i.  4.  Are  of  dif- 
ferent kinds,  i.  5.     • 

Ignition,  a  universal  effect  of  fire,  i.  355. — See  Fire. 

Impulse  the  only  material  cause  of  motion,  iv.  2  1 8. 

Ivgenhouz,  Dr.  first  discovered  the  power  which  plants  have  of 
emitting  vital  air,  i.  452.  Flis  experiments  confirmed  by 
others,  i.  455. — See  Light,  Air,  Vegetables. 

Insects,  the  manner  of  their  breathing  illustrated  in  the  larva  of 
the  musca  pendula,  i.  204. 

Instruments,  philosophical,  described  ;  general  remarks  on  them, 
i.  302.  Air-pump,  i.  38.  An  account  of  its  improvements, 
i.  127.  Philosophical  hammer,  i.  48.  Cupping,  i.  54.  Mag- 
deburg- hemispheres,  i.  55.  Common  pump,  i.  56.  Trans- 
ferrer of  air,  i.  56.  Fountain  of  command,  i.  57.  Anti-gug- 
gler,  i.  58.  Common  bellows,  i.  60.  Gage  to  the  air-pump, 
i.  69.  Bolt-head,  i.  76.  Smoke-jack,  i.  101.  Condensing 
engine,  i.  112.  Wind  or  air  gun,  i.  115.  Artificial  fountain 
by  condensed  air,  i.  116.  Common  pump,  i.  119.  Forcing 
pump,  i.  121.  Water-works  at  London  bridge,  i.  123.  Sy- 
phon, i.  123.  Tantalus's  cup,  i.  124.  Gages  for  the  air- 
pump,  i.  137.  Pear-gage,  i.  140.  Pyrometers,  i.  265.  Calo- 
rimeter, i.  321.  Wedgewood's  thermometer  for  ascertaining 
intense  degrees  of  heat,  i.  294.  Another,  invented  by  Mr. 
Jones,  i.  297.  Papin's  digester,  i.  324.  iftolipile  or  wind- 
ball,  i.  325.  Argand's,or  cylinder  lamps,  i.  363.  Pneumatic 
apparatus,  i.  434.  Eudiometer  tubes  and  measure*  i.  437. 
VOL.  IV.  4  F 


578  GENERAL  INDEX. 

Dr.  Nooth's  machine  to  impregnate  water  with  fixed  air,  i. 
485.  M.  Bettancourt's  contrivance  to  measure  the  force  of 
steam,  ii.  72.  Steam  engine,  ii.  73.  Inflammable  air  lamp, 
ii.  93.  Animated  optical  balls,  ii.  236.  The  boundless  gal- 
lery ;  the  magical  mirrors,  ii.  234.  Simple  camera  obscura, 
ii.  228.  Reflecting,  ii.  229.  Dioptrical  paradox,  ii.  232.  Op- 
tical paradox,  ii.  234.  Real  apparition,  ii.  239.  Optical  per- 
spective box,  ii.242.  Cylindrical  mirror,  ii.  243.  The  prism, 
ii.  329.  Telescopes,  ii.  255.  Microscopes,  ii.  478.  Atwood's 
friction  apparatus,  iii.  125.  Directions  for  construction,  iii. 
138.  Spirit  level,  iii.  148.  Plumb  line,  iii.  148.  Odometer 
or  way-wiser,  iii.  152.  Whirling  tables,  iii.  319.  Hydrosta- 
tic paradox,  iii.  353.  Hydrostatic  bellows,  iii.  355.  Hydro- 
meter, iii.  389.  Discharging  rods,  iv.  394.  Quadrant  elec- 
trometer, iv.  296.  Magic  picture,  iv.  31 1.  Spotted  bottle,  iv. 
3)2.  Thunder-house,  iv.  329. 

Jones,  Rev.  Mr.  his  observations  on  nature,  i.  176.  His  just  re- 
flexion on  the  origin  of  fire,  i.  405.  On  the  improvements 
in  philosophy,  iv.  310.  His  remark  on  the  electric  matter 
and  animal  spirits,  iv.  364.  His  observations  on  the  supe- 
riority of  the  northern  hemisphere  over  the  southern  in  se- 
veral particulars,  iv.  484. 

Iris* — See  Eye. 

Jupiter,  his  size,  distance,  revolutions,  iv.  25.  His  four  moons,  iv. 
26.  His  year,  and  motion  round  the  sun,  iv.  138.  His  belts, 
iv.  205.  Their  changes,  iv.  205. 


K 


Kepler,  his  laws  of  astronomy,  iv.  225. 

Knight,  Dr.  his  discoveries  in  magnetism,  iv.  386. 

K?ionvledge,  its  excellency,  i.  61. 


Language,  natural  and  artificial ;  language  was  not  invented  by 
man,  i.  199,  200. 

Latent  heat,  doctrine  of,  explained,  i.  305,  337,  414.  Is  of  two 
kinds,  of  fluidity  and  elasticity,  i.  378. — See  Phlogiston,  Fire, 
Heat, 

Lavoisier,  his  opinion  on  fire,  i.  256.  His  experiments  with  vital 
air,  i.  413.  His  mistakes  concerning  elastic  vapours,  i.  445 — 
446.  A  confutation  of  his  system  by  Mr.  Weiglib,  i.  507 — 
522. 

Laughter,  how  caused  ;  good  effects  of  moderate  laughter,  i.  201. 

Le?is,  may  be  formed  of  different  substances,  i.  409.  Various  ef- 
fects pro4uced  by  them,  i.  410. 

Lenses  of  various  sorts,  plano-convex,  plano-concave,  double  con- 
vex, double  concave,  concavo-convex,  ii.  163.  Their  different 


GENERAL  INDEX.  5T9 

properties,  ii.  165,  183.  Methods  to  find  their  focal  lengths 
by  experiments,  ii.  188.   The  properties  and  phenomena  of 
single  lenses,  ii.  192. 
Levelling,  the  principle  of  it,  iv.  42. 

Lever,  its  nature  and  properties,  iii.  229.     Levers  are  of  three 
kinds,  iii.  229—235.  Of  the  hammer  lever,  iii.  232.  Its  va- 
rious applications,  iii.  235 — 241.     Its  properties  applied  to 
various  subjects,  iii.  267. 
Leyden phial,  iv.  293.  How  to  charge  it,  iv.  294.    The  theory  of 
it,  iv.  296.     On  the  discharge  of  this,  the  two  electricities 
rush  into  union  from  opposite  directions,  iv.  304.  Different 
shocks  by  means  of  it,  iii.  304.     Confirmed  by  the  electric 
spark,  iv.  306.     The  two  powers  are  in  contrary  directions, 
iv.  307.  Various  effects  produced  by  it,  iv.  314. — See  Elec- 
s  tricity. 
Liberty,  singular  panegyric  en,  i.  390.  The  false  pretenders  to  it, 

i.  39. .  Genuine  liberty,  i.  392. 
Life,  or  the  animating  principle,  iii.  59.  The  analogy  between 
life  and  motion,  iii.  216 — 224.  Natural  life  depends  on  fire, 
iv.  346. 
Light  travels  at  the  rate  of  72,420  leagues  in  a  second,  i.  216.  Is 
the  mediating  substance  between  fire  and  air,  i.  242.  Light 
combined  with  fire  and  water  produces  an  aeriform  fluid,  i. 
447.  Light  extricates  vital  air  from  vegetables,  i.  452.  Rays 
of  light  are  extremely  minute,  ii.  129.  Its  operations  and 
analogy,  ii.  130.  The  advantages  of  it  to  man,  ii.  131.  Light 
a  property  of  fire,  ii.  132.  Light  is  a  material  real  substance, 
is  progressive,  may  be  stopped  and  diverted,  acts  on  all  bo- 
dies, ii.  134.  Light  moves  in  a  straight  line,  ii.  137.  Is  suc- 
cessive and  contemporary,  ii.  157.  The  rays  of  it  indefinitely 
small,  ii.  138.  They  carry  the  image  of  the  point  from  which 
they  proceed,  ii.  139.  Their  refiexibility  and  refrangibility, 
ii.  140.  In  different  mediums,  ii.  141.  The  light  of  the  moon 
is  300,000  times  fainter  than  the  light  of  the  sun,  ii.  146. 
The  quantity  of  light  decreases  as  it  recedes  from  the  radi- 
ant, ii.  148.  Light  is  suffocated  by  various  bodies,  ii.  148. 
A  table  of  the  quantity  of  light  dissipated  in  the  atmosphere, 
ii.  150.  Rays  of  light  are  parallel,  diverging  or  converging, 
ii.  151.  Are  reflected  before  they  touch  the  body,  ii.  198. 
Light  contracts  the  pupil  of  the  eye,  ii.  290.  Reflexions  on 
light,  ii.  323.  The  rays  of  light  are  not  homogenial,  ii.  327. 
The  rays  of  the  sun  consist  of  seven  different  coloured  rays, 
ii.  327.  The  compound  of  all  the  rays  exhibits  whiteness,  ii. 
328.  The  rays  are  of  different  refrangibility,  ii.  330.  Homo- 
genial  light  suffers  no  alteration  in  any  case,  ii.  334.  Rays 
which  differ  in  their  colour,  differ  also  in  their  refrangibi- 
lity, ii.  335.  Bodies  reflect  rays  of  one  colour,  and  transmit 
rays  of  another,  ii.  361.     The  rays  of  light  are  thought  to 


580  » GENERAL  INDEX. 

be  put  in  a  transient  state,  and  easily  reflected  and  trans- 
mitted, ii.  363.  This  Sir  Isaac  Newton  supposed  was  ow- 
ing to  the  vibrations  of  a  subtile  fluid,  ii.  363.  The  ana- 
logy between  the  reflexion  and  refraction  of  the  rays  of 
light,  ii.  365.  Is  imbibed  by  all  bodies,  except  water  and 
metals,  ii.  391.  Is  matter  moving-  in  a  straight  line  from  a 
body,  ii.  399.  Light  and  heat  are  different  modifications  of 
the  same  matter,  ii.  399 — 405.  Bodies  are  either  luminous 
or  illuminated,  ii.  400.  Without  heat  bodies  will  not  emit 
light,  ii.  401.  The  attractive  gravitating  matter  in  bodies 
has  no  power  to  resist  light,  ii.  405.  Light  is  acted  upon  by 
ooches  at  a  small  distance  by  attraction  and  repulsion,  ii. 
407.  The  rays  exhibit  three  fringes  of  coloured  light  round 
the  shadows  of  small  bodies,  ii.  109,  The  influence  of  light 
in  the  vegetable  kingdom  ;  it  produces  colours  and  smells, 
ii.  417.  Its  influence  on  animals,  ii.  417.  Its  effects  iu 
chemistry,  ii.  418.  Its  effects  on  colours  and  or.  wood,  ii. 
419.  The  opinions  of  the  ancients  concerning  light,  ii.  420. 
Questions  concerning  light,  ii.  493.  The  opinions  of  the 
ancients  concerning  it,  ii.  593.  Of  Plato,  ii.  493.  Its 
connexion  with  lire,  ii.  496.  And  with  electricity,  iv. 
332.  Its  energy  and  activity,  iv.  347.  Refraction  of,  iv. 
111.  Is  different  at  different  places,  iv.  112.  Effects  re- 
sulting from  it,  iv.  113.  Aberration  of,  discovered  by  Dr. 
Bradley,  iv.  1 19.  On  the  light  which  appears  in  the  eyes  of 
some  animals  in  the  dark,  whence  it  comes?  iv.  365.  The 
-  matter  of  light  first  formed,  iv.  371. — See  Fire,  Heat,  Co- 
lours, Kefraction^  Reflexion,  .Veivton, 

Light  ^inflammation  cud  light  of  ignition,  their  difference,  i.  368. 

Lightning,  on  the  phenomenon  of;  varieties  of  it,  iv.  317.  Its 
peculiar  property,  iv.  318.  Its  effects  are  limited,  iv.  318. 
A  remarkable  instance  of  it,  iv.  319.  Produces  whirlwinds, 
iv.  319.  The  identity  of  lightning  and  electricity,  iv.  319. 
There  is  a  reciprocal  exchange  from  the  earth  to  the  cloud, 
iv.  320.  The  extent  of  their  atmospheres,  iv.  320.  Causes 
concussions  on  the  earth,  iv.  323.  Imparts  magnetism,  iv. 
389. — See  Fire,  Electricity,  Magnetism, 

Luc,  M,  o'f,  his  admirable  reflexions  on  the  true  end  of  philoso- 
phy* i.  316.  His  remarks  on  infidelity,  i.  319.  His  just  ob- 
servations on  elastic  fluids,  i.  448.  His  observations  on  the 
change  of  ice  into  water,  and  vice  versa,  ii.  57.  On  the 
state  of  aqueous  vapour  in  the  atmosphere,  and  laws  of  eva- 
poration, ii.  74—86.  His  excellent  philosophical  works  ;  his 
refutation  of  materialism,  iii.  66.  His  observations  on  the 
hydrometer,  iii.  393.  An  account  of  his  whalebone  hygro- 
meter, iv.  431.  Was  in  a  storm  on  the  Buet,  iv.  454.  His 
remarks  on  barometers,  iv.  479. — See  Hygrometer, 

Lunarium,  description  of,  iv.  155. 


GENERAL  INDEX.  58i 

\ 

Lungs  described,  i.  195.  Their  situation  and  action,  i.  195.  How 
much  unknown,  i.  197.  Receive  great  quantities  of  blood,  i. 
]97-»  Their  correspondence  with  thought,  i.  197.  Their  con- 
nexion with  the  circulation  of  the  blood,  i.  198.  Express  va- 
rious affections,  i.  199. 


M 


Magic  lanthorn,  its  construction  and  use,  ii.  182. 

Magnetism  was  observed  by  the  ancients  ;  is  unknown,  iv.  374. 
Acts  universally  ;  natural  magnet ;  its  contents,  iv.  375.  The 
artificial  magnet  is  preferred,  iv.  375.  Its  poles,  iv.  376.  Its 
properties,  iv.  376.  Attracts  iron.  iv.  377.  The  sphere  of  its 
action  is  variable,  iv.  379.  The  similarity  between  magnet- 
ism and  electricity,  iv.  402.  On  the  magnetic  centre,  iv.  383. 
To  render  iron  and  steel  magnetic,  iv.  '85.  The  most  mag- 
netism may  be  communicated  to  steel,  iv.  388.  Is  commu- 
nicated by  lightning  and  percussion,  iv.  389.  On  the  magnet- 
ism of  the  earth,  iv.  392.  Effects  from  it,  iv.  393.  The  great 
uses  of  it,  iv.  394.  An  hypothesis  concerning  it,  iv.  403.  Is 
probably  supplied  by  the  sun,  iv.  404. — See  Electricity. 

Magnets,  the  manner  of  arming  them,  iv.  391. 

Man  received  an  untaught  language  from  nature,  i-  199.  Does 
not  require  the  brightest  evidence  of  truth  at  all  times,  i. 
246.  Is  at  first  led  by  his  senses,  i.  247.  Is  a  compound  be- 
ing, i.  282.  Is  an  imperfect  judge  of  heat  and  cold  from  his 
sensations,  i.  284.  Is  exposed  to  errors  from  various  causes, 
i.  6.  His  knowledge  is  power,  i.  24.  His  limited  views  of 
Divine  Providence,  ii.  388.  Collects  his  knowledge  from 
experiments  or  observations,  ii.  412.  The  variety  of  experi- 
ments he  necessarily  makes,  ii.  412.  His  pride,  ii.  413.  Re- 
ligion is  adapted  to  his  nature,  ii.  423.  His  want  of  a  Re- 
deemer, ii.  424.  Is  indebted  to  God  for  the  discoveries  which 
he  makes,  ii.  427.  His  unity  ;  he  continues  the  same  being, 
although  he  should  lose  different  members,  iii.  74.  His 
organs  only  channels  of  conveyance,  iii.  74.  How  much  he 
is  indebted  to  the  mechanical  powers,  iii.  83.  Men  do  not 
naturally  swim,  iii.  117.  On  walking  in  different  directions, 
iii.  161.  On  jumping,  skaiting,  and  running,  jii.  163.  Man 
considered  as  an  artificial  machine,  iii.  297.  The  vain  theo- 
ries for  ascertaining  the  strength  of  man,  iii.  298.  The 
strength  of  his  frame,  iii.  30 1 .  Is  able  to  carry  great  weights, 
iii.  302.  What  depends  on  the  posture  of  man,  iii.  303.  Me- 
thods by  which  he  draws  weights,  and.  instances  of  great 
strength,  iii.  304.  His  dependance ;  the  advantages  he  de- 
rives from  mechanics,  iii.  317.  A  source  of  his  errors,  iii. 
3^8.  Of  all  animals,  man  is  least  able  to  swim,  iii.  372.  His 
limited  powers  and  comparative  ignorance,  iii.  455.  His  rea- 


582  GENERAL  INDEX. 

son  is  to  correct  the  fallacies  of  the  senses,  iv.  43.  Is  apt 
to  be  forgetful  of  the  blessings  he  enjoys,  iv.  63.  The  bene- 
fits he  derives  from  the  animals,  iv.  170.  General  remarks 
on  man,  iv.  487.  The  means  of  his  understanding  the  works 
of  creation.,  iv.  488. — See"  God,  Providence,  Mind. 

Mariner*  a  comfiass,  a  description  of  it,  iv.  394.  When  discovered, 
and  by  whom,  iv.  395.  Its  variations,  iv.  397.  When  this 
variation  was  discovered,  iv.  397. — See  Magnetism. 

Mars,  his  size,  distance,  diameter,  revolutions,  iv.  23,  76.  His 
year  and  motion  round  the  sun,  iv.  140.  His  atmosphere 
and  poles,  iv.  204. 

Materialism,  danger  from  the  system  of,  ii.  249.  Considered  as  a 
system,  iii.  63.  Its  danger  and  misery,  iii.  63.  Particularly 
examined  and  confuted,  iii.  64 — 74.  Particularly  by  the 
unity  of  the  percipient  being,  iii.  74.  Perceptivity  cannot  be 
annexed  to  a  system  of  matter,  iii.  76. 

Matter  can  never  form  an  intelligent  being,  ii.  246.  The  use 
made  of  it  by  the  ancient  atheists,  ii.  246  ;  and  some  mo- 
dern philosophers,  ii.  246.  Is  the  object  of  the  five  senses, 
iii.  10.  An  inquiry  concerning  matter,  iii.  11.  The  com- 
mon properties  ascribed  to  it,  iii.  1 1.  The  properties  allow- 
ed to  matter  are,  impenetrability,  extension,  divisibility,  and 
hardness,  iii.  1 1 — 14.  Matter  is  not  infinitely  divisible,  iii. 
16.  Illustrated,  iii.  17.  The  great  divisibility  of  matter, 
iii.  19.  Sir  Isaac  Newton's  opinion  of  matter,  iii.  19.  Mat- 
ter hath  a  capacity  for  motion,  iii.  21.  Concerning  the  in- 
ertia of  matter,  how  understood,  iii.  23.  The  absurdities  re- 
sulting from  this,  iii.  23.  Matter  can  only  move  as  it  is 
moved,  iii.  24.  Is  gravity  an  essential  property  of  matter? 
iii.  25.  Matter  and  mind  totally  distinct,  iii.  50.  In  what 
this  difference  consists,  iii.  5  1.  The  opinions  of  the  an- 
cients concerning  matter;  its  visibility  is  supposed  to  arise 
from  its  form,  iii.  53.  The  first  matter  homogeneous,  iii. 
54.  This  original  matter  was  represented  by  Saturn  and 
Ops,  iii.  55.  The  primary  forms  of  matter  are  extension, 
figure,  organization,  iii.  56.  Matter  is  impressed  with  the 
marks  of  mind,  iii.  5  8.  Some  have  represented  matter  as 
without  impenetrability  and  inertia,  iii.  66. — See  Mind. 

Mcyoro,  Dr.  his  discoveries  of  airs  in  the  last  century,  i.  433. 

lilcasures,  philosophical,  remarks  on  them,  i.  301. 

Mechanics,  their  antiquity,  iii.  82.  The  wonderful  machines  of 
the  ancients,  iii.  82.  The  object  of  mechanics  is  motion, 
iii.  87. 

Mechanical  powers,  on,  iii.  224.  Their  use  to  man,  iii.  225.  Pos- 
tulata  for  the  consideration  of  mechanical  powers,  iii.  226. 
The  allusion  of  the  Platonists  and  Pythagoreans  to  these, 
iii.  272.  The  advantages  gained  by  them,  iii.  282.  Of  power 
and  time,  iii.  282.  Of  the  difference  between  practice  and 
theory,  iii.  288.  Caused  by  the  weight  and  friction,  iii.  289. 
Their  Use  to  manufactures  and  merchants,  iii.  317. 


GENERAL  INDEX.  583 

Mercury  congeals  at  40°  below  0,  ii.  54.  Mercury  congealed 
by  a  frigorific  mixture,  ii.  55.  A  long  column  of  it  is  sup- 
ported in  a  glass  tube,  iii.  37* 

Mercury,  his  size,  distance,  annual  revolution,  iv.  16. 

Meridian,  iii.   465.    The  degrees  on  it.  iii.  487. 

Metals,  a  table  of  the  different  expansions  of  different  metals,  i. 
273.  The  analogy  between  them  and  transparent  media,  ii. 
382. 

Meteorological  diaries,  importance  of  them,  iv.  410. 

Meteors,  their  appearance  at  great  heights  in  the  atmosphere ; 
difficulty  of  accounting  for  them,  i.  86. 

Microscopes,  their  several  kinds,  ii.  478.  The  advantages  to  be 
derived  from  them,  ii.  479.  Of  their  optical  effects,  ii.  480. 
Of  the  single  microscope,  ii.  485.  Its  properties,  ii.  486. 
Of  the  compound  microscope,  ii.  487.  Its  properties,  ii. 
488.  Of  the  solar  miscroscope,  ii.  489.  General  observa- 
tions on  them,  ii.  490.    Their  imperfections,  ii.  492. 

Milky  way,  iii.  501.  The  computation  of  the  number  of  suns  in 
it,  by  Dr.  Herschel,  iv.  192. 

Mind  and  matter  totally  distinct,  iii.  50.  Mind  always  has  some 
end  in  view,  iii.  51.  The  powers  and  qualities  of  mind,  iii. 
52.  Mind,  its  strong  desire  after  truth,  iii.  58.  Forms  exist 
in  mind  before  they  are  exhibited  in  matter,  iii.  60.  Every- 
thing excellent  is  an  emanation  from  mind,  iii.  60.  The 
mindofmanisnotacompound,iii.  75.  Its  immortality, iii. 79. 

Mirrors,  plane,  ii.  203.  Their  nature  and  properties,  ii.  204. 
Observations  on  them,  ii.  225.  How  to  judge  of  their  good- 
ness, ii.  227.  Of  convex  mirrors,  ii.  206.  Of  concave  mir- 
rors, ii.  207.  Deceptions  and  experiments  by  these,  ii.  210. 
Increase  heat  and  kindle  fire,  ii.  212.  Of  pictures  seen  in 
them  ;  to  find  the  focal  length  of  a  spherical  mirror,  ii.  216. 
General  properties  of  mirrors,  ii.  217. 

Moisture  is  invisible  water,  iv.  437,  Totally  absent ;  extreme,  iv. 
437. 

Monsoons,  or  periodical  winds,  iv.  458.  An  account  of  them, 
iv.  458.    How  caused,  iv.  459. 

Montgolfiers,  M.  discovered  the  air-balloon,  iii.  401.  Their  ex- 
periments, iii.  402. 

Moon,  phenomena  of  ;  her  periodical  motion,  iii.  474.  Her  vari- 
ous uses,  iv.  20.  Her  diameter,  distance,  revolution,  ap- 
pearances, iv.  21.  Her  orbit,  iv.  84.  Her  nodes  ;  her  con- 
junction with  the  sun,  iv.  85.  The  periodic  month,  and 
synodical,  iv.  87.  Her  different  phases,  iv.  89.  Eclipses 
of,  when  caused,  iv.  93.  The  nodes  of  the  moon,  iv.  94. 
Is  eclipsed  by  the  shadow  of  the  atmosphere  of  the  earth* 
iv.  95.  Sometimes  the  moon  totally  disappears,  iv.  95. 
Her  appearance  in  an  eclipse,  iv.  96.  The  beginning  or 
end  discovers  the  longitude,  iv.  97.  On  what  the  quantity 
and  the  duration  of  the  eclipse  depends,  iv.  98.  She  moves 
2077  miles  in  an  hour,  iv.  103.  Is  about  240,000  miles  from 
the  earth,  iv.    103v  The  moon  only  intersects  the  plane  of 


584  GENERAL  INDEX. 


. 


the  ecliptic  in  two  points,  iv.  105.  General  phenomena 
the  moon,  iv.  156.  Her  diffe rent  phases  explained,  iv.  157. 
Has  always  the  same  face  to  the  earth,  iv.  158.  Is  always 
half  enlightened  by  the  sun,  iv.  159.  Her  clays  and  nights 
equal  14-|  of  our  days,  iv.  159.  May  be  in  conjunction  or 
opposition  without  an  eclipse;  the  cause  of  this  explained, 
iv.  160.  Her  appearance  when  viewed  through  a  telescope; 
consists  of  mountains  and  cavities,  iv.  203.  Volcanoes 
have  been  seen  on  her  surface,  iv.  203.  Her  atmosphere,  iv. 
204.  She  gravitates  towards  the  earth,  iv.  230.  Is  acted 
on  with  the  greatest  force  when  nearest  the  earth,  iv.  233. 
Her  orbit  equal  to  60  times  the  earth's  semidiameter,  iv, 
236.  Her  irregularities,  iv.  248.  Whence  caused,  iv.  24£ — 
253. 

Motion,  improperly  considered  as  the  cause  of  fire,  i.  352.  On  the 
communication  of  motion  by  collision,  iii.  200.  Is  supposed 
to  cause  elasticity,  iii.  203.  The  laws  of  the  communication 
of  motion,  iii.  205.  In  elastic  and  non-elastic  bodies,  iii.  207. 
The  inexhaustible  source  of  motion  and  impuse,  iii.  211. 
The  cause  of  motion,  iii.  218.  Impulse  is  the  material 
cause  of  motion,  iv.  218. 

Motion,  apparent,  observation  on  it,  ii.  316.  In  what  degree  it 
must  be  to  become  visible,  ii.  317.  Is  change  of  place,  iii. 
87.  Involves  the  idea  of  space  and  time,  iii.  88.  Velocity 
is  the  quantity  of  motion,  iii.  89.  The  sources  of  motion,  iii. 
91.  Of  simple  motion,  iii.  93.  Circumstances  observed  in 
this,  iii.  93.  Of  the  quantity  of  motion,  iii.  97.  To  compute 
the  momentum,  iii.  98.  The  laws  of  motion,  iii.  101.  Ob- 
jection to  the  first  law  of  motion,  iii.  101.  Motion  is  not  a 
property  of  matter,  iii.  24.  Of  compound  motion,  iii.  104. 
Its  general  laws,  iii.  104.  Instances  of  compound  motion, 
iii.  113.  Of  accelerated  motion,  iii.  118.  An  inquiry  whe- 
ther motion  be  a  cause  or  an  effect,  iii.  184.  On  the  perma- 
nent motions  in  nature,  iii.  216.  Fire  and  light  are  the  in- 
struments of  motion  in  nature,  iii.  222.  The  permanency 
of  motions,  iii.  220.  There  is  no  motion  independent  of  the 
action  of  any  medium,  iii.  223.  Motion,  whence  produced  ; 
varieties  of  motion,  iv.  371. 

Munro,  Dr.  his  objections  to  the  nervous  and  electric  fluid  being 
the  same  answered  iv.  363. 

Musical  sounds,  effects  of,  i.  365.  Organs  in  man-  to  produce 
these,  i.  238. 

N 

Xadn;  iii.  463. 

Mature,  the  views  of  it  infinite,  i.  27.  Is  inexhaustible  on  every 
side,  i.  27.  Is  a  mere  name,  when  considered  as  independ- 
ent of  God,  ii.  244.  Is  the  benevolence  of  the  Almighty  pro- 
viding for  all  the  inhabitants  of  the  earth,  ii.  412.  Appears 
more  excellent  the  more  it  is  examined,  iii.  9.-    The  opera- 


GENERAL  INDEX.  585 

tions  in  nature  are  carried  on  mechanically,  iii.  22.  There 
is  nothing  insulated  in  nature,  iii.  214.  A  general  circula- 
tion through  all  nature,  iii.  220.  The  immensity  of  the  works 
of  npture,  iv.  38.  The  perfection  of  them,  iv.  137.  All 
the  works  of  nature  are  connected,  iv.  257.  Remarks  on. 
the  chemistry  of  nature,  iv.  407. — See  God,  Providence, 
Man,   Sun,  Air,    Water, 

Nebula  of  fixed  stars,  iv.    196.     Planetary  nebulse,  iv.   198. 

Needle,  magnetic,  its  diurnal  variation,  iv.  399.  Is  disturbed 
by  the  aurora  borealis,  iv.  399.  Its  dip  ;  by  whom  discovered, 
iv.  400.  'The  variations  in  the  dip,  iv.  401.  The  needle 
is  affected  by  the  aurora  borealis,  iv.  402. — -See  Magnetism, 
Mariner's  compass, 

Newton,  Sir  Isaac,  his  first  rule  of  philosophizing,  i.  91.  His 
discoveries  of  the  aerial  pulses  ;  the  manner  in  which  they 
are  propagated,  i.  211.  His  works  ;  his  rules  of  philoso- 
phizing, i.  31. "  His  grand  discoveries  concerning  light  and 
colours,  ii.  326.  His  optics,  ii.  332.  His  experimentum 
crucis,  ii.  335.  An  eulogium  on  him,  ii.  347.  He  sup- 
posed that  bodies  of  different  densities  reflected  different  rays 
of  light,  ii.  3->5.  His  conjectures  on  the  fits  of  easy  reflec- 
tion and  transmission  of  a  ray  of  light,  ii.  362.  He  disco- 
vered that  inflammable  bodies,  possessed  the  refractive  power, 
more  than  bodies  not  inflammable,  ii.  383.  Constructed  a 
reflecting  telescope,  ii.  470.  His  great  modesty,  ii.  471. 
His  opinion  concerning  the  original  atoms,  iii.  19.  His 
discoveries  of  gravitation,  iii.  45.  Not  very  consistent  in 
hydrostatics,  iii.  341.  His  theories  on  the  subject,  iii.  451. 
An  account  of  his  principles,  iii.  452.  His  observation  on 
the  curvilineal  motion  of  the  moon,  iv.  232.  His  mathe- 
matical astronomy,  iv.  225. — See  Light,  Colours,  Gravita- 
tion. 

Nodes  of  the  moon,  iv.  105.  Go  backwards  nineteen  degrees  and 
an  half  in  every  year,  iv.   108. 

Nonius,  scale  to  estimate  the  divisions  on  it,  iv.  417. 


O 

Observer  of  Nature,  character  of  i.   175 — 177. 

Opacity  arises  from  the  discontinuity  of  the  particles  of  bodies, 

and  the  different  density  of  the  intervening  medium,  ii.  366. 

How   destroyed,  ii.   366.     Different   significations,  ii.  397. 

Of  opacity,  considered  as  a   positive  quality  in   bodies,  ii. 

402.  It  does  not  depend  on  the  solid  matter  in  bodies,  ii.  404. 

— See  Light. 
Optics,  the  excellency  of  the  knowledge  of  them,  ii.   133. 
Oxygenation,  or  acidifying,  is  produced  by   the    combination  of 

any  substance  with  vital  air,  i.  462. 
VOL.  IV.  4  G 


486  GENERAL  INDEX. 


Parallax,  annual  diurnal,  horizontal,  iv.  113 — 119.  The  accu- 
racy necessary  in  finding  it,  iv.   118. 

Pascal^  M.  his  character  ;  he  first  applied  the  barometer  to 
measure  mountains,  i.  67. 

Pendulum,  its  vibration  explained,  isochronous,  i.  212.  The 
analogy  between  a  pendulum  and  a  musical  string,  i.  212. 
Account  of  pendulums,  iii.  175.  Their  oscillations,  iii. 
176.  Their  isochronism,  iii.  179.  Pendulums  are  simple 
and  compound,  iii.  181.  Of  the  centre  of  oscillation  in  com- 
pound pendulums,  iii.  181.  Of  the  time  of  their  oscillation,  iii. 
186.  Are  affected  by  heat  and  cold,  iii.  187  ;  by  their  place 
on  the  globe,  iii.  187.  Huygens  adapted  them  to  clocks,  iii. 
1 91.  Wooden  pendulums,  their  properties,  iii.  193.  The 
gridiron  pendulum,  its  construction  and  advantages,  iii.  193. 

Penumbra  of  an  eclipse,  iv.  9->. 

Percussion,  centre  of,  iii.   187. 

Perspiration,  great  quantities   of   food  carried  off  by  it,  i.   181. 

Philosopher,  the  universality  of  knowledge  which  ought  to  form 
his  character,  i,  2.  A  picture  of  a  true  philosopher,  i.  2. 
His  character,  as  drawrn  by  Lord  Bacon,  i.  19.  Studies 
the  intention  of  nature,  i.  20.  He  proceeds  by  induction,  i. 
22  ;  and  thus  forms  general  axioms,  i.  22.  He  makes  use 
of  every  help,  particularly  of  analogy,  i.  27.  He  proceeds 
with  great  caution,  i.  30.  The  error  of  the  modern  philo- 
sophers, ii.  410.  The  weakness  of  vanity  in  a  philosopher, 
ii.  425 — See  Truth. 

Philosophy,  natural,  excellence  and  advantage  of  it,  i.  37,  94.  Ori* 
gin  of  the  name,  i.  37.  Its  tendency  to  elevate  the  mind, 
i.  126  ;  and  promote  religion,  i.  127.  The  business  of 
it,  i.  175.  Its  tendency  to  cultivate  sublime  taste,  i.  243. 
Its  grand  object,  i.  247.  Is  concerned  with  final  causes,  i. 
385.  Is  continually  presenting  scenes  of  beauty  to  the  mind 
of  man,  i.  281.  It  advances  the  cause  of  religion,  ii.  9. 
The  method  of  reasoning  in  it,  i.  1.  Leads  us  to  the  know- 
ledge of  God,  i.  20.  Its  connexion  with  religion,  ii.  388. 
The  discoveries  of  philosophy  gradual,  iv.  258. — See  Air, 
Astronomy,  Colours,  Elastic  Fluids,  Electricity,  Fire,  Gravity, 
Light,  Magnetism,  Matter,  Mechanics,  Meteorology,  Micros- 
copes, Phosphorus,  Telescopes,   Water. 

Philosophy,  inductive,  an  account  of,  i.  25. 

Philosophy,  false,  its  errors  and  dangers,  i.   19. 

Phlogiston,  or  the  principle  of  inflammability,  i.  369.  Denied 
by  the  French  philosophers,  i.  369.  Is  a  substance  sui generis  ; 
the  matter  of  light  and  heat,  i.  370.  Proved  by  the  de- 
composition of  water,  and  the  luminous  appearance  then  ex- 
hibited, i.  37 1 — 379.  It  is  the  solar  substance  detained  in  the 


GENERAL  INDEX.  587 

phlogistic  composition,!.  378.  Is  restored  by  animal  and 
vegetable  substances,  i.  380;  particularly  by  the  influence 
of  light  for  the  phlogistication  of  vegetable  bodies,  i.  382  ; 
and  by  the  mass  of  colour  which  they  obtain,  i.  382.  Is  im- 
parted from  vegetables  to  animals,  i.  384.  Is  maintained  by 
Mr.  YYeiglib,  in  opposition  to  the  French  chemists,  i.  507 — 
522.  Its  existence  proved,  i.  508  ;  particularly  by  the  re- 
production of  the  metallic  calces,  i.  509.  Other  considera- 
tions in  support  of  it,  i.  517.  Its  universality  and  energy, 
i.  519.  The  analogy  between  phlogiston,  or  fixed  fire,  and 
fixed  air,  i.  519 — 522. — See  Fire,  Heat. 

Phosphorus,  the  several  kinds  of,  ii.  385.  The  Bolognian  phos- 
phorus was  discovered  by  Vincenzo  Cascariolo  ;  its  proper- 
ties, ii.  390.  Artificial  phosphorus,  how  formed,  ii.  392. 
Phosphori  generally  diffused,  ii.  392.  Of  Canton's  phos- 
phorus, how  prepared,  ii.  393.  Imbibes  its  property  from 
light,  ii.  394.  Mr.  Wilson's  phosphorus  exhibited  vivid  co- 
lours, ii.  394v  Phosphorus  is  an  incipient  ignition  in  certain 
bodies,  ii.  395.  Different  kinds  of  phosphori,  ii.  396.  They 
do  not  emit  the  identical  light  which  they  have  received,  ii. 
397.  The  agreement  and  disagreement  of  phosphoretic  and 
phlogistic  bodies,  ii.  398.  The  change  it  undergoes  when 
burnt  in  vital  air,  i.  460. — See  Fire. 

Physicians,  the  error  into  which  some  of  them  have  fallen,  ii.  245. 

Plane,  inclined,  descent  of  bodies  upon  it,  iii.  139.  Has  a  rela- 
tive and  absolute  gravity,  iii.  141 — 146.     Its  use,  iii.  257. 

Planets,  on,  iii.  480.  They  are  spherical  opake  bodies,  iv.  11. 
Inferior  and  superior  planets,  iv.  15,  23.  A  table  of  their 
diameters  and  distances,  iv.  31.  Revolutions  round  the  sun, 
iv.  32 ;  and  their  own  axes,  iv.  33.  Their  proportional  mag- 
nitude, iv.  34.  Their  heliocentric  and  geocentric  latitude, 
iv.  65.  Their  conjunction  and  opposition,  iv.  66.  Appear- 
ances of  the  inferior  planets,  iv.  73  ;  and  of  the  superior,  iv. 
76.  Their  direct  and  retrograde  motion,  and  stationary  si- 
tuation, iv.  77.  Their  satellites,  iv.  79.  Inferior  planets, 
their  superior  and  inferior  conjunctions,  iv.  141.  Their  ap- 
parent irregularities  explained,  iv.  143.  Of  the  superior 
planets,  as  seen  from  the  earth,  iv.  144;  are  most  probably 
inhabited  worlds,  iv.  214.  They  gravitate  towards  the  sun, 
iv.  238.  The  irregularity  produced  among  them  by  gravi- 
tation, iv.  243. 

Planetarium,  its  antiquity  and  use,  iv.  135.  Proves  the  truth  of 
the  Copernican  system,  iv.  145.  How  to  rectify  it  for  the 
true  places  of  the  planets,  iv.  146.  To  use  it  as  a  tellurian, 
iv.  147.  To  explain  the  changes  of  the  seasons,  iv.  148. 
The  parallel,  direct,  and  right  spheres,  iv.  15  1. 
Plants  are  sensibly  affected  by  light,  ii.  417.  Plants  exposed  to 
light  emit  vital  air,  ii.  417.  Are  aflected  by  vital  air,  ii. 
419. — See  Air,  Light,  Vegetables. 
Plato's  idea  of  the  intertexture  of  air  and  fire  in  the  human  frame, 
i.  245.  His  observation  on  colours,  ii.  347.  On  the  present 
state  of  human  knowledge,  iii.  55. 


588  GENERAL  INDEX. 

Plenum,  a,  necessary  for  motion  by  impulse,  iv.  222.  Bodies  are 
able  to  move  in  a  plenum,  iv.  222. 

Plurality  of  worlds,  reasons  for  them,  iv.  211. 

Pneumatics* — See  Air. 

Points,  cardinal,  and  points  of  the  compass,  iii.  462. 

Pole  star,  iii.  460.  Its  position,  iii.  460.  How  to  be  found,  iii. 
460.     It  describes  a  small  circle  round  the  pole,  iii.  473. 

Poles,  or  arctic  and  antarctic  circles,  iii.  473. 

Poles  of  the  magnet,  iv.  379.  Their  action  on  each  other,  iv. 
380.  Their  action  on  steel  filings,  iv.  381.  The  poles  should 
always  be  left  connected,  iv.  39  1. 

Prayer,  a,  for  wisdom  and  virtue,  i.  33. 

Prejudice,  its  mischiefs  and  effects,  i.  62. 

Priestley,  Dr.  his  discoveries  of  airs,  i.  433.  His  system  of  ma- 
terialism fully  examined  and  confuted,  iii.  63 — 74. 

Projectiles,  motion  of,  iii.  165.  Galileo's  discoveries  in  them,  iii. 
166.  Are  opposed  by  the  air's  elasticity,  iii.  174.  The  great 
quantity  of  motion  which  they  lose,  iii.  171. 

Providence,  discoverable  in  the  smallest  as  well  as  greatest  events; 
no  such  thing  as  chance,  ii.  386. 

Providence,  reflections  on  the  wisdom  and  goodness  of,  in  the 
suction  of  animals  and  the  swallowing  of  food,  i.  60.  In 
the  pressure  of  the  air,  i.  72.  In  the  universal  good  de- 
signed in  all  his  works,  i.  94.  In  the  admirable  provision 
made  for  breathing,  i.  195.  In  the  blessing  of  speech,  i. 
199.  In  the  singing  of  birds,  i.  206.  In  the  admirable 
construction  of  the  human  ear,  i.  240.  In  the  creation  of 
fhe  universe,  and  particularly  of  the  air,  for  the  most  uni- 
versal good,  i.  243.  In  the  provision  made  for  the  warmth 
of  different  animals,  i.  262.  In  guarding  against  the  too 
sudden  changes  of  heat  or  cold,  i.  313.  In  the  continued 
agency  of  the  Divine  Mind,  i.  318.  In  the  insensible  opera- 
tions of  the  rise  of  vapours,  i.  548.  In  the  provisions  made 
for  supplying  heat  and  light,  i.  38  4.  In  rendering  evtry 
part  of  matter  active  and  useful,  i.  384.  In  the  great  and 
benevolent  ends  which  are  obtained  in  nature  by  simple 
means,  i.  388.  In  the  agency  and  operations  of  fire,  i.  429. 
In  the  abundant  productions  of  vital  air,  and  in  the  preser- 
vation of  the  equilibrium  of  the  atmosphere,  i.  457.  In  the 
uses  resulting  from  the  vegetable  kingdom,  i.  45  7.  In  the 
provision  made  against  cold,  i.  467.  In  the  ocean,  and  its 
inhabitants,  ii.  90.  In  the  various  benefits  bestowed  by 
means  of  water,  ii.  90.  In  the  construction,  form,  and  uses 
of  the  eye  ;  and  in  the  blessings  of  sight,  ii.  255 — 258,  321, 
322.  In  restoring  the  purity  of  the  air  by  means  of  vege- 
tables, ii.  417.  In  the  simplicity  and  energy  of  his  works, 
iii.  50.  In  the  divine  agency  exhibited  in  nature,  iii.  58.  In 
the  powers  and  excellency  of  the  soul  of  man,  iii.  79.  In  the 
regular  order  and  establishment  of  the  Divine  Mechanic,  iii. 
S3;  both  in  the  natural  and  moral  world,  iii.  84.  In  the 
starry  heavens,  iii.  5 10.  In  the  gradual  progress  of  arts  and 


GENERAL  INDEX.  589 

sciences,  iv.  3.  In  the  immensity  of  his  works,  and  their 
continual  preservation,  iv.  38.  In  the  various  changes  of 
the  seasons,  iv.  62.  In  the  clear  discoveries  of  divine  intel- 
ligence and  design,  and  in  the  supplies  of  the  numerous 
wants  of  man,  iv.  166.  In  the  wonderful  structure  of  the 
human  frame,  iv.  168.  In  the  vegetable  kingdom,  iv.  169. 
In  the  animal  kingdom,  iv,  169.  In  the  universal  distribu- 
tion and  management  of  fire,  iv.  353.  In  the  degree  of  heat 
which  every  country  enjoys  in  the  course  of  the  year,  iv. 
472.  In  the  arrangement  of  mountains  and  seas,  iv.  472. 
In  the  perfection  of  the  word  and  works  of  God,  iv.  488. — 
See  God,  Man,  Natural  Philosophy. 

Pulley,  its  properties,  iii.  252.  Are  fixed  and  moveable,  iii.  252. 
Of  the  moveable  pulleyriii.  253.  Of  Smeaton's  pullies,  iii. 
276.     Of  their  immense  force,  Iii.  277. 

Pulses  of  the  air,  propagated  by  sound,  i.  211.  Are  alternately 
condensed  and  rarefied,  i.  213.  All  pulses  move  at  an  equal 
rate,  1142  feet  in  a  second,  i.  214. 

Pump,  common,  invented  by  Ctesebes,  i.  119.  Raises  water  thirty- 
four  feet,  i.  120. 
forcing,  acts  by  condensed  air,  i.  122. 

Pumps,  of.  Of  the  chain  pump,  iii.  433.  Its  construction  and 
use,  iii.  433.  Of  the  common  pump,  iii.  434.  Its  construc- 
tion and  action,  iii.  434.  The  piston  must  be  less  than  thirty- 
three  feet  from  the  water,  iii.  435.  uf  the  forcing  pump  its 
construction  and  use,  iii.  437.  Of  dc  hi  Hire's  pump)  iii. 
439.  Of  Taylor's  pump,  in.  440.  Of  the  Hessian  pump,  iii. 
,    441.  Of  Vera's  pump,  iii.  442. — See  dir<  Water, 

Pupil  of  the  eye,  an  account  of,  ii.  289.  lis  motions;  is  naturally 
dilated,  ii.  289.  Is  contracted  by  light,  ii.  290. — bee  Eye, 
Light. 

Pyrometer,  an  instrument  for  measuring  theflegree  of  heat,  i.  265. 


Quadrant,  astronomical,  its  use  and  description,  iii.  467.  To  find 

the  altitude  of  any  celestial  body,  iii.  471. 
Quadrant  of  altitude,  iii.  490. 

R 

Rain  is  supposed  to  proceed  from  the  decomposition  of  the  air 
resulting  from  the  aqueous  vapours  being  converted  into  an 
aeriform  fluid,  ii.  78.  Heavy  showers  caused  by  clouds  of 
different  electricities  being  driven  together,  iv.  320.  How 
much  we  are  ignorant  of  it;  on  what  it  depends,  iv.  439. 
Rain  is  not  the  precipitation  of  water  simply  evaporated  in 
the  air,  iv.  440.  Is  indicated  by  a  hollow  noise,  iv.  457. 
Rains  in  the  West  Indies,  iv.  463.   Most  rain  falls  in  woody 


520  GENERAL  INDEX. 

and  mountainous  countries,  iv.  46S.  Rain  and  snow  gene- 
rally give  vitreous  electricity,  iv.  476.  Generally  follows 
sudden  changes  of  the  weather,  iv.  481 — See  Vafiour. 

Rainbow,  the  ignorance  of  the  arrcients  concerning  it ;  explained 
by  Sir  Isaac  Newton,  ii.  348.  The  order  of  colours;  the 
varieties  of  them,  ii.  £50.  Illustrated;  the  second  bow  ex- 
plained, ii.  351.  On  what  the  size  of  the  bow  depends^  ii. 
352. — See  Light,  Colours,  Refraction, 

Rain-gage,  its  construction  and  use,  iv.  430.  Explanation  of,  by 
the  Editor,  iv.  495. 

Read,  Mr.  his  experiments  on  electricity,  iv.  305,  306. 

Reaumur,  M.  his  discoveries  on  eggs,  i.  180. 

Rejiexion  of  light,  ii.  198.  All  reflexion  reciprocal,  ii.  200.  Laws 
of  reflexion,  ii.  202.  No  colours  are  displayed  by  reflected 
light,  ii.  375 — See  Light. 

Refraction  of  light,  laws  of,  ii.  143.  Refraction  at  a  convex  sur- 
face, ii.  15  6.     At  a  concave  surface,  ii.  160. — See  Light. 

Religion,  it  requires  a  sobriety  of  mind,  i.  320.  Religion  and  phi- 
losophy agree  together,  iv.  370.  The  arts  and  sciences  flou- 
rish most  where  religion  is  cultivated,  iv.  486. — See  God, 
Providence,  Man. 

Resinous  electricity. — Sec  Electricity. 

Respiration  receives  vital  air,  and  mixes  it  with  the  blood  in  the 
lungs,  i.  465.  Respiration  is  similar  to  combustion;  respi- 
ration explained,  i.  188,  196.  Concerned  in  smelling,  laugh- 
ing, speaking,  weeping,  i.  201. 

Rods,  conducting,  iv.  326.  Are  the  means  of  restoring  the  equili- 
brium, iv.  326.  Observations  against  pointed  conductors,  iv. 
328,  They  only  draw  off  the  electric  matter  when  immersed 
in  its  atmosphere,  iv.  329.  Cannot  attract  the  lightning  out 
of  its  direction,  iv.  329.  Objections  against  them,  iv.  329. 
— See  Electricity. 


Salts  produce  great  degrees  of  cold,  ii.  5  3.  How  they  form  sa- 
line liquids,  ii.  65. 

Saturn,  his  size,  distance,  revolution,  iv.  26.  His  ring  and  moons, 
iv.  27,  206.  His  year,  and  motion  round  the  sun,  iv.  138. 
His  belts,  iv.  206. 

Screw,  male  and  female,  iii.  261.  Of  the  endless  screw,  iii.  263. 
Of  the  micrometer  screw,  iii.  265. 

Sea.  Saltness  of  the  sea,  inquiries  into  the  cause  of  it,  ii.  32.  It 
was  originally  salt,  ii.  33.  Dr.  Halley's  weak  opinion,  ii.  33. 
The  water  is  most  salt  where  the  sun  is  vertical ;  an  easy 
method  to  ascertain  the  saltness,  ii.  34.  The  advantages 
derived  from  the  sea  to  temperate  the  air,  iv.  473. 

Seasons  of  the  year  accounted  for,  iv.  55.  Summer  is  longer  than 
winter,  iv.  61. 


GENERAL  INDEX.  '  59 1 

Senses  lead  to  all  physical  knowledge,  i.  247.  The  imperfec- 
tions attending  their  information,  i.  248.  This  to  be  judg- 
ed of  from  experiment,  i.  249. 

Sight.  Of  imperiled  sight,  ii.  293.  Of  old,  or  long-sighted 
eyes,  ii.  294.  Of  short-sighted  eyes,  ii.  296.  How  as- 
sisted, ii.  305. — See  Eye. 

Smeaton,  Mr.  an  account  of  his  pyrometer,  i.  271. 

Smoke. — See  Chimnies. 

Smith,  Dr.  his  observation  on  the  division  of  labour  illustrated, 
iii.  216. 

Snoiv  keeps  the  ground  warm  in  winter,  i.  260,  262.  The  form 
of  its  flakes,  ii.  46. 

Solution,  an  effect  of  fire,  i.  397.  Description  of  its  operation, 
i.  398.     Illustrated  in  the  solution  of  salts,  i.  399. 

Sound,  benefits  resulting  from  it,  i.  205.  Cause  of,  i.  206, 
233.  Of  musical  sounds,  i.  235.  Of  sympathetic  sounds, 
i.  237.  Sounds  of  metals,  how  improved,  i.  207.  Classes 
of  sonorous  bodies,  i.  207.  Sound  is  besfeonducted  in  a  dense 
medium,  i.  208.  May  be  conveyed  through  wood  or  water, 
i.  208.  It  does  not  proceed  from  a  flux  of  air,  but  from  a 
vibratory  motion  of  the  particles  of  air  in  their  proper  place, 
i.  210.  Sound  vibrates  according  to  the  motion  of  a  cycloi- 
dal  pendulum,  i.  214.  Differences  among  sounds,  i.  214. 
The  intensity  is  inversely  as  the  squares  of  the  distance,  i.  2 1 5. 
The  velocity  of  sound  continues  always  the  same,  i.  215. 
S^und  diminishes  for  want  of  perfect  elasticity  in  the  air,  i. 
215.  Is  more  perfect  in  some  winds  that  in  others,  i.  216. 
Its  effects  on  solid  bodies,  iv.  349. — See  Air. 

Sound  judgment,  the  means  to  form  it,  i.  62. 

Space,  the  idea  of  it  from  extension,  iii.  15.  Space,  absolute 
and  relative,  iii.  88.  The  analogy  between  time  and  space, 
iii.  88.      , 

Sfieaking  trumpet  explained,  i.  217. 

Spectacles,  their  use,  ii.  297.  Directions  in  the  choice  of  them^ 
ii,  297.     Directions  to  discover  if  they  be  wanted,  ti.  301. 

Speech,  the  blessing  of  it ;  the  various  parts  which  fofm  it,  i. 
199. 

Spheres,  right,  parallel,  or  oblique,  iii.  493. 

Spirit  of  man,  the  opinions  which  the  ancients  entertained  of  it, 
iii.  62.  The  gospel  does  not  treat  of  its  natural  immorta- 
lity, iii.  64.  Dr.  Hartley  represented  the  soul  as  uniform- 
ly passive,  iii.  64.     The  excellencies  of  the  soul,  iii.  79. 

Springs  of  water,  different  opinions  concerning  them,  ii.  23. 
Are  not  supplied  by  rains  and  dews,  ii.  24.  Some  run  the 
same  in  a  wet  or  dry  season,  ii.  27.  Springs  are  princi- 
pally supplied  from  the  subterraneous  stores  of  water,  ii.  28. 

Stars,  the  numbers  of,  discovered  by  Herschel,ii.  427.  Their  ap- 
parent diurnal  motion,  iii.  459.  Of  the  fixed  stars,  their 
twinkling,  iii.  497.  Are  arranged  in  constellations,  iii.  497. 
Are  divided  into  different  classes,  from  their  size,  iii.  498. 
The  catalogues  of  them,  iii.  500,  by  Hipparchus,  by  Bayer,  by 
Flamstead,  by  de  la  Caille,  bv  Wollaston,  *i.  500.     De- 


592  GENERAL  INDEX. 

picted  on  a  new  18-inch  celestial  ^lobe  by  W.  Jones,  iii 
501.  The  immense  number  of  the  stars,  iii.  502.  The 
numbers  discovered  by  Dr.  Herschtl,  iii.  503.  How  to 
obtain  a  knowledge  of  the  constellations,  in.  503.  The 
vast  distance  of  the  fixed  stars  from  us,  of  the  first  mag- 
nitude, iv.  36,  of  the  second,  iv.  36.  Their  parallax  iv. 
113.  Their  apparent  motions,  from  the  aberration  of  light, 
iv.  119.  Their  motion,  iv.  121.  Different  stars  appear  at 
different  times  of  the  year,  iv.  139.  Their  distance  great 
beyond  computation,  iv.  187.  Have  a  general  motion,  iv. 
188.  The  variety  in  these  ;  some  appearing,  others  vanish- 
ing, iv.  188.  New  stars;  catalogues  formed,  iv.  189.  Remark- 
able new  stars,  iv.  190.  Stars,  their  proper  motion,  iv. 
190.  Stars  of  different  lustre  supposed  to  be  at  different 
distances  from  us,  iv.  194.  Nebulae  of  stars,  iv.  196.  A 
perforated  nebula,  iv.  198.  All  the  universes  of  stars  or 
suns  connected  together,  iv.  199.  Are  probably  suns; 
their  use,  iv.  21  1.  There  are  more  stars  in  the  northern 
than  in  the  southern  hemisphere,  iv.  485. 

Steam  of  boiling  water  occupies  1800  times  more  space  than  water, 
i.  331.     Its  nature  and  properties,  ii,  80. 

Steel-yard,  an  account  of  the,  iii.  247. 

Suction,  improperly  applied  to  account  for  some  phenomena  o 
air,  i.  5  1. 

Sun.  The  emanation  of  matter  from  the  sun  one  of  the  prime 
movers  of  the  machine  of  the  world,  i.  384.  Its  influence 
under  different  forms,  i.  384.  The  solar  fluid  is  absorbed 
in  vegetables,  and  is  the  cause  of  colour,  flavour,  &c.  i. 
45  3.  The  solar  substance  in  one  place  is  fire,  in  another 
light,  in  a  third,  electricity,  ii.  401.  The  sun  animates 
and  quickens  the  globe  of  the  earth,  as  the  seminal  bed  of 
his  rays,  ii.  414.  The  source  of  natural  life,  ii.  415.  His 
influence  on  the  earth,  ii.  415.  particularly  in  the  vegetable 
kingdom,  ii.  415.  His  rising  and  setting,  iii.  465.  His 
annual  motion,  iii.  476.  He  rises  and  sets  in  different  parts, 
iii.  476.  He  moves- about  a  degree  every  day,  iii.  477.  The 
centre  of  the  system  ;•  the  heart  of  heaven,  iv.  12.  His  in- 
fluence, size,  distance,  motion,  iv.  13.  His  supposed  at- 
mosphere, iv.  14.  Is  the  centre  of  the  system,  iv.  49. 
ijis  apparent  motion,  iv.  50.  The  motion  of  the  sun  ap- 
pears differently  to  inhabitants  of  different  planets,  iv.  52. 
He  appears  to  move  in  the  ecliptic,  iv.  53.  His  apparent 
diameter  greater  in  winter  than  in  summer,  iv.  62.  Eclipses 
of  the  sun,  how  caused,  iv.  98,  are  visible  to  only  a  few  in- 
habitants of  the  earth,  iv.  99.  Total  eclipse  remarkable, 
iv.  100.  Are  total,  annular,  or  partial,  iv.  101.  On  what 
the  quantity  and  duration  of  the  eclipse  depends,  iv.  102. 
His  parallax,  iv.  118.  His  tropical  and  sidereal  year,  iv. 
122.  A  measurer  of  time,  iv.  123.  The  inequality  o:' his 
apparent  motion,  iv.  132.  Appears  to  pass  through  the  si^ns 
of  the  zodiac  in  a  year,  iv.  1 39.  Dr.  Herschel  conjectures  that 
our  sun  belongs  to  the  milky  way,  iv.  192.  The  spots  on  its 


: 


GENERAL  INDEX.  sOo 

surface,  their  variety,  iv.  200.  Peculiarities  of  their  nucleus 
and  umbra,  iv.  200.  Sometimes  they  appear  to  burst,  iv.  20 1 . 
Their  directions  different,  iv.  201.  Conjectures  concerning 
them, iv.  201.  His  centre  of  gravity,  iv.  242.  Is  the  source  of 
the  electric  fluid,  iv.  339.  Is  the  cause  of  natural  life,'iv.  347. 
Probably  supplies  the  magnetic  fluid,  iv.  404.  Is  a  principal 
source  of  heat,  iv.  467.  His  rays  act  as  lire,  and  increase  the 
expansive  force  of  fire,  iv.  467.  Causes  an  undulating  motion 
in  the  atmosphere,  iv.  478.  Shines  more  on  the  northern 
than  on  the  southern  hemisphere,  iv.  484. — See  Light)  Heat-, 
Colours,  Electricity,  Magnetism. 

Sun-dial,  universal  or  equatorial,  description  of,  iv.  180. 

Swimmiiig,  on,  Hi.  369. 

Sympathetic  inks,  experiments  with,  ii.  122. 

Syphon  explained,  i.  123.  Forms  Tantalus's  cup,  i.  124.  Ac- 
counts for  intermitting  springs,  i.  125.  Fuller  account  of; 
principles  on  which  they  act,  iii.  446.  Distiller's  syphon,  iii. 
447.     Of  s'Gravesande's  syphon,  iii.  448. 


Telescopes,  observations  on  their  use,  ii.  426.  Lord  Bacon's  re- 
mark on  them,  ii.  427.  Are  supposed  to  have  been  disco- 
vered by  Roger  Bacon,  ii.  428  ;  and  by  Jansen,  ii.  430.  Were 
improved  by  Galileo,  ii.  430.  Of  refracting  telescopes,  ii. 
431.  Their  properties,  ii.  434 — 440.  Their  apparent  field, 
ii.  437.  Of  the  astronomical  telescope,  its  properties,  ii. 
440 — 443.  Imperfections  arising  from  the  dispersion  of  the 
rays  of  light  in  them,  ii.  448.  Of  the  compound  object  glass, 
ii.  449.  From  the  refrangibility  of  the  light,  ii.  452.  How 
corrected,  ii.  453.  Of  telescopes  with  several  eye-glasses, 
ii.  457.  Of  achromatic  telescopes,  ii.  461,  Were  invented 
by  Mr.  Dollond,  ii.  463.  The  invention  has  been  ascribed  to 
Mr.  Hall,  ii.  468.  Are  composed  of  different  kinds  of  glasses, 
ii.  466.  This  discovery  was  claimed  for  Euler,  ii.  467.  Of 
the  reflecting  telescope,  ii.  468.  By  whom  discovered,  ii. 
469.  Of  the  Gregorian  telescope  ;  its  properties,  ii.  472. 
Of  the  Newtonian  telescope,  ii.  476»  The  most  improved 
constructions  of,  described  by  the  Editor,  ii.  496.  Transit 
telescope,  ii.  508. 

Temperature  of  the  earth,  observations  on  it,  iv.  471.  On  what  it 
depends,  iv.  472. 

Tests,  chemical,  list  of,  ii.  115. 

Thermometers,  the  principle  on  which  they  are  constructed,  i.  286. 
Of  Fahrenheit,  Keaumur,  and  Celsius,  the  relation  between 
them,  i.  287.  Mercurial  thermometer,  an  accurate  mea- 
sure of  heat,  i.  288.  Experiments  on  it,  i.  340.  May  be 
reduced  by  ether,  ii.  5  1.  Its  construction  and  use,  iv.  422. 
The  requisites  for  a  good  one,  iv.  424.  The  manner  of  fill- 
VOL.IV.  4H 


594  GENERAL  INDEX. 

ing  it,  iv.  427.  How  to  graduate  it,  iv.  428.  To  seal  it 
hermetically,  iv.  428.  The  thermometer  is  a  scale  of  ex- 
pansion, indicating  the  transfusion  of  the  igneous  fluid,  iv, 
429.     Six's  thermometer,  iv.  495. — See  Fire,  heat* 

Thunder,  remarks  on,  iv.  452. 

Time,  of,  iii.  88.  The  analogy  between  time  and  space,  Hi.  88. 
The  measure  of  it,  iii.  190.  Observations  on  time,  iii.  195. 
Mr.  Locke's  opinion  of  it,  iii.  197.  Dr.  Clarke's  mistake 
concerning  successive  and  unsuccessive  duration,  iii.  197. 
Quotation  from  Tucker,  iii.  198.  Time,  true  and  apparent, 
and  mean,  iv.  128.  Equation  of  time,  iv.  129.  Whence 
the  difference  arises,  iv.  130.  Reflexion  on  the  lapse  of 
time,  iv.  133. 

Tin,  its  peculiar  quality  in  rendering  other  metals  more  sono- 
rous, i.  207. 

Torricellius,  his  invention  of  the  barometer,  i.  65. 

Transfxarency,  the  least  part  of  all  bodies  are  transparent,  ii.  365. 
Transparency  depends  on  homogeneity,  ii.  367.  Transpa- 
rent bodies  reflect  rays  of  one  colour,  and  transmit  rays  of 
another,  ii.  367.  Transparency  acquired,  ii.  384.  The  ad- 
vantages from  the  transparency  of  glass,  ii.  384. — See  Light, 
Opacity, 

Tropics  y  iii.  491. 

Truth,  love  of,  i.  03.  In  what  manner  it  should  be  sought  for,  i. 
63.  Its  gradual  advances,  i.  70.  Nature  of,  i.  92.  The 
cause  of,  injured  by  a  deference  to  the  authority  of  names, 
i.  255.  Its  analogy  or  correspondence  with  water,  ii.  14. 
Its  nature,  ii.  249. 

Tschirnhauscn,  1V1.  eft'tcls  produced  by  his  burning  glass,  ii.  172. 

Tubes,  eudiometer,  and  measure,  i.  437. 


Vacuum,  no  perfect  vacuum  of  air  can  be  produced  by  the  air- 
pump,  i.  135.     A  vacuum  in  nature  disproved,  iii.  221. 

Vapours,  not  permanently  elastic  fluids,  i.  441.  Are  destroyed 
by  pressure  and  cold,  i.  442.  The  accidents  occasioned  by 
their  sudden  expansion,  i.  324.  Occupy  14,000  times  more 
space  as  vapours  than  as  water,  ii.  73. 

Vapours,  vesicular  and  concrete,  an  account  of,  ii.  83.  In  the  form 
of  spherical  balls,  ii.  84.  Only  the  3600th  part  of  an  inch  in 
size,  ii.  84.  The  difference  between  vapours  and  elastic 
fluids,  iv.  432.  Watery  vapours  are  one  half  less  than  a 
like  volume  of  air,  iv.  433.  Vapours  consist  of  fire  and  wa- 
ter united,  iv.  435.  How  they  are  decomposed,  part  with 
their  water  to  hygroscopic  substances,  iv.  436.  The  conden- 
sation of  them  a  source  of  heat,  iv.  468. — See  Fire,  Air, 
Water. 

Vegetables,  when  acted  upon  by  the  solar  light,  afford  abundance  of 
vital  air,  i.  452  ;  but  in  the  shade,  the  air  they  yield  is  impure, 


GENERAL  INDEX.  595 

i.  454-.  They  imbibe  mephitic  and  emit  vital  air,  i.  457. 
Admirable  reflexions  on  their  uses,  i.  457.  They  consume 
more  water  than  falls  in  rain,  ii.  26.  Th,eir  influence  on 
the  climate  and  weather,  iv.  473. — See  Lights  Air,   Water, 

Velocity,  relative  and  absolute,  iii.  95.  Of  the  velocity  of  falling 
bodies,  iii.   130. 

Venus,  her  size,  distance,  diurnal  and  annual  revolutions,  differ- 
ent appearances,  atmosphere,  iv.  17.  Her  conjunctions 
with  the  sun,  iv.  67.  When  she  appears  stationary,  iv.  71. 
Her  phases,  iv.  73. 

Vince,  Mr.  his  observations  on  friction,  iii.  290 — 295.  His  ob- 
servations on  wheel  carriages  on  a  plain  ground,  iii.  311. 

Vision  is  caused  by  the  refraction  of  the  rays  of  light,  ii.  267. 
Why  are  not  objects  seen  in  an  inverted  position  \  ii.  272. 
Vision  is  not  produced  on  the  optic  nerve,  ii.  274.  Of  the 
extent  and  limits  of  vision,  ii.  276.  Is  limited  by  various 
means,  ii.  277.  Vision  is  confused  by  the  undulating  mo- 
tion of  the  air,  ii.  278.  The  angle  of  the  least  vision,  ii, 
280.  Of  distinct  and  clear  vision,  ii.  280.  On  what  it  de- 
pends, ii.  281.  At  what  distance  it  is  perceptible,  ii.  284. 
The  appearance  of  distance  affected  by  light  and  colours,  ii. 

310.  Mistakes  concerning  distances,  ii.  314.  Fallaciesof 
vision  explained,  ii.  319.  Of  vision  by  images,  ii.  320. — 
See  Light  eye. 

Vitreous  electricity. — See  Electricity* 

Voice  of  man,  wonders  and  variety  of  it,  i.  238. 

W 

Walker,  Mr.  of  Oxford,  his  experiments  for  freezing  mercury, 
ii.    55. 

Water  is  converted  into  vapour  whenever  the  pressure  of  the  at- 
mosphere is  diminished  to  a  certain  degree,  i.  142.  Gra- 
dually parts  with  its  latent  fire  whilst  it  is  freezing,  i.  311, 
May  be  cooled  several  degrees  below  the  freezing  point,  i. 

311.  Receives  a  less  quantity  of  heat  than  quicksilver 
does,  i.  316.  Water  boils  with  a  small  degree  of  heat 
when  the  pressure  is  removed,  and  vice  versa,  i.  329.  Water 
is  not  dissolved  by  air,  i.  333.  One-thousand  sixrhundred 
gallons  of  water  raised  from  an  acre  of  ground  in  a  hot 
summer's  day,  i.  348.  Water  constitutes  the  ponderable 
part  of  all  aeriform  fluids,  i.  418,  ii.  59.  Its  nature  and 
properties,  ii.  10.  Its  various  uses,  ii.  10.  Is  not  a  com- 
pound of  vital  and  inflammable  airs,  ii.  11.  Of  water  in  a 
fluid  state,  ii.  13.  Is  compressible  in  a  small  degree,  ii.  13. 
Enters  into  the  composition  of  all  bodies,  ii.  14.  Its  analogy 
to  truth,  ii.  14.  The  quantity  of  it  suspended  in  the  atmos- 
phere, ii.  15.  It  increases  the  weight  of  certain  bodies  ex- 
posed to  it,  ii.  16.  Has  a  similar  effect  on  the  human  frame, 
ii.  16.  Water  in  mixture  or  combination  with  bodies,  ii. 
20.     Is  a  general  cement,  ii.  20,     Is  never  obtained  pure. 


596  GENERAL  INDEX. 

ii.  20.  Is  of  different  degrees  of  softness,  ii.  21.  Is  puri- 
fied by  distillation,  ii.  22,  The  water  from  rain  is  not  a 
sufficient  supply  for  springs,  ii.  24.  The  subteraneous 
stores  of  water,  ii.  2$,  These  supply  springs  and  vegeta- 
bles, ii.  30.  Peacock's  filtration  of  it  by  ascent,  ii.  29. 
Sea  water  deposits  its  salt  by  freezing,  ii.  35,  According 
to  the  quantity  of  heat  is  the  quantity  of  salt  which  water 
can  dissolve,  ii.  36.  Water  becomes  ice  by  losing  its  fire, 
ii.  3P.  Boiled  water  does  not  so  easily  freeze  as  unboiled, 
ii.  39.  Water  may  be  cooled  below  the  point  of  congelation 
without  freezing,  ii.  40.  It  increases  in  bulk  just  as  it 
freezes,  ii.  41.  Ice  is  changed  into  water  by  means  of  fire, 
ii.  59.  Is  the  ponderable  part  of  all  aeriform  fluids,  ii.  59. 
Its  simple  particles  are  of  a  certain -form,  ii.  65.  By 
means  of  acids  the  particles  of  water  are  brought  nearer 
together,  without  losing  the  fire  of  liquifaction,  ii.  66.  Is 
probably  the  principal  constituent  in  oils  and  salts,  ii.  67. 
Is  the  universal  menstruum,  ii.  67.  Water  combines  with 
all  other  substances,  ii.  68.  \\  ater  expanded  in  vsipour  is 
800  times  rarer  than  air,  ii.  70  ;  and  14,000  times  rarer 
than  itself,  ii.  72.  Is  a  principal  ingredient  in  vegetable 
and  animal  substances,  ii.  86.  The  varieties  cf  the  neather 
depend  on  the  changes  of  water,  iv.  410.  Can  receive  a 
greater  degree  of  heat  before  it  boils,  than  when  ii  boils,  iv. 
425.  Is  not  held  in  solution  by  air,  iv.  433.  In  what  man- 
ner it  is  received  by  different  hygroscopic  substances,  iv.  436. 
— See  Fire,  Air,   Vapour,  Evaporation, 

Waters,  mineral,  their  nature  and  properties,  ii.  86.  Their  dif- 
ferent qualities,  ii.  88.     Are  artificially  made,  ii.  89. 

Weather,  knowledge  of,  very  interesting,  iv.  405.  But  at  pre- 
sent is  uncertain,  iv.  407.  The  phenomena  which  are  to 
be  observed,  iv.  477.  Depend  on  the  circulation  of  matter, 
iv.  410.  Inquiries  concerning  it  ;  instruments  to  be  used, 
iv.  411.  A  barometer,  iv.  411.  How  to  attain  a  more 
perfect  knowledge  of  the  weather,  iv.  478.  Signs  of  the 
weather  from  the  barometer,  iv.  479.  From  the  thermome- 
ter and  hygrometer,  iv.  483.  From  the  appearance  and 
different  currents  of  clouds,  iv.  483. 

Wedge,  its  use,  iii.  256.  A  simple  instrument  to  illustrate  its 
theory,  iii.  261. 

Whalebone,  slips  of,  best  substance  for  a  hygrometer,  iv.  491. 

Wheels,  of  their  work,  iii.  279.  How  to  compute  their  forces,  iii. 
280. 

Wheel  and  axis,  its  properties,  iii.  248.  Acts  as  a  perpetual  lever, 
iii.  250.  Crane-wheel,  capstan,  iii.  251.  Watch  spring, 
iii.  252.     Of  the  fly-wheels,  iii.  284. 

Wheel-carriages,  on,  iii.  306.  Their  utility,  iii.  308.  On  the 
centre  of  gravity  in  wheel  carriages,  iii.  310.  Observations 
on  them  on  plain  ground,  iii.  311.     On   hard   ground,  with 


GENERAL  INDEX.  597 

obstacles,  iii.  311.  On  sand,  iii.  314.  The  advantages  of 
springs,  iii.  314.     The  reason  of  this,  iii.  315. 

Whirling  table,  Ferguson's  description  of,  iii.  319.  Description 
of  an  improved  one,  iii.  336. 

Wieglib,  Mr.  a  German  chemist,  an  abstract  of  his  Dissertation 
on  Phlogiston,  i.  507.  Supported  by  the  experiments  of  Mr. 
Green,  i.  517.  His  analysis  of  mineral  waters,  ii.  97. 

Wilson,  Mr.  his  experiments  on  phosphoric  bodies  in  a  dark 
chamber,  ii.  392. 

Wind-gage,  by  Dr.  Lind,  iv.  495. 

Winds,  cause,  i.  99.  Bacon's  suggestion  for  a  history  of  them, 
iv.  455.  His  queries  concerning  them,  iv.  455.  Different 
causes  which  affect  them,  iv.  455.  Are  influenced  by  the 
return  of  air  to  a  state  of  vapour,  iv.  457.  On  the  origin  of 
winds,  iv.  457.  Their  irregularities,  iv.  457.  Are  affected 
by  the  diurnal  rotation  of  the  earth,  iv.  459.  Various  tem- 
pests produced  by  winds,  iv.  463.  Are  affected  by  the  soil 
over  which  thev  blow,  iv.  464.  Remarkable  unhealthy  winds, 
iv.  464.  Their  indication  of  a  change  of  weather,  iv.481. — 
See  Air, 

Wood  is  pervious  to  air,  i.  180.     Effects  of  this,  i.  181. 

Woods,  their  utility  in  a  country  respecting  rain,  iv.  469. 

World,  the  great  powers  of  it,  heat  and  gravitation  mutually 
counterbalance  each  other,  i.  386.  The  influence  of  these, 
i.  387.  The  northern  hemisphere  of  the  world  superior  to 
the  southern,  iv.  484. 


Zenith,  iii.  463. 
Zodiac ,  iii.  4-91. 


SUBSCRIBERS'  NAMES, 


DAVID  ALLEN,  attorney,  Winchester,  Virg. 
Jonathan  Aikin,  student,  D.  College,  N.  H. 
Samuel  Ayer,         do.  do.  do. 

Dr.  Joel  Abbot,  Washington,  Georgia. 
Rev.  Timothy  Alden,  Portsmouth,  N.  H. 
Alexander  Addison,  Esq. 

B 
Rev.  Samuel  Brown,  V.  D.  M.  Rockbridge. 
Hon.  John  Brown,  (Chancelor). 
William  Bernard,  Port  Royal,  Virginia. 
Rev.  James  Blythe,  Lexington,  Kentucky. 
Thomas  Billings,  Philadelphia. 
Laurence  Bassaile,  Caroline  county,  Virginia. 
John  C.  Beeler. 

Cornelius  Baldwin,  M.  D.  Winchester,  Virginia. 
Jonathan  Bryan,  merchant,  Charleston. 
Rev.  Duncan  Brown,  Pee-Dee,  S.  C. 
Jedidiah  Baldwin,  Esq.  P.  M.  Hanover,  N.  H. 
Gregori  Burnside,  student,  D.  College,  N.  H. 


Zedekiah  Belknap, 
Ichabod  Bartlett, 

do. 
do. 

do. 
do. 

do. 
do. 

John  Bontell, 
Edmund  Bayley, 
Jesse  Bliss, 

do. 

do. 
do. 

do. 
do. 
do. 

do. 
do. 
do. 

Charles  Boyd,  Chester  district,  S.  C. 
Ralph  Barkshire,  Esq.  Morgantown,  Virginia. 
John  Boggess,  do.  do. 

H.  H.  Brackenridge,  Esq. 
C 
John  Coalter,  attorney  at  law,  Staunton,  Virginia. 
Dr  Samuel  L.  Campbell,  Lexington,  Virginia.  (3) 
John  Carson,  sea  captain,  Philadelphia. 
Daniel  Conrad,  M.  D.  Winchester,  Virginia. 
William  Connell,  Esq   Pee-Dee,  S.  C. 
Increase  Cook  &  Co.  New  Haven. 


subscribers'  names, 


do. 

do. 

do. 

do. 

do. 

do. 

do. 

do. 

do. 

do. 

do. 

do. 

Josiah  P  Cooke,  student,  D.  College,  N  H. 

Hercules  Cushman,  do. 

William  Crawford,  jun  do. 

John  Chandler,  do. 

Ichabod  R.  Chadbourne,  do. 

William  Claggett,  do. 

Solomon  Cummings,       do. 

Rev.  Francis  Cummins,  Washington,  Georgia. 

Ebenezer  H.  Cummins,  attorney,  do.     do. 

Dr.  Thomas  Casey,  Vienna,  S   C. 

Rev.  Robert  M.Cunningham,  Greene  county,  Geo. 

JohnM.  Creary,  Esq.  sheriff,  C  D. 

Alexander  Cobean. 

Thomas  A   Clarke  Staunton,  Virginia. 

Edward  Cutts,  jun.  Esq.  Portsmouth,  N  H. 

Mathew  Carey.  (12) 

D 

Robert  Doak,  Esq.  farmer,  Augusta. 
Joseph  Downing,  Wilmington. 
Timothy  Dargan,  Esq.  Pee-Dee,  S.  C. 
Robert  Davidson,  S.  T.  P.  Carlisle. 

E 

Josiah  Espy,  Esq.  Bedford,  Pennsylvania    (2) 
Dr.  William  Edwards,  New-London,  Virginia. 
Nimrod  Evans,  Esq.  Morgantown. 

F 
Timothy  Farrar,  student,  D.  C.  N.  H. 
Luther  Fitch,  do.  do. 

William  P.  Farrand. 

G 

David  Gregg,  Pee-Dee. 
John  Golton,' student,  D.  C.  N.  H. 
Messrs.  Gold  &  Riely,  merchants,  Winchester,  V. 

H 

Henry  Harford,  Esq.  Darien,  Georgia. 
Samuel  Harper,  P.  G.  Augusta. 
John  Hoomes,  Esq.  B.  Green  Car.  county,  Virg. 


subscribers'  names. 

Wade  Hampton,  Esq   Columbia,  S.  C. 

Rev  Joseph  Henderson,  Indiana. 

Dr.  George  Hayes,  do. 

John  M.  Hanckel,  teacher  Burltngton  Academy. 

Solomon  Henkel,  P.  M.  New-Market,  Shannan- 
doah,  Virginia. 

Hugh  Holmes,  attorney  at  law,  Winchester,  Vir- 
ginia. 

Robert  Huntington,  student,  D.  C.  N.  H. 

Jonathan  Hunt,  do.         do.       do. 

Thomas  Hardy,  do.      .    do.       do. 

Eliphalet  Hardy,  do.         do.       do. 

Levi  Heyward,  do.         do.       do. 

Rev.  John  Hodge,  Greene  county,  Georgia. 

Arthur  Hicklin,  Chester  district,  S.  C. 

Rev.  Dr.  William  Hollinshead,  Charleston.  (3) 

Rev.  William  Hill   Winchester,  Virginia. 

Austin  Hazen,  student,  D.  C.  N.  H. 

David  Hogan,  Philadelphia.  (2) 

j 

Thomas  C.  James,  M.  D.  Philadelphia. 
John  Jamison,  farmer,  Augusta. 
Thomas  B.  Jansen,  bookseller,  N.  Y. 
Thomas  Jenkins,  Esq.  Chester  district,  S.  C. 
William  N.  Jarrett,  Esq. 
Rev.  John  Jones. 

K 
George  Kennedy,  P.  M.  Chester  C.  House,  S.  C. 
Robert  Kimball,  student,  D.  C.  N.  H. 
Kimber,  Conrad,  &  Co. 
Rev.  Dr.  I.  S.  Keith,  Charleston. 

L 

Captain  Elisha  Leak,  Goochland. 
Thomas  Lomax,  St.  Tobago,  Caro.  co.  Virginia. 
James  Lewis,  student,  D.  C.  N.  H. 
Samuel  Lowrey,  C.  L   C.  D. 
W.  G.  Lyford,  Staunton,  Virginia. 
E.  Larkin,  Boston.  (10) 

VOL,  IV.  4  1 


subscribers'  names. 

M 

B.  k  C.  Morris,  merchants,  Staunton,  V. 

Philip  H.  Mattes,  Easton,  Pennsylvania. 

Rev.  Dr.  Jedidiah  Morse. 

Major  James  Morton,  Prince  Edward.  (18) 

Samuel  Means,  Man's  Lick,  New  Lewisville,  Ken- 
tucky. 

Rev  William  M  Theeters. 

Charles  Magill,  attorney,  Winchester. 

David  M'Clure,  Shippensburgh. 

William  F.  Morrison,  student,  D.  C.  N  H. 

Royal  A.  Merriam,         do.  do. 

Rev.  Samuel  Monett,  Staunton,  Virginia. 

George  Mitchell,  do. 

Manning  &  Loring,  Boston. 

Alexander  lYrCleland,  merchant,  Morgantown. 

William  M'Cleery,  Esq. 

Joseph  E.  Muse,  Cambridge,  Maryland. 

James  M'Cormick,  Esq.  A.  B.  professor  of  mathe- 
matics and  natural  philosophy,  D.  C.  N.  H. 

John  M'Mullin,  goldsmith  and  jeweller. 

Robert  MTarlin. 

N 
Alexander  Nelson,  Esq.  Staunton,  Virginia. 

Thomas  Nelson,  student  at  Washington  Academy. 

James  P.  Nelson,  merchant,  Middlebrook. 

Rejoice  Newton,  student,  D.  C.  N.  H. 

George  Newton,     do.  do. 

Samuel  M.  Neal,  Chester,  D.  S.  C. 

Francis  Nichols,  Philadelphia. 

O 

Samuel  Olden,  N.  Jersey. 
Samuel  Osgood,  student,  D.  C.  N.  H. 
John  Ormrod.  (3) 
Walter  Oliver,  Carlisle. 
Joseph  Olden,  student,  Princeton. 

P 

John  Pratt,  Cambden,  Virginia. 
Gen.  Robert  Porterfield,  Augusta. 
Comegys  Paul>  Trenton. 


Jacob  Peck,  Staunton. 
Thomas  Park,  A.  B.  Pee-Dee. 
Phineas  Parkhurst,  student,  D.  C.  N.  H. 
Albion  K.  Paris,  do.  do. 

James  EL  Parmeti,       do.  do. 

William  Partridge,      do.  do. 

R 

Matthias  Roush,  No.  1 S,  N.  Front-street. 
Rev.  John  H.  Rice,  Prince  Edward. 
John  Reily,  principal  of  the  academy  at  Frankford. 
Alpheus  Roberts,  D.  C.  N.  H. 
John  Rosborough,  Esq.  C.  D. 
William  Reynolds,  Esq.  Bedford  county,  Penn. 
James  Robb,  Philadelphia. 

S 

David  Steel,  miller,  Augusta. 

Godfrey  Souders,  C.  P.  Printer,  Philadelphia. 

Valentine  Sevier,  Esq.  Tennessee. 

Dennison  Smith,  student,  D.  C.  N.  H. 

Benjamin  Sawyer,     do.    .  do. 

Ruggles  Sylvester,    do.  do. 

Amos  Spaldins,         do.  do. 

William  &  Archibald  Simpson,  Esqrs.  Wilkes 
county,  Georgia. 

Rev.  I.  W.  Stephenson,  South 'Carolina. 

Rev.  David  Snodgrass,  Greenville,  Mississippi  Ter- 
ritory. (2) 

William  Snodgrass,  Esq.  do. 

John  Stealey,  Esq. 

Dr.  Samuel  M.  Shute,  Bridgetown,  West  N.  Jersey. 

George  Sullivan,  Esq.  Exeter,  N.  H. 

Samuel  H.  Smith,  Clinton,  near  Zaneville,  Ohioc 

Micajah  Speakman,  yeoman,  do. 

T 

David  Thomson,  student,  D.  C.  N.  H. 
Silvanus  Thayer,      do.  do. 

Arad  Thompson,      do.  do^ 

David  Terril,  Esq.  Wilkes  county,  Georgia, 


subscribers'  names. 

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U 
Timothy  Upham,  Portsmouth,  N.  H. 

V 

Thomas  Vickray,  student,  D.  C.  N.  H. 
Dr.  John  Vaughn,  Wilmington. 

W 
Rev.  Abner  Waugh,  rector  of  St.  Mary's,  Bow- 
ling-Green,  Caroline  county,  Virginia. 
Henry  W.  Weston,  Philadelphia. 
Lawrence  A.  Washington,  Winchester,  Virginia. 
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,  Samuel  C.  Webster,    do.  do. 

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C.  W.  Wever,  Prospect  Hill,  Virginia. 
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Y 

William  Young,  Rockland,  Delaware, 
John  Young,  Esq. 

Z 

Col.  William  Zimmerman,  Pee-Dee. 


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ARK    ALSO   ISSUED    FOR    PRINTING, 

A  THEOLOGICAL  DICTIONARY, 

In  two  octavo  volumes, 
Containing  Definitions  of  all  religious  terms ;  a  comprehensive 
view  of  every  article  in  the  system  of  Divinity  ;  an  impartial  ac- 
count of  all  the  principal  denominations  which  have  subsisted  in  the 
world,  from  the  birth  of  Christ  to  the  present  day.  Together  with 
an  accurate  statement  of  the  most  remarkable  transactions  and 

events  recorded  in  Ecclesiastical  History By  Charles  Buck 

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Commendations  of  the  London  Reviewers. 

"  A  very  excellent  and  useful  book,  the  result  of  much  labour 
and  investigation,  and  a  remarkable  talent  for  clearness  of  defini- 
tion and  description.  This  undertaking,  in  its  own  nature  very  com- 
plicated and  extensive,  has  not  here  fallen  into  unworthy  hands. 
The  diligence  of  the  author  has  rendered  it  very  copious  ;  and  the 
soundness  of  his  understanding  has  made  it  abundantly  instructive. 
It  is  in  general  free  from  bigotry,  and  may  be  used  advantageously 
by  Protestants  of  all  descriptions,  and  indeed  by  all  Christians." 

Brithh  Critic 

**  In  former  periods,  when  religious  controversy  was  more  in 
vogue  than  it  is  in  these  days,  a  publication  like  the  present  would 
have  been  in  great  request,  and  secure  of  a  rapid  sale.  It  possesses 
value  however,  independently  of  temporary  circumstances.  In  these 
volumes  a  neat  snd  succinct  account  of  various  religious  opinions 
is  given,  and  which  seems  to  us  to  possess  much  correctness." 

Monthly  Reviev.\ 

"  The  compiler  seems  to  have  undertaken  this  work  with  a  full 
view  of  the  danger  attending  the  enterprise.  He  observes,  *  that, 
while  he  has  endeavoured  to  carry  the  torch  of  truth  in  his  hand, 
he  has  not  forgotten  to  walk  in  the  path  of  candour.'  To  this  decla- 
ration we  give  every  degree  of  credit.  He  has  laboured  for  his 
information,  and  in  general  has  obtained  it.  He  is  strictly  ortho- 
dox in  his  opinions,  yet  is  candid  in  dilating  on  those  of  others.  Ho 
is  in  general  very  clear,  as  well  as  candid,  in  the  explanation  of  most 
opinions.  The  work  possesses  considerable  merit." 

Critical  Review. 

"  A  work  of  this  nature  has  long  been  a  desideratum  in  the  Chris- 
tian world;  yet  this,  we  believe,  is  the  first  attempt  that  has  been 
made  to  furnish  the  public  with  a  compendious  dictionary,  explain- 
ing the  various  terms  which  have  obtained  general  currency  in  di- 
vinity. It  is  very  different  from  a  Dictionary  of  the  Bible  ;  such  as 
Calmet's,  Wilson's,  Brown's,  &c....Here  we  have  an  interesting  im- 
partial account  of  the  various  sects  and  denominations  that  have 
arisen  and  flourished  in  the  visible  Kingdom  of  Christ,  in  every  age 
and  nation.  The  principal  events  in  ecclesiastical  history  are  briefly 
related  with  candour,  fairness,  and  undeviating  regard  to  truth.  A 
mass  of  useful  information  is  laboriously  collected  and  judiciously 
compressed.  The  definitions  of  terms  are,  in  general,  concise  and 
accurate  ;  and  though  we  are  not  partial  to  the  method  of  those  who 
always  lay  a  considerable  stress  on  the  etymology  of  words,  in  or- 
der to  determine  their  precise  meaning,  yet  we  think  Mr.  B.  has 
manifested  considerable  judgment,  attention,  and  care,  in  the  use 

he  makes  of  it We  also  very  highly  approve  and  commend  the 

Chi-istian  Spirit  which  it  uniformly  breathes From  a  careful  pe- 


iusal  of  this  volume,  we  most  cordially  recommend  it  to  our  read- 
ers, as  well  calculated  to  inform  the  inquiring-,  to  instruct  the  ig- 
norant, and  to  establish  the  Man  of  God  in  his  attachment  to  the 
Lord  Jesus,  as  revealed  in  the  Holy  Scriptures." 

Theological  Review. 
««  Though  we  have  had  various  Dictionaries  of  the  Bible,  we 
have  never  before  seen  Divinity  and  Ecclesiastical  History  redu» 
ced  to  this  convenient  form.  Mr.  Buck  is  certainly  entitled  to 
much  praise  for  the  labour  and  care  with  which  he  has  collected 
and  arranged  a  body  of  information  that  will  be  found  highly  use- 
ful for  ministers  and  private  Christians,  especially  such  as  are  not 
accommodated  with  extensive  libraries." 

Evangelical  Review. 

W.  W.  Woodward  has  likewise  issued  Proposals  for  printing  by 
Subscription, 

[In  three  vols.  12mo.  at  §1  per  vol.  handsomely  bound  and  lettered] 

'      THE  MISCELLANEOUS  WORKS 

OF    THE 

REV.  CHARLES  BUCK, 

Minister  of  an  Independent  Church  in  London,  and  Author  of  the 
deservedly  celebrated  and  highly  useful  Theological  Dictionary, 

CONTAINING    THE 

YOUNG  CHRISTIAN'S  GUIDE, 

OR  I 

Suitable  Directions,  Cautions,  and  Encouragement) 

THE  BELIEVER, 

On  his  First  Entrance  into  Divine  Life, 

This  work  contains,  among-  other  things,  Rules  for  understand- 
ing- the  Scriptures  ;  Advice  as  to  reading  —Hearing — Joining  a 
Church — Receiving  the  Ordinance  of  the  Lord's  Supper — The  Im- 
provement of  Time — Zeal — Leadings  of  Providence — Prayer — 
Usefulness,  &c.  Cautions  as  to  forming*  Connexions — Marriage — 
Novelty — Curiosity — Anger — Discontent — Bigotry,  &c. — Dress — 
Recreations — Spiritual  Declension,  &c.  Encouragement  under 
Despondency — Temptations — Satanic  Suggestions — Variety  of 
Opinions — Persecution — Desertion — Fear  of  Death,  &c.  &c.  &c. 

A  TREATISE 
RELIGIOUS  EXPERIENCE, 

In  which  its  True  Nature,  Evidences,  and  Advantages,  are 
considered, 

ANECDOTES, 

Religions,  Moral,  and  Entertaining, 

Alphabetically  arranged,  and  interspersed  with  a  variety 

of  Useful  Observations. 

"  Seize  every  opportunity  of  introducing  or  maintaining  spiritual 

converse.     In  order  to  this,  furnish  your  mind  with  an  extensive 

stock  of  interesting  anecdotes  and  striking  hiiats." 

Brown. 
i 


LIKEWISE, 

Proposals  are  issued,  for  Printing  by  Subscription. 
(going  to  press) 

A  COMPLETE  HISTORY 

UF    THE 

HOLY  BIBLE, 

In  two  octavo  volumes,  as  contained  in  the  Old  and  New  Tes- 
taments, including  also  the  occurrences  of  four  hundred  years,  from 
the  last  of  the  prophets  to  the  birth  of  Christ,  and  the  life  of  our 
blessed  Saviour  and  his  apostles,  Sec.  With  copious  Notes  critical 
and  explanatory,  practical  and  devotional.  From  the  text  of  the 
Rev.  Lawrence  Bowel,  A.  M.  with  considerable  additions  and  im- 
provements, by  the  Rev.  George  Burder,  author  of  the  Village 
Sermons,  Notes  to  Pilgrim's  Progress,  £cc.    Price  to  subscribers,, 

g  2  25  per  volume,  bound  and  lettered one  for  every  five  sub- 

scribed  for, 

Mr.  Burder's  Remarks. 

•«  The  History  of  the  Bible,  by  the  Rev.  Mr.  Howell,  being 
much  esteemed,  and  having  become  very  scarce,  I  was  desired 
by  the  publishers  of  this  edition  to  prepare  it  for  the  press  :  in  do- 
ing which,  I  found  much  more  labour  than  I  expected ;  for  Mr. 
Howell's  style  was  frequently  negligent,  and  required  some  im- 
provement to  render  it  agreeable  to  modern  and  intelligent  read- 
ers. Many  events,  recorded  both  in  the  Old  and  New  Testaments, 
appear  to  me  to  have  been  passed  over  too  slightly.  To  his  account 
of  these  things  I  have  made  considerable  additions;  and  have 
sometimes  ventured  to  intermingle  a  few  practical  reflections.  I 
have  also  endeavoured  to  throw'  that  light  upon  some  of  the  ob- 
scurer passages  of  the  Old  Testament  with  which  we  are  furnish- 
ed, by  the  New.  The  history  of  our  Saviour's  sufferings,  death, 
and  resurrection,  is  much  enlarged,  for  which  I  am  indebted  chiefly 
to  those  excellent  writers,  Drs.  Doddridge  and  Macknight;  from 
whom,  as  well  as  from  several  other  able  critics,  I  have  borrowed 
many  explanatory  notes,  which,  I  trust,  have  contributed  greatly 
to  enrich  the  work;  and  throughout  the  whole,  I  have  laboured  to 
render  the  history  uniformly  Evangelical.  In  a  word,  if  Mr.  How- 
ell's original  work  received  the  approbation  of  the  public,  I  hope 
this  improved  edition  will  still  be  more  acceptable,  and  be  found 
generally  useful  to  Christians  of  all  denominations. 

GEORGE  BURDER 

London,  January,  1807". 

The  following  valuable  Works  are  aho  printed. 
Scott's  Essays  on  Important  Subjects  in  Religion,  price  g  l....Rev. 
Roland  Hill's  Village  Dialogues,  in  two  vols,  price  g  2.,..Village' 
Sermons,  by  George  Bur-der,  2  vols*-  g  2....Baxter's  Miscellaneous 
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This  publication  is  highly  commended  by  the  Reviewers Bon-. 

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four  8yo.  volumes,  g  8  50. 


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